Chronic Effects of Acidic Precipitation
and Heavy Metals on Forest
Ecosystems
The Acidity Problem-Its Nature,
Causes, and Possible Solutions1
Lowell smith2 Abstract: Interest within the scientific community in North America and Europe about the nature, effects, and causes of atmospheric acid deposition has grown rapidly over the past decade. This interest has recently intensified because of the explosion in public awareness of, and concern over, the acid deposition problem, and a growing political will to address the problem within appropriate national and international forums. his paper sketches the nature of the acid deposi- tion problem; describes the atmospheric processes that convert precursor emissions into acidic compounds as these are transported over distances ranging from a few to more than a thousand kilometers; discusses past and possible future trends in geo- graphical distribution and rate of acid deposition; and summarizes the governmental activities which have been initiated to address the problem. The scientific study of acid deposition is
archtypical of many contemporary environmental
problems, in that it necessarily covers a wide spectrum of disciplinary interests. Simply
listing the many subdisciplines involved would fill more than a page. It is important for the active research worker in the
field to recognize the many interconnections
between her or his own endeavors and other
research areas. This involves a careful
balance because, at the same time one is
encouraging a cross-fertilization of ideas
among various disciplinary efforts, one must guard against extending scientific judgements
beyond one's own limits of competency. Since
the author is fully aware of this hazard, he
'presented at the Symposium on Effects of
Air Pollutants on Mediterranean and Temperate
Forest Ecosystems, June 22-27, 1980, Riverside, California, U.S.A.
'~irector, Program Integration and Policy
Staff, Office of Research and Development, U.S.
Environmental Protection Agency, Washington, D.C. invites reaction from any who take exception to the summaries presented in this paper.3 The major features of acid deposition are: 0
acid deposition results primarily from the combustion of fossil fuels which releases sulfur dioxide (SO2) and nitrogen oxides (NOx) in the form of nitric oxide (NO) and nitrogen dioxide (NO?) to the atmosphere; depositing acidic material is formed out of these precursor emissions by means of a large number of chemical reactions as the emissions are transported away from their source region; he views expressed in this paper are those of the author and do not neces- sarily reflect those of the Environmental Protection Agency. acidic material is deposited dry in the form o f fine particu- late matter and is incorporated into all of the possible physical forms of precipitation; transport distances between source and receptor regions can exceed a thousand kilometers, although a major source can, on occasion, significantly affect the rate of acid deposition within the first few kilometers downwind of the source; and the effects of the deposited acid can vary markedly, depending upon the form in which the acid is deposited, the biologic, geologic and hydrologic pathways between deposition site and the receptor of interest, and the sensitivity of the receptor. This paper explores these major features of acid deposition. Nature of Acid Deposition Acidic compounds may be deposited by several forms of precipitation (rain, snow, hail, dew, rime, and mist) as well as by fine particles that settle out of the atmosphere on to biologic, mineral, and aquatic surfaces, and on to man made materials, buildings, and artistic objects. Because monitoring techniques are not well developed for measuring and character- izing the dryfall component of acid deposi- tion, comparatively little is known about its extent and variability (~urham and Hicks, 1980). Monitoring data from eastern North America indicates that the sulfate ion is the predominant anion present in acidic precipitation; the nitrate ion is associated with about half as much acidity as the sulfate ion, and chloride and other anions make sub- stantially lower contributions (Hales, 1980).
In some areas of the West the nitrate ion contributes an equal or larger share than the sulfate ion (McColl, 1980; and Morgan and Liljestrand, 1980). Some regional scale atmospheric models estimate that up to half of the sulfate component may be deposited dry, while other models estimate a lower percentage (Whelpdale and Galloway, 1979).
The variance results from a basic lack of understanding of the physical, as well as biological processes, and their varia- tion over space and time, as these processes transfer gases and particles across the atmosphere/surface interface (~urhamand Hicks, 1980). In addition, nitric acid depositing onto surfaces out of its vapor phase may at times be an important contributor to dry deposition; but even less is known about its ambient concen- tration and geographical distribution (Altshuller, 1979). Thus, until monitor- ing techniques are developed for routine reliable measurements of dryfall deposition, we are compelled to rely on isolated spot measurements and tentative inferences to characterize this important phenomenon. The characterization of wet deposition is a more tractable problem. While it is doubtful that precipitation at any place on the globe is entirely free of anthropogenic contaminants, relatively "clean" rainfall is generally found on the windward edge of continental land masses and where frontal storm systems have traversed large expanses of sparsely populated land surfaces (Granat, 1978). Because of the formation of carbonic acid from the hydrolization of background atmospheric carbon dioxide, such clean rainfall is thought to have a pH of about 5.6, approximately 25 times more acidic than a neutral pH of 7 (Likens, 1979). Yet even the pH of et. g.,
rainfall relatively unaffected by anthro- pogenic emissions can vary by several pH units due to the entrainment of alkaline soil particles, reaction with atmospheric ammonia, and possibly other little under- stood factors (Stensland, 1979). -
The concentration of acidity in precipi- tation is observed to be quite episodic (Smith and Hunt, 1979), which combined with the episodicity of precipitation rates, leads to great variability over space within a particular rain event and over time at a particular location of the rate of acid deposition (~ales,1980). A range of over seven orders of magnitude of acidity (pH 2 to pH 9) has been recorded for various isolated rainfall events in North America. Averaged over time, spatial variations tend to vanish at locations distant from large sources. An excep-
tion is the orographic effect produced by high terrain features on precipitation rates. This effect can be further augment- ed by the increasing acidity of cloudwater at higher elevations within a cloud structure (Falconer and Falconer, 1980). Terrain subject to orographic precipitation in North America and Europe, such as the Nothern Alps, that is downwind of high emission areas has been observed to sustain substantially increased quanti- ties of deposited acidic material than similarly situated low lying terrain (Schrimpff, 1980). upper soil horizons of forest soils subjected to heavy rates of acid deposition. In extreme cases ground water quality may be adversely affected (Hultberg and Wenblad, 1980). Abnormally low pH for drinking water supplies drawn from wells in some of these regions has been observed. This can result in unacceptably high concentrations of copper and lead in drinking water in those .
homes which are equipped with copper plumbing systems (~ultbergand Wenbald, 1980). Averaging wet acid deposition rates over an annual cycle for monitoring stations in eastern North America produces a pattern of depressed pH values in the northeast United States (Pack, 1980) extending northward into southern Ontario and Quebec, westward to the eastern Midwest, and southward along the Appalachian ridge into Tennessee and the Carolinas. Less depressed average pH levels extend deep into Florida &. , 1980), west to Arkansas and
(~rezonik,
Missouri, and northward an undetermined distance into Canada (Likens and Butler, 1980).
The average acidity of precipitation in the central portion of this pattern approaches pH 4, approxi-
mately a factor of forty more than what many consider to be the "normal" value of pH 5.6 (Likens and Butler, 1980). s.
Due to the spottiness of available monitoring information less is known about acid deposition rates west of the Mississippi River. Measurements made in the Boundary Waters Canoe Wilderness Area (northern Minnesota) (Glass and Loucks, 19801, the Colorado Rockies (northwest of ~enver) (Lewis and Grant, 19801, and California (the Sierra Nevada, and the Los Angeles and San Fran- cisco Bay basins) (McColl, 1980; and Morgan and Liljestrand, 1980) all suggest that this problem is not unique to the eastern half of the continent. Agricultural crop species cultured in pots and subjected to lowered pH simulated acid rain irriga- tion water have displayed mixed responses in yield (Lee,
1980).
Some cultivars exhibit enhanced yields and others depressed yields while many cultivars show no discernible effect on yield. The effect of acidic deposition on forest growth rates is more uncertain. Early stages of acidifi- cation can accelerate growth rates for a few years, possibly due to the increased mobilization of nutrients within the soil structure. But sustained elevated rates of acid deposition are hypothesized to depress forest growth rates in geologically sensitive areas due to nutrient depletion and elevated concentrations of AI^+
ion within the root zone. Other ecological effects are discussed elsewhere (Overrein, 1980, Norton, et. G . , 1980, and Cowling, 1980).
-
Effects of Acid Deposition Mechanisms of Atmospheric Formation Acid deposition creates a public policy problem to the extent that potentially sensitive receptors are harmed by the deposition rates they sustain. Research results on acid deposition effects are multiplying rapidly in North America and Europe (International Conference on the Ecological An impressive Impact of Acid Precipitation, 1980).
body of information now exists as to the impact of acidifying surface waters on the ecosystems they support (Gorham, 1976).
Some fish species show adverse effects below pH 6; while other more tolerant fish species do not evidence serious effects until pH 4 or lower has been reached. Eggs and especially the fry of sensitive species are more susceptible than are adult fish (Johannson, et. &., 1977).
Thus, during the initial melting
of a snowpack, acid pulses may be released which abnormally skew subsequent fish population distri- butions to the extent that one or more generations may be completely absent in some instances. Other portions of aquatic food chains can also be affected to the detriment of wildlife species at the top of these chains, including fish eating birds (Peakall, 1980) and freshwater wading birds (~oucks,1980). As previously stated, acid deposition is primarily the result of the anthropogenic release of SO2 and NOx to the atmosphere where these react through several available chemical pathways to sulfuric and nitric acids. To a lesser extent hydrochloric acid can also be involved. Coal and oil fired electric utility generating stations, predominantly in the central and eastern portions of the U.S. are responsible for two-thirds of the national SO2 emission inventory (U.S. EPA, 1979a). Nine-tenths of these emissions are from coal-fired stations. Industrial and commerical boilers account for nearly half of the remaining emissions. SO2 emissions in the U.S. are likely to increase slowly over the next few decades as emissions from well-controlled new sources slightly overbalance emission reductions expected to be achieved by the control or retirement of existing sources (Altshuller and McBean, 1979).
Canadian SO2 emissions come predominantly from their non-ferrous smelter industry at present, but by the end of the century coal fired utility boiler emissions are expected to about equal those from the smelter industry (Choquette, 1980). Recent evidence from Europe suggests that leaching of calcium and other nutrients from the lower horizons of sensitive soils and the concomitant
mobilization of Al3+ ion within these soils (Ulrich,
1980) is promoted by atmospheric acid deposition. Also observed is a reduction of the decomposition rate of forest litter and decreased numbers of micro-organisms (Baath, s. &., 1978) within the
z. e.,
Two-fifths of the nation's NOx emissions come from motor vehicles. Geographically, these mobile sources are clustered around large population centers. Nine-tenths of the remaining NOx emissions are released by the same large utility and industrial boilers which are responsible for the predominant portion of the SO2 inventory (U.S. EPA, 1979a). NOx emissions are expected to increase significantly in the U.S. and Canada over the remainder of the century, unless some promising breakthrough in NOx emission controls is achieved and rapidly implemented by industry (Altshuller and McBean, 1979). inventory has yet to be constructed. Stockyards, municipal waste water treatment plants, certain industrial processes, and decaying vegetable matter are among its important sources. Although no emission inventory exists for chloride ions, these emissions are likely to be primarily the result of burning high chloride coals produced from many coal seams in the midwestern and eastern U.S. Therefore, chloride emissions are likely to be colocational with large SO2 and NOx emitting sources. Natural emissio s of SO2 (Adarns,s. &., 1980)
and:ON
make only modest additions to the anthropogenic sources in eastern North America, though natural emissions may make an appreciable contribution to the global background of these
atmospheric constituents (Husar, s. al., 1978).
Some investigators have hypothesized that fly ash from coal combustion has historically played a major role in reducing atmospheric acidity and that recent efforts to control fly ash emissions have noticeably worsened the acid deposition Such an problem (Frohlinger and Kane, 1975).
effect is unlikely to have been nearly as important as was once supposed. As will be discussed later in this paper it is unclear what the historic trends for deposited acid are in the Northeast. Thus, such an ad hoc explanation may not be relevant to explain what is an inconclusive trend. More importantly, considerations such as the size of fly ash particles relative to the size of sulfate particles, the chemical composition of fly ash from midwestern and eastern coals, and the change from stoker-fired coal boilers to pulverized coal boilers during this period, all suggest that the neutralizing effect of emitted fly ash could only have been important in the immediate locality of a relatively few heavy fly ash emitting sources. These precursor emissions react in the atmos- phere with water vapor, other minor atmospheric constituents and sunlight to produce fine sulfate particles, nitric acid vapor and dilute acids in cloud droplets. These submicron particles agglomerate into larger particles until a natural barrier to further growth is reached at slightly above one micron mean diameter. The growing particle may be deposited in dry form onto a surface, incorporated into a cloud droplet (rainout), or scavenged by a falling raindrop (washout) (Hales, 1980). The hydroxyl ion in conjunction with sunlight is believed to promote the formation of particulate sulfate (Davis,
&., 19741, while
the peroxyl ion is believed to promote the conv r- sion of SO2 to sulfuric acid in cloud droplets.
Mechanisms of Atmospheric Acid Transport Several physical processes are important to the atmospheric transport of acid precursors and their acidic products. Particularly important for the S~~/sulfuric
acidlsulfate
complex is the elevated height of injection into the atmosphere level for SO2 emissions from most major power plant sources. A nocturnal inversion layer frequently isolates these tall stack plumes from In the summer months, substantially enhanced concentration of sulfate ion are found in deposited the gr~und(Smith, s. &., 1978) until after
sunrise when the incident solar energy begins to rainwater while nitrate ion concentrations tend mix the atmosphere through the activation of to be more constant throughout the year (Hales, convective cells. In the Midwest during summertime 1980). There is some evidence to suggest that conditions, a nocturnal bulge in the wind speed nitrogen oxides are deposited more rapidly from vertical profile is frequently observed at normal the atmosphere on the average than are sulfates tall stack plume heights. This condition can (Mueller, s.&. , 1979), so nitric acid -deposition may be relatively less important for receptor transport emissions from a tall stack several &.,
hundred kilometers overnight (Smith,
sites far from emission regions than is sulfuric 1978). The gas to particle conversion of the acid deposition. emissions in this displaced plume can be greatly Another complicating feature in the atmospheric accelerated the next day as these transported emissions are mixed with a polluted urban air mass. chemistry of acid deposition is the atmosphere's ability to partially neutralize its acid load. The highest ambient concentrations of particulate Alkaline fine wind blown soil particles, particu- sulfate are observed under summertime conditions larly over arid regions, appear to neutralize when a synoptic scale high pressure system stalls , 1980),
the acid load (Eisenreich,
for more than a day over a region of high SO2 emission or create aerosols with basic chemical properties. density, such as the Ohio River Basin (Hidy, s. al.,
The ammonium ion is also effective in partially 1978).
In this situation weakly circulating winds can neutralizing dry and wet atmospheric acidic materials. Many natural and anthropogenic sources trap a large air mass while it is being continually filled with precursor emissions (Vukovich, 1979).
The produce ammonia, but a complete ammonia emissions higher temperature, moisture and sunlight levels generally encountered under these conditions tend 4~ersonalcommunication from Dr. Rudolph to increase the chemical reactivity of the atmosphere, Husar, August 15, 1980, Washington, D. C. so the higher concentration of precursor emissions 'personal communication from Dr. A. L. Lqzrus, can more rapidly be converted to particulate sulfate. May 21, 1980, Washington, D. C.
s.
5
s.
s.
&..
I
A common atmospheric cleansing mechanism for this condition is for a cold front to approach the high pressure center from the north or northwest (Whelpdale, 1978).
This creates a strong pressure gradient which sweeps much of the polluted air mass parallel to the line of the front for many hundreds of kilometers, frequently to the northeast. (LaFleur and Whelpdate, 1977).
Frontal storm activity can further remove considerable amounts of pollutants as rainout and washout. Similarly, large convective storms are believed to be an efficient mechanism for processing and removing pollutants from the large volume of air they entrain. These storms are capable of pumping large quantities of polluted air from the planetry boundary layer to high elevations where these can be left as isolated patchy layers to be transported considerable distances and eventually deposited. Alternating periods of stagnation and ventila- tion over a high emission area produce episodic concentrations of pollutants (~ttar,1978) which result in highly variable acid deposition rates in downwind regions. Likewise, the variability in wind direction, as it guides and mixes isolated plumes from major sources and the regional scale plumes described above, adds to the temporal variability of deposition rates at any monitoring site. Thus, acid deposition must be viewed as a stochastic process which, in areas not under the influence of a major source and not affected by orographic terrain effects, is temporally and spatially chaotic over a small scale but is rather spatially homogeneous within a given region when averaged over a large number of events. Historical Trends in Acid Deposition Rates While the routine measurement in a scienti- fically reliable manner of wet deposited acid has only recently, with the exception of a very few monitoring stations, been undertaken .
in this country, it has been practiced in
several European countries over a longer period of time (Granat, 1978).
Both the European and North American experiences have demonstrated the need for strict quality assurance procedures for collecting and analyzing the rain water samples. Failure to establish sufficiently stringent quality assurance procedures early in monitoring programs has created questions about the validity of much of the early data (Tyree, 1980). The need to calculate retroactively the acidity of monitored rainfall using one of several ion balancing procedures, has compounded these measurement difficulties since acidity or pH was usually not measured directly (Kramer, 1978).
Such procedures can propogate the experimental uncertainties, which were introduced by the collection and laboratory procedures employed, into rather sizable uncertainties in the calculated hydrogen ion concentra- tion. Further careful analysis of all available rainwater chemistry monitoring data is required in order to establish the level of certainty with which historical trends of acid deposition in North America can be determined. Fortunately, it may be possible to describe the gross features of deposition trends by relying on other related physical phenomena for corroborative support. Since sulfate aerosols are highly efficient light scatterers due to their characteristic submicron size, and since sulfate is the dominant component of Eastern aerosols (U.S. EPA, 1979b), visibility trends probably serve as a useful indicator for dry sulfate deposition trends. The ratio of dry to wet sulfate deposition should have remained relatively constant for a particular area unless a climate change has occurred. Visibility measurements have been routinely made at medium and large size airports in the U.S. for several decades. Recent analysis of these data shows that summertime visibility has significantly deteriorated throughout large portions of the eastern U.S. (Trijonis and Shapland, 1979). Some regions such as the Tennessee Valley appear to have sustained nearly a factor of two decrease in average summertime visibility over a twenty- five year period (Husar,
&., 1979). Similar trends
in summertime solar insolation are also suggestive of a increase in atmospheric turbidity during this same period. Further analysis effort is required to assess the possible causal relationships between trends in precursor emission rates and these trends in environmental conditions. =.
Although the trends of such surrogates for acid deposition may only be used to corroborate an inconclusive record for monitored acid deposited, other studies strongly support the conclusion that anthropogenic emissions are deposited on ecosystems far from their point of origin. Dated lake bottom cores from remote lakes in North America and Greenland icecap cores show a marked increase in deposition rates for fossil fuel combustion-related pollutants shortly after the beginning of the industrial revolution. A review of emission trends from U.S. sources over the past forty years indicates several clear trends. First, SO2 emissions increased about forty percent. ~ l t h o u ~ hemissions
~ 0 ~ have decreased from most economic sectors during this period, the electric utility sector's emissions increased by more than a factor of six during this same period (EPA, 1978).
Second, the increase in SO2 emissions from this one sector occurred concurrently with a substantial increase (by approximately a factor of five) in the stack height for utility sources. Third, SO2 emissions from coal burning changed from a wintertime peak to a summertime peak in emission &. , 1979). Fourth, the precursor
rate (Husar,
emissions for photochemical oxidants increased markedly during this time (EPA, 1978).
At the beginning of the period photochemical smog was hardly recognizable as a problem, whereas, currently urban plumes of photochemical oxidants now frequently blanket the Northeast during the summer months (Altshuller, 1978). Fifth, total NOx emissions approximately quadrupled during this period (EPA, 1978). =.
These trends suggest a situation in which the atmosphere has become chemically more reactive, and for which greater quantities of acid-forming precursors are added to the atmosphere during its most reactive period. Further, a substantially greater quantity of these emissions is now injected high into the mixed layer where the emissions and their reaction products have much longer residence times as they travel to areas remote from their point of origin. Possible Solutions At present there are no regulatory requirements that are primarily directed at reducing emissions to control acid deposition. However, the Clean Air Act's requirement to reduce ground level SO2 concentrations has achieved modest overall reductions in SO2 emissions during the first part of the past decade (u.S. EPA, But, average stack heights continued l979a).
to increase, as did the emission rates for NOv, over this period. Regulatory requirements established for new coal-fired utility boilers mandate control of seventy to ninty percent of their SO2 emissions and require NOx emission reductions of approximately forty percent (U.S. Federal Register, 1979).
EPA is currently developing new source control performance standards for industrial boilers which could require similar levels of control for this important source category. From a regulatory perspective the principal problem for SO2 emission control is the control of emissions from existing sources. In addition, there are large opportunities for improved NOx control requirements for new and existing sources. Others have maintained that the costs of emission control are so high that the only cost-effective mitigation measure is to raise artifically the pH of affected lake water by Repeated the addition of lime (~arnes,1979).
treatments would be required as long as acid deposition rates exceeded the geologically controlled release rates for a given area (Horn, s. &., 1980). Recent Swedish experience indicates that liming the entire watershed for a lake may be necessary, while others believe that even though it is possible to raise the pH of an acidified lake, it is not possible to restore the lake to a natural condition containing its original food chains. Others have observed that the potentially sensitive areas in North America cover millions of hectares, and question the feasibility of liming such a vast area for hundreds of years. Federal government efforts to address the acid deposition problem include: monitoring activities to establish more fully the geographical var-
iation and temporal trends in rainwater chemistry; research activities to determine more completely the range of environmental effects produced by acid deposition and the atmospheric processes which transport and convert emissions into acid deposition; assessment activities to determine the potential seriousness of the acid deposition problem in North America and the most cost-effective measures that could be employed as first steps to combat the problem; the creation of a ten year inter-agency Federal Acid Rain Assessment Program to coordinate the above activities; working with the states in order to promote a mutual understanding of the nature and causes of the acid deposition problem, and to encourage the states to engage in collective problem solving on this issue; and becoming a signatory to an inter- national Convention on Long-Range Transboundary Air Pollution, developed under the auspices of the United Nations Economic Commission for Europe, and pursuing bilateral discussions with Canada which are expected to evolve into formal negotiations on a U.S.-Canada bilateral transboundary air pollu- tion agreement. Such efforts cannot be expected to bring about immediate and complete relief from the acid deposition problem nor are they designed to achieve this
objective. Rather the goals are to establish as quickly as is feasible a scientific basis for under- standing the full range of receptors at risk and the extent of risk to each receptor category; to determine the extent of the control measures which are required to reduce the acid deposition problem to an acceptable level; to develop new air pollution control policies and strategies as necessary and appropriate; and to promote full cooperation and understanding across interstate and international borders to deal effectively with this complex and challenging problem. -
Acknowledgments The author is deeply indebted to many colleagues and research workers for the opportunity to learn from and share ideas with them. Particular credit is given to Dr. Brand L. Niemann for ^ ~ o t eadded in editing: A Memorandum of Intent was signed in Washington, D. C., on Aug. 5, 1980, by the two governments in which they agreed to initiate formal negotiations by June 1, 1981. his helpful comments, to Ms. Barbara H. Brandon and Mr. Paul Schwengels for their editorial advice, and to Ms. Mable Scales and Ms. Veronica Parker for their special efforts in typing the manuscript. Finally, I wish to thank Dr. Paul R. Miller for the extraordinary patience he displayed while coaxing me to complete this paper. -
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1980. Acid Precipitation and Wildlife. In Proc. of Action Seminar Acid Pre- cipitation (Nov. 1-3, 1979), Toronto, Ontario. Schrimpff, E. 1980. The Relationship Between Relief and the Precipitation Impact of Polycyclic Aromatic Hydrocarbons, Pesticides, Trace Metals and Some Macro-Elements in Northern Bavaria. In Proc. of the Int. Conf. on the Ecological Impact of Acid Precipitation (March 11-14, 1980, Sandefjord, Norway), in press. Smith, T. B., D. K. Blumenthal, J. A.
Anderson, and A. H. Vanderpol. 1978. Transport of SO2 in Power Plant Plumes: Day and Night. Atmos. Environ. 12: 605-611. Smith, F. B., and Hunt R. D. 1979. The Dispersion of Sulfur Pollutants Over Western Europe. Phil. Trans. R. Soc. Lond., A. 290: 523-542. Stensland, G.
1979. Precipitation Chemistry Trends in the Northeastern United States. Proc. of Twelth Annual Rochester Inter. Conf. on Environmental Toxicity: Polluted Rain (May 21-23), Rochester, New York. Trijonis, J. and T. Shapland. 1979. Existing Visibility Levels In the U.S. Report prepared for the U.S. EPA, Research Triangle Park, NC. Tyree, S. Y., Jr. 1980. Rainwater Acidity Measurement Problems. Atmospheric Environment (to be published). Ulrich, B.
1980. Deposition, Production and Consump- tion of Hydrogen Ions In a Beech and Spruce Ecosystem In the Soiling District. In Proc. of the Int. Conf. on the Eco- logical Impact of Acid Precipitation (March 11-14, 1980, Sandefjord, Norway), in press. U. S. Environmental Protection Agency. 1978. National Air Pollutant Emission Estimates, 1940-1976, Report No. EPA- 45011-78-003 Office of Air, Noise and Radiation, Research Triangle Park, N.C. U. S. Environmental Protection Agency. l979a. National Air Pollutant Emission Estimates, 1970-1978, Monitoring and Data Analysis Division Report, EPA-4501 4-80-002, Research Triangle Park, N.C. U. S. Environmental Protection Agency. 1979b. Protecting Visibility: An EPA Report to Congress. Report No. EPA- 45015-9-008 Office of Air, Noise, and Radiation, Research Triangle Park, N.C. U. S. Federal Register. 1979. Standards of Performance for New Stationary Sources: Electric Utility Steam Generating Units (June 11, 1979) 44:33580. '
Vukovich, T. 1979. A Note On Air Quality in High Pressure Systems. Atmos. Environ. 13: 255-265. Whelpdale, D. M.
1978. Large Scale Atmospheric Sulfur Studies. Atmos. Environ. 12: 661-670. Whelpdale, D. M., and J. N. Galloway. 1979. An Atmospheric Sulfur Budget for Eastern North America, Proc. of the W O
Symposium on Long Range Transport, October 1-5, Sofia, Bulgaria, World Meteorological Organization, Geneva, Switzerland. Acid Precipitation Impact on Terrestrial
and Aquatic Systems in Norway'
Lars N. Overrein
2
Abstract: In r e c e n t decades t h e a c i d i t y of r a i n and snow
has increased sharply over wide areas. The p r i n c i p a l
cause i s t h e r e l e a s e of sulphur and n i t r o g e n oxides by
t h e burning of f o s s i l f u e l s . The a i r q u a l i t y i n any one
European country i s measurably a f f e c t e d by emissions i n
o t h e r European countries. Strong a c i d s have lowered t h e
annual mean pH of p r e c i p i t a t i o n i n much of northern Europe t o between 4 and 5. In southern c o a s t a l a r e a s of
Norway, t h e annual mean a c i d i t y i n p r e c i p i t a t i o n i s now
4.3 pH-units, o r even more a c i d i c .
Acid p r e c i p i t a t i o n has increased leaching of n u t r i e n t s from t h e uppermost s o i l l a y e r s . These l o s s e s of
n u t r i e n t s may be expected t o decrease p l a n t growth, b u t
f i e l d evidence i n Norway and elsewhere, has not y e t
been obtained. I t i s p o s s i b l e t h a t p o l l u t e d a i r and
p r e c i p i t a t i o n over a period of years can influence p l a n t
production.
Atmospheric t r a n s p o r t of sulphur and o t h e r a c i d i f y i n g components has l e d t o extensive regional a c i d i f i c a t i o n of water courses i n a r e a s with very l i t t l e neutral i z a t i o n capacity. A c i d i f i c a t i o n of watercourses had
had major e f f e c t s on l i f e i n r i v e r s and l a k e s . Lakes
i n an a r e a of 13,000 km2 i n southern Norway have become
empty of f i s h i n r e c e n t decades, and a f u r t h e r a r e a of
approx. 20,000 km2 contains l a k e s with s i g n i f i c a n t l y r e duced f i s h stocks.
The e c o l o g i c a l impact of a c i d p r e c i p i t a t i o n has
been a m a t t e r of growing concern over t h e l a s t decade p a r t i c u l a r l y i n t h e i n d u s t r i a l i z e d c o u n t r i e s
of t h e Northern Hemisphere.
The Norwegian I n t e r d i s c i p l i n a r y Research ProE f f e c t s on F o r e s t and
gramme "Acid P r e c i p i t a t i o n
Fish", (The SNSF-project) was i n i t i a t e d i n 1972.
-
Presented a t t h e Symposium on E f f e c t s of A i r
P o l l u t a n t s on Mediterranean and Temperate F o r e s t
Ecosystems, June 22-27, 1980, Riverside,
C a l i f o r n i a , U.S.A.
2 ~ e s e a r c hD i r e c t o r ,The Norwegian I n t e r d i s c i p l i nary Research Program, Acid P r e c i p i t a t i o n
E f f e c t s on F o r e s t and Fish.
1432 Aas-NLH, Norway.
The SNSF-project
-
-
In Norway a c i d p r e c i p i t a t i o n was a t t h a t time seen
a s a p o s s i b l e cause of i n c r e a s i n g a c i d i t y of t h e
watercourses i n t h e southern p a r t of t h e country,
and of t h e gradual disappearence of valuable f i s h
populations from many l a k e s and r i v e r s . I t was a l so feared t h a t t h e i n p u t s of a c i d might over time
reduce f o r e s t growth p a r t i c u l a r l y through increased
leaching of n u t r i e n t elements from t h e s o i l .
The SNSF-project has t h i s year marked t h e conc l u s i o n of e i g h t years of research by organizing
t h e I n t e r n a t i o n a l Conference on t h e Ecological Imp a c t of Acid P r e c i p i t a t i o n . This Conference, which
was held i n Sandefjord, Norway, March 11 14, 1980,
a t t r a c t e d more than 300 p a r t i c i p a n t s from some 20
countries.
-
The p r e s e n t r e p o r t i s mainly a summary and b r i e f
discussion of the ecological impact of acid precipitation in Scandinavia with particular reference to Norway.
EMISSIONS, TRANSPORT, AND DEPOSITION
The concept of acid precipitation is now used
to denote precipitation with high amounts not only of the hydronium ion (H+ for short), but with
enhanced concentrations of the acidifying anions
sulphate, 304 and nitrate, NO3, which predominantly stem from anthropogenic sources. "Acid" precipitation regularly also contains high amounts
of ammonium, NH4, and of various heavy metals
and trace elements including organic micropollutants.
It should also be kept in mind that comparable
amounts of anthropogenic pollution may be deposited by dry deposition processes from the same air
masses giving acid precipitation. In studies of
effects of acid precipitation the contribution
from dry deposition is often difficult to quantify. In areas remote from the main industrial
centres, like in Southern Norway, acid precipitation is more or less synonymous with long-range
transported air pollution, but the distinction
should always be observed. In our context, sulphur and nitrogen compounds giving rise to the
acidifying properties of precipitation, are of
prime interest, and their sources should be described with a view to finding their geographical
distribution, emission rates and seasonal variation. This is necessary both for enabling modelling of their transport, and for formulating
abatement policies against their negative effects.
Sulphur Emissions
Most of the man-made sulphur emissions occur
as SO2 from combustion of coal and petroleum products. Comparatively less stems from smelting of
sulphur-containing mineral ores and other industrial processes, on a global basis about 10 percent. Knowledge of the location of industrial
and powerproducing units in addition to population distribution has allowed quantification of
annual emissions in a 150 km x 150 km grid over
Europe (Semb, 1979) The main area of SO2 emissions corresponds to the industrial belt from the
Midlands in U.K., the Netherlands and Belgium,
central and southern parts of Germany and Poland.
Parts of northern France, Czechoslovakia and USSR
also have industrial concentrations with very high
emission rates. On a country basis, total SO2
emissions in 1973 were estimated at e.g. 2.8
lo6
tonnes S from UK, 2.0 from the Federal Republic
of Germany and 1.6 from France (OECD, 1977) Norwegian annual emissions were estimated at 91,000
tonnes of S.
.
.
Seasonal variations in SO2 emissions occur as
the demand for energy fluctuates through the year.
At high latitudes the variable component of emis-
sions is related to space heating and illumination
during winter. In warmer climates, the maximum
demand may occur during summer due to the use of
air conditioning. In most of Europe, the fuel
consumption will probably be at its peak in January - February. The seasonal variation is about
30 percent of the annual mean emission.
Nitrogen Emissions
Both nitrogen oxides, NOx, and ammonium, NH4,
play important roles for the composition of acid
precipitation. Although global budgets of nitrogen compounds are even more uncertain than those of
sulphur, natural sources seem to be larger than the
man-made. However, major man-made emissions also
of nitrogen compounds occur in Europe and thus influence strongly precipitation chemistry in
Scandinavia. The main anthropogenic source of nitrogen oxides is combustion of fossil fuels, by
oxidation of nitrogen compounds in the fuel and
oxidation of nitrogen in the combustion air.
It is well known that the NOx emissions depend
on fuel types, combustion chamber design and operating conditions. High combustion temperatures
favour the emissions. There is, naturally, a high
spatial correlation between SO2 and NOx emissions.
OECD studies indicated almost a doubling of anthropogenic nitrogen oxides emissions in Europe
from 1959 to 19%73,thus increasing more than sulphur dioxide emissions in the same period (OECD,
1977).
Trace-Element Emissions
Acid precipitation contains a wide range of
minor and trace elements associated with the major chemical constitutents. Heavy metals, other
trace elements and organic micropollutants of manmade origin are receiving increasing interest as
some of these are enriched in living organisms.
Emission rates for trace elements and micropollutants are largely unknown, but the main sourMany of the organic micropolluces are known
tants in the atmosphere are products of human activity, including industrial and waste products,
and also chemicals used in industry as solvents
or intermediate products. The emissions are very
complex mixtures of chemical compounds.
.
Transport and Deposition
Of particular interest for long-range pollutant transport is the build-up of high concentrations in stagnant air near the ground during inversion situations. Observations show that such
parcels of contaminated air may subsequently move
over long distances without much dilution. A
much used technique in analyzing source areas and
transport directions of air pollutants, is the
sector analysis, grouping together trajectories
belonging to the same sector.
Of considerable i n t e r e s t f o r t h e p r o p o r t i o n of
s u l p h a t e i n a c i d p r e c i p i t a t i o n t o dry deposited
sulphur components, i s t h e o x i d a t i o n r a t e from
sulphur dioxide t o s u l p h a t e s . The o x i d a t i o n
t a k e s p l a c e both by a b s o r p t i o n of SO2 i n cloud
d r o p l e t s with subsequent o x i d a t i o n , and by oxid a t i o n i n t h e gas phase with oxygen compounds i n
t h e atmosphere. High c o n c e n t r a t i o n s of ozone and
photochemical o x i d a n t s , which a r e observed over
l a r g e a r e a s of Europe, w i l l i n c r e a s e t h e t r a n s formation r a t e . The t r a n s f e r of gases and part i c l e s from t h e a i r t o n a t u r a l s u r f a c e s , and t h e
a d s o r p t i o n , a r e u s u a l l y d e s c r i b e d i n analogy with
t h e t h e o r y of e l e c t r i c a l r e s i s t a n c e . The t r a n s p o r t from t h e atmosphere t o t h e boundary l a y e r
c l o s e t o t h e s u r f a c e , t a k e s p l a c e by t u r b u l e n t
d i f f u s i o n . The a e r o s o l p a r t i c l e s i n q u e s t i o n ,
i - e . , s u l p h u r and n i t r o g e n a e r o s o l s , a r e mostly
1.0 pm s i z e range with low g r a v i t a i n t h e 0.1
t i o n a l s e t t i n g , and t h e t u r b u l e n t t r a n s p o r t w i l l
depend on meteorological c o n d i t i o n s .
-
Model c a l c u l a t i o n s of wet and dry d e p o s i t i o n
p a t t e r n s over Europe show t h a t i n Scandinavia,
p a r t i c u l a r l y i n Norway, t h e wet d e p o s i t i o n outweighs t h e e s t i m a t e d dry d e p o s i t i o n . For southern
Norway dry d e p o s i t i o n i s e s t i m a t e d t o account f o r
about 30 p e r c e n t of t h e t o t a l d e p o s i t i o n of excess sulphate.
The p r e c i p i t a t i o n i n Norway i s l a r g e l y d e t e r mined by p o l a r f r o n t lows b r i n g i n g moist maritime
a i r i n w e s t e r l y t o s o u t h - e a s t e r l y d i r e c t i o n s . Of
p a r t i c u l a r importance i s t h e orographic enhancement o f p r e c i p i t a t i o n , caused by t h e l i f t i n g and
subsequent c o o l i n g of t h e a i r masses when flowing
a c r o s s t h e Scandinavian mountain chain. This
g i v e s r i s e t o a maximum zone of e l e v a t e d p r e c i p i t a t i o n some 40
50 km from t h e c o a s t l i n e .
In
this zone, annual mean p r e c i p i t a t i o n exceeds
1000 mm along t h e SE c o a s t i n c r e a s i n g t o an absol u t e maximum of perhaps more than 5000 mrn i n n o r t h
western Norway. S t i l l f u r t h e r n o r t h , i n northern
Norway, annual p r e c i p i t a t i o n i n t h e maximum zone
exceeds 2000 mm.
-
The group of macrocomponents t y p i c a l of a c i d
p r e c i p i t a t i o n , i . e . H+, NH4, SO4 and NO3, has a
marked north-south g r a d i e n t . The c o r r e l a t i o n s
between s u l p h a t e , n i t r a t e and ammonium a r e high,
and t h e r e a r e roughly e q u i v a l e n t amounts o f t h e
and
p r i n c i p a l c a t i o n s H + NH4+ and anions SO4
NO3
The c o n t e n t of s t r o n g mineral a c i d i n prec i p i t a t i o n i s s t r o n g l y c o r r e l a t e d with excess
s u l p h a t e . The c o r r e l a t i o n c o e f f i c i e n t i s 0.7
0.9 a t Norwegian s t a t i o n s . A t Norwegian s t a t i o n s
No3 makes about 30 p e r c e n t o f t h e sum SO4 + NO3.
Ammonium and n i t r a t e occur i n Norwegian p r e c i p i t a t i o n i n about e q u i v a l e n t c o n c e n t r a t i o n s , lowest
a t mountain s t a t i o n s . The c o n c e n t r a t i o n s of
~ 0 4 ~
and
- H+ i n p r e c i p i t a t i o n a r e h i g h e s t along
t h e south-east c o a s t . The mountain p l a t e a u i n
northernmost Norway i s a f f e c t e d by a i r t r a n s p o r t
from t h e s o u t h , which g i v e s a c i d p r e c i p i t a t i o n
5.0) a s f a r n o r t h a s 7 0 O ~ . l a t . In
(pH 4.5
southernmost Norway about 10 p e r c e n t of t h e pre-
-.
-
-
c i p i t a t i o n h a s a pH below 4.0 and about 5 p e r c e n t
P r e c i p i t a t i o n a c i d i t y below 3.5
above pH 5.0.
has been observed s e v e r a l times a t s e v e r a l p l a c e s ,
Dovland and Semb, (1980)
.
P r e s e n t knowledge o f atmospheric d e p o s i t i o n of
inorganic t r a c e elements i n Norway i s compiled by
Semb (1978). They a r e found i n t h e w a t e r s o l u b l e
f r a c t i o n of a e r o s o l s , o r absorbed on t h e o t h e r
p a r t i c l e s , and a r e mostly contained i n t h e a e r o s o l
s i z e f r a c t i o n with aerodynamic mass diameter below 2 urn. Available d a t a f o r l e a d , z i n c and cadm i u m i n p r e c i p i t a t i o n shows a d e p o s i t i o n p a t t e r n
s i m i l a r t o t h a t of excess s u l p h a t e . Also antimony, a r s e n i c , selenium and vanadium seem t o have
s i m i l a r d e p o s i t i o n p a t t e r n s . Influence from met a l l u r g i c a l i n d u s t r i a l centres is evident f o r ins t a n c e f o r chromium (western Norway) and a r s e n i c ,
selenium, n i c k e l , chromium and copper (smelting
i n d u s t r y i n t h e Murmansk a r e a , U.S.S.R.).
I n nort h e r n Norway t r a j e c t o r y a n a l y s i s shows t h a t h i g h l y
p o l l u t e d episodes a r e o f t e n a s s o c i a t e d with t h e
r e t u r n flow p a t t e r n d i s c u s s e d by Rahn and McCaffr e y (1980). Several s t u d i e s of o r g a n i c micropoll u t a n t s i n p r e c i p i t a t i o n and i n a e r o s o l s have
been performed w i t h i n t h e SNSF p r o j e c t . They
have i d e n t i f i e d a wide range of compounds i n t h e
same a i r masses b r i n g i n g a c i d p r e c i p i t a t i o n t o
Norway.
ECOLOGICAL IMPACT
The e f f e c t s of a i r p o l l u t i o n have h i s t o r i c a l l y
been considered l o c a l problems, o c c u r r i n g n e a r poll u t a n t s s o u r c e s , u s u a l l y urban a r e a s . This conc e p t of p o l l u t e d c i t i e s versus c l e a n r u r a l a r e a s
i s no longer a p p l i c a b l e . The i n c r e a s e i n anthropogenic emission sources coupled with t h e i n c r e a s ed h e i g h t of emissions have enhanced t h e phenomena of a i r p o l l u t i o n e f f e c t s on r u r a l a r e a s . The
most s t a r t l i n g e f f e c t s discovered s o f a r of t h e
long-range transmission of p o l l u t a n t s , have appeared i n r e l a t i v e l y remote, p r i s i t i n e a r e a s of
Norway, Sweden and t h e Eastern United S t a t e s and
Canada.
Despite t h e f a c t t h a t sulphur dioxide emissions
t o a large extent contribute t o acid precipitation
t h e two p o l l u t a n t s show g r e a t d i f f e r e n c e s i n e f f e c t s . Sulphur dioxide i s a primary a i r p o l l u t a n t
a s well a s a primary t o x i c a n t . Acid p r e c i p i t a t i o n
on t h e o t h e r hand, i s a secondary p o l l u t a n t causi n g mainly i n d i r e c t e f f e c t s on ecosystems.
F o r e s t Ecosystems
The e f f e c t s of a i r p o l l u t a n t s on p l a n t s i s extremely d i v e r s i f i e d ; it depends upon s p e c i e s l i n k e d t o l e r a n c e o r s u s c e p t i b i l i t y , and i s a funct i o n of many exposure parameters (frequency, time,
concentration, e t c . )
Many t y p e s of response have
been d e s c r i b e d on t h e b a s i s of l a b o r a t o r y e x p e r i ments, where known chemicals were t e s t e d under
c o n t r o l l e d c o n d i t i o n s with d i f f e r e n t p l a n t s p e c i e s .
.
However, t h e i n v e r s e o p e r a t i o n - namely i d e n t i f y i n g and e s t i m a t i n g t h e n a t u r e and importance of
an e x i s t i n g source on t h e b a s i s of response symptoms - i s o f t e n d i f f i c u l t , except i n t h e case of
a c u t e i n j u r y and when a p o l l u t i o n source i s known
o r suspected i n t h e v i c i n i t y .
Vegetation damage due t o t h e emission of a c i d
and poisonous substances has long been observed
i n t h e v i c i n i t y of emission sources. V i s i b l e
symptoms have been decribed and a r e o f t e n assoc i a t e d with t h e decrease i n growth. Recently,
however, concern has been expressed t h a t f o r e s t
growth may a k o be a f f e c t e d f a r away from emission
sources. Even i f t h e d i r e c t evidence i s meagre,
T a m , (1976), t h e r e i s f a i r l y s u b s t a n t i a l i n d i r e c t evidence t h a t continued exposure t o a c i d
r a i n has a growth-decreasing e f f e c t . The most
s i g n i f i c a n t i n d i r e c t evidence i s t h e p o s i t i v e
c o r r e l a t i o n between f o r e s t y i e l d and t h e s o i l
base s t a t u s . Jonsson and Sundberg (1972) c l a s s i f i e d a r e a s i n southern Sweden a s r e l a t i v e l y r e s i s t a n t t o a c i d r a i n and r e l a t i v e l y s u s c e p t i b l e
t o a c i d r a i n , r e s p e c t i v e l y , and compared t h e
growth t r e n d s i n both a r e a s by measuring annual
r i n g s on increment c o r e s from groups of t r e e s
which were otherwise a s i d e n t i c a l a s p o s s i b l e .
They found a s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e
and "found no reason f o r a t t r i b u t i n g t h e reduct i o n i n growth t o any cause o t h e r than a c i d i f i c a t i o n . " These r e s u l t s however, have n o t been
confirmed by Norwegian r e s e a r c h e r s , Abrahamsen
and o t h e r s , (1976) ; Abrahamsen, (1980), S t r a n d ,
(1980).
When e v a l u a t i n g t h e e f f e c t of a c i d p r e c i p i t a t i o n on t h e supply of p l a n t n u t r i e n t s i n a f o r e s t , a b a s i s could be t o c o n s i d e r t h e n u t r i e n t
c y c l e i n a t e r r e s t r i a l ecosystem.
Plant availa b l e n u t r i e n t s a r e g e n e r a l l y s u p p l i e d t o t h e system from two s o u r c e s ; from t h e atmosphere, a s
f o r N and S, and from t h e m i n e r a l s , a s f o r Ca,
In natural
Mg, P I K, S, and t h e m i c r o n u t r i e n t s .
systems n o t h a r v e s t e d by man, n u t r i e n t s a r e a l s o
l o s t i n two ways: To t h e atmosphere by v o l a t i l i z a t i o n and t o t h e s e a by leaching. Evaluation of
t h e e f f e c t of a c i d p r e c i p i t a t i o n on t h e amount of
p l a n t n u t r i e n t s i n a f o r e s t ecosystem can t h e r e f o r e be r e s t r i c t e d t o t h e c o n s i d e r a t i o n of f o u r
p r o c e s s e s ; d e p o s i t i o n from t h e atmosphere, weat h e r i n g , v o l a t i l i z a t i o n and l e a c h i n g from t h e
s o i l . Obviously many p r o c e s s e s i n t h e s o i l and
t h e p l a n t s can a f f e c t t h e a c c e s s i b i l i t y of p l a n t
nutrients.
Acid r a i n may a f f e c t some of t h e s e
processes.
Experimental s t u d i e s on t r e e growth i n r e l a t i o n t o a c i d r a i n have been conducted i n r e c e n t
y e a r s i n s e v e r a l c o u n t r i e s . A s y e t no conclus i v e evidence of decreased growth has evolved.
On t h e c o n t r a r y , a s l i g h t l y p o s i t i v e growth e f f e c t on t h e s e e d l i n g , which was e x p l a i n e d a s a
n i t r o g e n f e r t i l i z e r e f f e c t , was r e p o r t e d by
Wood and Bormann, (1975). Such i n c r e a s e s , though,
a r e l i k e l y t o be temporary, a s d e p l e t i o n of nut r i e n t c a t i o n s through a c c e l e r a t e d l e a c h i n g
should e v e n t u a l l y r e t a r d growth. Experiments on
t h e e f f e c t of a r t i f i c i a l a c i d i f i c a t i o n on f o r e s t
growth under f i e l d c o n d i t i o n s have been c a r r i e d
Out i n Sweden and Norway. The Swedish experiments
have shown t h a t i n c r e a s i n g a p p l i c a t i o n of
d i l u t e H2S04 has s i g n i f i c a n t l y i n c r e a s e d t h e baThe Norwes a l a r e a growth, Tanun e t a l . , (1980)
g i a n s t u d i e s c o n s i s t of f i v e f i e l d p l o t e x p e r i ments where a r t i f i c i a l r a i n has been produced by
mixing groundwater and H2S04 t o pH values from 6
I n one experiment with Scots p i n e , i n c r e a s t o 2.
ed h e i g h t and diameter growth was observed i n
1976 and 1977 a t t h e p l o t s s u p p l i e d with 250 mm
of water p e r y e a r of pH 3 , 2.5 and 2.
I n 1979
however, t h e most a c i d i f i e d p l o t s showed s i g n i f i c a n t l y l e s s growth than t h e o t h e r experiments.
(See Abrahamsen 1980).
.
The experiments t h u s show i n c r e a s e d growth t h e
f i r s t couple of y e a r s i n t h e a c i d i f i e d p l o t s ,
Simfollowed by decreased growth t h e l a s t y e a r .
i l a r p a t t e r n s , though n o t s i g n i f i c a n t , have been
found i n some of t h e o t h e r experiments. Chemical
analyses of t h e f o l i a g e have revealed t h a t t h e
most l i k e l y e x p l a n a t i o n of t h e i n c r e a s e d growth
i s i n c r e a s e d N uptake. The decrease i n growth
observed i n 1979 might be r e l a t e d t o reduced av a i l a b l i t y of Mg a s t h e f o l i a r c o n c e n t r a t i o n i s
c l o s e t o values g i v i n g v i s u a l d e f i c i e n c y symptoms.
Short-term growth r e s u l t s from a c i d i f i c a t i o n
experiments must be t r e a t e d with c a u t i o n . They
i n d i c a t e , however, t h a t t r e e growth may be reasonably s t a b l e when t h e p l a n t - s o i l system i s
s t r e s s e d by a c i d r a i n . Another d i f f i c u l t y t o be
k e p t i n mind i s t h a t p a r t of t h e a c i d i t y of r a i n
i s due t o n i t r i c a c i d o r n i t r o g e n o x i d e s , which
means t h a t t h e p o s i t i v e f e r t i l i z e r e f f e c t of n i trogen may p a r t l y o r f u l l y compensate f o r any
harmful e f f e c t s .
T h e o r e t i c a l l y t h e r e might be c a s e s where a c i d i t y caused by sulphur oxides i s c o u n t e r a c t e d by
f e r t i l i z e r e f f e c t s , s i n c e sulphur i s an indispensable plant nutrient.
However, d e f i c i e n c y i n
sulphur has never been observed i n f o r e s t t r e e s
under n a t u r a l c o n d i t i o n s i n Scandinavia, and considering the rather t i g h t nutrient circulation
i n t h e f o r e s t ecosystem, it i s n o t l i k e l y t o occur
except p o s s i b l y on very extreme s i t e s .
A number of p o s s i b l e e f f e c t s of a c i d r a i n on
b i o l o g i c a l p r o c e s s e s i n t h e f o r e s t s o i l has been
considered by T a m , (1976) , Abrahamsen, (1980)
Most f o r e s t s o i l s have a considerable b u f f e r cap a c i t y . Therefore, we may assume t h a t t h e supply
of a c i d i t y , measured e i t h e r a s hydrogen i o n s
o r a s " s t r o n g a c i d " of i n d u s t r i a l o r i g i n , cannot
y e t have a f f e c t e d t h e e n t i r e s o i l p r o f i l e , exc e p t p o s s i b l y i n t h e immediate v i c i n i t y of emiss i o n sources. S t i l l , e f f e c t s may be found on
processes o c c u r r i n g i n t h e t o p - s o i l o r on t h e
s u r f a c e of s o i l p a r t i c l e s .
S o i l organisms, i n cluding r o o t s , i n t h e upper s o i l horizons may
a l s o be a f f e c t e d .
Soil respiration, nitrogen
.
turnover which i s intimately connected t o organic
matter decomposition i n s o i l , n i t r i f i c a t i o n , nitrogen f i x a t i o n and nitrogen immobilization a r e
some of the processes apparently affected by increasing s o i l a c i d i t y . There a r e a number of
other biological processes which may be affected
by a change i n s o i l a c i d i t y o r sulphur supply
which have not y e t been studied.
Several comparative and experimental investigations have yielded evidence i n support of the
t h e o r e t i c a l assumption t h a t a c i d i f i e d precipit a t i o n , l i k e any other change i n climate, w i l l
r e s u l t i n changes i n the properties of s o i l . Influences from t h i s changed chemical climate on
s o i l conditions have been indicated through decreases i n pH and base s a t u r a t i o n a s well as increased leaching. From the studies performed up
t o now, however, it i s d i f f i c u l t t o draw any def i n i t e conclusions on the time required f o r the
reactions and t h e i r i n t e n s i t i e s . Many s o i l s a r e
f a r from the s t a b l e and mature stage, and it i s
a well-known f a c t t h a t considerable changes due
t o f a c t o r s other than acid p r e c i p i t a t i o n may simultaneously be a f f e c t i n g the properties of the
s o i l . The g r e a t variation i n s o i l types and i n
t h e i r s u s c e p t i b i l i t y t o acid p r e c i p i t a i t o n make
detection even more d i f f i c u l t before the expect e d e f f e c t s have become extensive.
The r e l a t i v e significance of strong acids and
associated heavy metals found i n heavily pollut e d areas has not been c l e a r l y established i n
terms of t o x i c e f f e c t s on p l a n t s and s o i l organi s m s . The most serious consequence of regional
a c i d i f i c a t i o n a t currently observed l e v e l s may
be the increased r a t e of leaching of major elements and t r a c e metals from f o r e s t s o i l s and vege t a t i o n . This i s t r u e f o r t h e f o r e s t ecosystem
and a l s o has a bearing on the aquatic systems
receiving these e f f l u e n t s .
Aquatic Ecosystems
Freshwater bodies i n many areas of northern
Europe and eastern North America, t h a t today l i e
i n and adjacent t o the areas where p r e c i p i t a t i o n
i s most a c i d , a r e threatened by the continued
deposition and f u r t h e r expansion of acid precip i t a t i o n . Many of these bodies of fresh water
a r e poorly buffered and vulnerable t o acid inputs. These ecosystems appear fated t o s u f f e r
a c i d i f i c a t i o n and l o s s of f i s h populations. Equally a s serious a s damage t o f i s h a r e the l e s s
conspicuous e f f e c t s of the a c i d i f i c a t i o n of
fresh water including changes occurring i n commun i t i e s of aquatic organisms such a s microdecomposers, algae, aquatic macrophytes, zooplankton
and zoobenthos.
Water chemistry
The composition of the lakes which a r e discussed i n connection with a c i d i f i c a t i o n depends
on three principal sources of chemical components
atmospheric inputs of sea-water s a l t s , atmospheri c inputs of acid p r e c i p i t a t i o n and t e r r e s t r i a l
inputs of chemical-weathering products.
-
Unpolluted, s o f t water lakes a r e generally
d i l u t e solutions of Ca and Mg bicarbonate. The
bicarbonate system c o n s t i t u t e s the main buffering
system i n the water. Lakes i n regions underlain
by highly r e s i s t a n t , carbonate-poor rocks have
lower buffer capacities, and a r e vulnerable t o
the input of acid p r e c i p i t a t i o n . A major number
of the lakes i n Scandinavia f a l l within t h i s category, especially above the postglacial marine
l i m i t , where the bedrock over large areas i s
covered by only t h i n g l a c i a l deposits. A continuous supply of acid substances t o lakes and
streams eventually leads t o the depletion and l o s s
of the normal buffer system. The pH f a l l s t o below 5.0, and sulphate becomes the major anion.
Such lakes have only minimal capacity t o neurali z e additional inputs of acid; and new inputs of
acid cause sharp drops i n pH, Wright and Gjessing,
(1976) , Henriksen, (1980)
.
Acid p r e c i p i t a t i o n a l s o causes other changes i n
lake water chemistry a s well. The a c i d i c , high
sulphate lakes a l s o have high aluminum concent r a t i o n s . Since p r e c i p i t a t i o n contains very
l i t t l e A l l the A 1 i n the l a k e water must come from
the drainage basins. That has been shown t o be
the case i n investigations conducted on 9 small
drainage basins i n southern Norway. I t has been
shown t h a t l o s s of calcium, magnesium and aluminum
from the basins is p a r t l y due t o natural weathering processes, but a major f r a c t i o n probably
r e s u l t s from the massive inputs of acid precipit a t i o n . In g r a n i t i c basins there i s approximatel y equivalence between net H+ input and Ca + Mg +
A 1 output.
The deposition of acid p r e c i p i t a t i o n occurs episodically. Acid p r e c i p i t a t i o n generally causes
two seasons of increased a c i d i t y i n streams and
the f a l l , a season of frequent r a i n , and
rivers
the spring, when p o l l u t a n t s stored i n the snowpack a r e released i n the f i r s t p a r t of snowmelt.
Laboratory and f i e l d studies of polluted snow have
shown t h a t the f i r s t f r a c t i o n s of meltwater due
t o concentration e f f e c t s within the snowpack cont a i n higher concentrations of p o l l u t a n t s than the
bulk snow. The f i r s t 30 per cent of the meltwater
80 per cent of the t o t a l acontains up t o 70
mount of H',
~ 0 and
~ "s
~
~
~
.
-
-
The episodic deposition of a i r p o l l u t a n t s and
a c e r t a i n temporary accumulation of sulphate i n
the summer r e s u l t s i n major short-term increases
i n the a c i d i t y of lakes and r i v e r s and these
changes are most frequent i n t h e f a l l and spring.
From a biological point of view these periods a r e
often c r i t i c a l because they a r e spawning and hatching seasons f o r many aquatic organisms.
Regional surveys
A large number of surveys have been conducted
t o give a p i c t u r e of the a c i d i f i c a t i o n of Scandinavian lakes and r i v e r s . A systematic survey
of 155 lakes i n southern Norway was conducted i n
October 1974, and repeated every year since, on
varying number of lakes.
Excess sulphate i s sulphate t h a t does not come
from sea water s a l t s . Although i n some lakes,
t h i s excess sulphate comes from a t e r r e s t r i a l
source i n the drainage basin, the r e g u l a r i t y of
a SE-NW gradient most probably i s due t o chronic
inputs of anthropogenic sulphur through precipit a t i o n and dry deposition.
Indeed, the d i s t r i bution of excess sulphate i n lakes i s remarkably
similar t o the weighted average concentration of
excess sulphate measured i n p r e c i p i t a t i o n over
southern Norway.
The pH l e v e l s i n the lakes can be largely explained by inputs of acid p r e c i p i t a t i o n i n the
SE-NW gradient superimposed upon the variations
i n buffer c a p a c i t i e s due t o the geology of the
drainage basin.
Aquatic organisms
Acid s t r e s s on l i f e i n r i v e r s and lakes has
e f f e c t s on a l l stages i n the food chain. Primar y producers communities, l i k e phytoplankton,
a r e simplified by a reduction i n the number of
species. The composition of a community may
a l t e r with a s h i f t t o more acid-tolerant species.
Macrophytic vegetation has been observed t o
change i n a c i d i f i e d lakes. In some lakes Sphagnum occurs i n dense mats, and epiphytes a r e well
developed. Generally, t h e vegetation i n acidif i e d lakes i s poor i n species.
The same tendency of an acid-tolerant " s h i f t i s
observed i n diatom communities i n 7 locations i n
southern Norway described i n 1949 and r e v i s i t e d
i n 1975. There was an increase i n the proportion
of species which p r e f e r o r require acid water.
In some a c i d i f i e d lakes, an increased accumul a t i o n of organic bottom sediment has been observed, indicating a reduced r a t e of decomposit i o n . There a r e strong suggestions t h a t microb i a l decomposition i s reduced i n acid water and
t h a t slow-acting fungi take over. This w i l l influence t h e n u t r i e n t exchange with t h e lake sediments.
In the invertebrate freshwater fauna the
same trends a r e observed: a reduced number of
species and a t o t a l reduction i n biomass i n the
acid locations. Experiments on the tolerance
of c e r t a i n crustaceans which a r e important a s
f i s h food (Gammarus l a c u s t r i s and Lepidurus
t i c u s ) have shown a d i r e c t mortality of eggs a t
pH 5.5 o r lower. There a r e a l s o delays i n the
development from stage t o stage of surviving animals. Exposure below pH 5.5 w i l l k i l l a majority
of adult individuals even a f t e r a short time, 1
2 days. In Norwegian lakes, s n a i l s a r e r a r e a t
pH below 5.8 and disappear below 5.2.
-
In s p i t e of the strong e f f e c t s on f i s h food
organisms, the indications a r e t h a t changes i n
f i s h food supply play a small r o l e i n the elimination of f i s h from acid r i v e r s and lakes. Instead,lack of recruitment seems t o be the dominant
factor. The tolerance against acid water i s lowe s t i n newly hatched larvae. This f a c t makes the
spring flood a p a r t i c u l a r l y c r i t i c a l time f o r the
f i s h population.
The physiological mechanism o r mechanisms responsible f o r f i s h death i n acid water a r e not
f u l l y understood,, but it has been well established t h a t acid s t r e s s i s accompanied by a f a i l ure i n s a l t regulation within t h e f i s h body.
Metabolism and osmotic c e l l regulation seem t o
be affected. I t i s a l s o q u i t e c l e a r t h a t the
s a l t uptake and l o s s i s influenced by the ion
content of the water. Elevated concentrations
of aluminum, manganese, zinc, cadmium, lead, copper, and nickel have frequently been observed i n
a c i d i f i e d lakes. The abnormally high concentrat i o n s are apparently due i n p a r t t o d i r e c t depos i t i o n with p r e c i p i t a t i o n a s well a s t o increase
release ( s o l u b i l i t y ) from the sediments. These
metals may represent a major physiological s t r e s s
f o r various aquatic organisms.
Fish population s t a t i s t i c s from nearly 1000
lakes i n southern Norway show t h a t when both pH
i s low (e.9. pH 4.7) and s a l i n i t y i s low (e.g.
K 10 IJS cm) almost a l l lakes a r e empty of f i s h .
A t higher s a l i n i t y (K 20pS/cm) several lakes
have sparce populations and a few even good.
The recent a c i d i f i c a t i o n of freshwater i n
p a r t s of Europe and eastern North-America has
profound impacts on aquatic l i f e . It can be
s t a t e d with r e l i a b i l i t y t h a t a l l trophic l e v e l s
are affected. Of immediate concern t o the people
l i v i n g i n the a c i d i f i e d regions i s the major dec l i n e i n f i s h populations. In the four southernmost counties i n Norway more than half of the
f i s h populations have been l o s t during the 1940 1980 period. Today, lakes i n more than 13,000
km2 of south Norway a r e p r a c t i c a l l y devoid of
f i s h , and i n an additional 20,000 km2 the f i s h
stocks a r e reduced. Continued water a c i d i f i c a t i o n i s a t h r e a t t o hundreds of lakes s t i l l harbouring valuable f i s h populations(Muniz and
Leivestad, 1980).
Semb, A.
1979. Sulphur emissions i n Europe.
A t m . Env. 12 p. 455-460.
LITERATURE CITED
Abrahamsen, G.
1980. Acid p r e c i p i t a t i o n , p l a n t n u t r i e n t s and
f o r e s t growth. I n Proc. I n t e r n a t i o n a l conference on t h e e c o l o g i c a l impact of a c i d
p r e c i p i t a t i o n , Norway, March, 1980.
SNSF,1432 Aas-NLH, Norway ( I n p r e s s )
Abrahamsen, G. , Bjor, K. , Horntvedt, R. and
B. Tveite.
1976. E f f e c t s of a c i d p r e c i p i t a t i o n on conif e r o u s f o r e s t . I n SNSF-project FR 6/76,
SNSF, 1432
Norway
as-NL~
Strand, L.
1980. The e f f e c t of a c i d p r e c i p i t a t i o n on t h e
growth. I n Proc.
I n t e r n a t i o n a l conference
on t h e e c o l o g i c a l impact of a c i d p r e c i p i t a t i o n , Norway, March, 1980.
SNSF, 1432 Aas-NLH, Norway ( I n p r e s s )
Tamm, C. 0.
1976. Acid p r e c i p i t a t i o n : Biological E f f e c t s
i n s o i l and on f o r e s t vegetation. Ambio,
5,6: 235-238.
Tamm, C. O., F a r e l l , E. P.
, Nilsson,
J.
, and
G. Wicklander.
Dovland, H. and A. Semb.
1980. Atmospheric t r a n s p o r t of p o l l u t a n t s .
I n Proc. I n t e r n a t i o n a l conference on t h e
e c o l o g i c a l impact o f a c i d p r e c i p i t a t i o n ,
Norway, March, 1980.
SNSF, 1432 Aas-NLH, Norway ( I n p r e s s )
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Henriksen , A.
1980. A c i d i f i c a t i o n of freshwaters
a large
s c a l e t i t r a t i o n . In Proc. I n t e r n a t i o n a l
conference on t h e e c o l o g i c a l impact of a c i d
p r e c i p i t a t i o n , Norway, March, 1980.
SNSF, 1432 Aas-NLH, Norway ( I n p r e s s )
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Jonssson, B. and R. Sundberg.
1972. Has t h e a c i d i f i c a t i o n by atmospheric
p o l l u t i o n caused a growth reduction i n Swedi s h f o r e s t s ? A comparison between regions
with d i f f e r e n t s o i l p r o p e r t i e s . In Rapp.
Uppsatser I n s t . f o r skogproduktion, Skoghogskolan, N r . 20, 48 p.
Muniz, I. and H. Leivestad.
e f f e c t s on freshwater
1980. A c i d i f i c a t i o n
f i s h . In Proc.
I n t e r n a t i o n a l conference on
t h e e c o l o g i c a l impact of a c i d p r e c i p i t a t i o n ,
Norway, March, 1980.
SNSF, 1432 Aas-NLH, Norway ( I n p r e s s )
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OECD
1977. The OECD Programme on Long Range Transp o r t of A i r P o l l u t a n t s . OECD, 2 rue Andre
Pascal, Paris
Rahn, K.A. and R . J . McCaffrey.
1980. On t h e o r i g i n and t r a n s p o r t of t h e wint e r A r c t i c a e r o s o l . I n Proceedings of t h e
conference on ~ e r o s o l ;
New York Academy
of Sciences, New York C i t y , 9-12 January 1979
(In press)
Semb, A.
1978. Deposition of t r a c e elements from t h e
atmosphere i n Norway. SNSF-project FR 13/78,
SNSF, 1432 Aas-NLH, Norway
1980. E f f e c t s o f a r t i f i c i a l a c i d i f i c a t i o n with
sulphuric a c i d on t r e e growth and s o i l chemist r y i n Scots Pine f o r e s t . In Proc. I n t e r n a t i o n a l conference on e c o l o g i c a l impact of
a c i d p r e c i p i t a t i o n , Norway, March, 1980.
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Wood, T. and F. H. Bormann.
1975. Increases i n f o l i a r leaching caused by
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chemical 'composition o f lakes. Ambio , 5 : 219.
The Impact of Acidic Precipitation and
Heavy Metals on Soils in Relation to
Forest Ecosystems
2
Stephen A. Norton, Denis W. Hanson, and Richard J. Campana Abstract: Normal terrestrial cycling of metals in eastern North America and the Pacific Coast states has been altered by the increasing acidity of precipitation, and associated heavy metal deposition and mobilization. Pb and chemically similar metals are accumulating in soils. Al, Ca, K, Mg, and Mn are being leached from soils. The mobilities of Fe, Zn, and P vary with site characteristics. Biological recycling of nutrients by decomposition and uptake is impeded by lowered pH and elevated levels of toxic metals in soils. Increased leaching of nutrients in the 0 and A horizonscaused by in- creased H"1" inputs, decreases percent base saturation and thus
decreases nutrient pools for shallow rooted plants, especially seedlings. Deeper rooted plants are subjected to elevated, potentially toxic, concentrations of dissolved metals (e.g., A1 and Mn). In many contemporary forest ecosystems, nutrient availability is barely adequate for sustained yield with bole harvesting techniques. Our work indicates that nutrient pools are dimin- ishing in the northeastern United States, suggesting that de- creases in forest productivity will occur. Large areas of the northern hemisphere are receiving precipitation which is more acidic than
would be predicted by equilibration of rain and
atmospheric CO2: The resulting pH should be about 5.6. This may be
modified by the hydrolysis of particulates or the
addition of naturally occurring.organic and inor- ganic acids. Precipitation pH's in the U.S.,
prior to pollution of the atmosphere, probably
presented at the Symposium on Effects of Air
Pollutants on Mediterranean and Temperate Forest
Ecosystems, June 22-27, 1980, Riverside, California, U.S.A.
Professor and Chairman of Geological Sciences;
M.S. degree candidate in Geological Sciences;
Professor of Botany and Plant Pathology; Univer- sity of Maine at Orono, Orono, Maine 04469.
ranged up to about 8.2 (corresponding to semi- arid to arid regions where CaC03 dust, or its equivalent, dominatesthe rain chemistry). Atmospheric concentrations of CO2 should result in a pH of about 8.2. In eastern North America, where vegetation cover minimizes particulate in- jection into the atmosphere, unpolluted precipi- tation would probably have a pH of about 5.6. Associated with the precipitation are numerous metals and plant nutrients (e.g., Na, K, Ca, Mg, NO;, NQ, H2P04, Pb, Zn, etc.). Precipitation (wet and dry) is one of three inputs into the nutrient budgets for forest eco- systems. The others are chemical weathering of inorganic soil and nutrient cycling within the canopy/root space. Both are closely linked to precipitation chemistry. Simply put, from a nutritional point of view: Input
-
Output = Net accumulation of organic
material. Output consists of leaching to groundwater (below the root zone), injection of particulates into the atmosphere, volatilization of certain elements, and loss of particulates "downstream", including harvesting. If the mass balance (above) is posi- tive for all limiting nutrients, growth and accu- mulation of organic matter will occur as living biomass or as soil organic matter. If the mass balance is negative, growth can occur only so long as the decreasing reservoirs of organic matter and nutrients from precipitation and chemical weather- ing can supply necessary nutrients; organic matter must decrease. The amount of organic material on the forest floor and contained in the rooting zone of the mineral soil is generally of the same order of magnitude orlarger than the organic material contained in the above ground biomass. Removal of this biomass disrupts the recycling of nutrients. Continued growth of new biomass must occur from the reservoir within the soil, forest floor, and from precipitation. A forest ecosystem can subsist on nutrients de- livered solely by precipitation, both wet and dry (Art et al. 1974). However, it is doubtful that this precipitation-based forest ecosystem could be harvested periodically and still have sustained growth. If there are changes in the chemistry of precipitation, specific inputs/outputs of the forest nutrient budget may be altered so as to affect both sustained growth yields and net orga- nic matter accumulation. Many forest ecosystems are not forced to sub- sist on atmospheric inputs alone. They receive additional primary inputs of nutrients from chemi- cal weathering of mineral matter. However, in unglaciated areas of the eastern U.S. where chemi- cal weathering dominates over mechanical weather- ing or in glaciated granitic (nutrient poor) ter- rains characteristic of large areas of eastern North America, nutrient pools are largely con- tained within the organic litter of the forest floor. Thus, these areas are the most vulnerable to depletion of nutrient pools due to acidic pre- cipitation. Nutrient availability is commonly classified as deficient (where addition of the limiting nutrient elicits a positive response), adequate (where ad- dition of a nutrient does not elicit a response), and excessive (where addition of the nutrient eli- cits a negative response). This paper investigates some of the consequences of increased acidity and metal availability, associated with acidic precipi- tation to nutrient availability (and thus to for- est productivity).
Excellent reviews of potential problems are given by Voigt (1979) and in Hutchin- son and Havas (1980). Figure I~GeneralizedpH (dotted) (NADP 1979, 1980; CANSAP 1979),Pb (solid) and Zn (dashed) isopleths (Davis and Galloway 1980) for eastern U.S. precipitation. Fluxes are g/h/mo. area receiving acidified precipitation. Recent data (NADP 1979; NADP 1980 a,b; CANSAP 1980) shows a continued decrease in the pH to average values of less than 4.0 for the "bull's eye"; the eastern half of the U.S. and the eastern half of southern Canada is receiving precipitation with a pH less than 5.0 (fig. 1). Most of the lowering of the pH of precipitation from "normal" to present levels occurred over the last 40 years. In North America, there are no long term regional studies of the acidification of soils showing trends related to regional pH gradients. Linzon and Temple (1980) (in Ontario) found acidification of soils over a 16 year ob- servation period. Many studies have been made of soil acidification and nutrient status in areas heavily affected by point sources. Expected changes in the inorganic aspects of soil chemistry associated with acidification in- clude desorption of metals from organic and inor- ganic cation exchange surfaces, increased solu- tion of "mineral" colloids and crystalline min- erals, and changes in metal speciation and thus biological availability. Desorption of metals EFFECTS OF DECREASED pH Although early precipitation chemistry is sparse for North America, Cogbill and Likens (1974) con- structed 3 pH isopleth maps spanning 1952 to 1972 which suggested increasing acidity for precipita- tion in the eastern U.S. and a broadening of the Regardless of the nature of the substrate, ion exchange (for a monovalent ion) may be represented as where X i s a n exchange
m e t a l , H+ i s a p r o t o n ,
specie. This reaction
r e c t i o n by changing H"1'
+
s i t e , R i s any monovalent
and aq r e f e r s t o a n aqueous
can be f o r c e d i n e i t h e r d i o r R+ a c t i v i t i e s .
P r e - p o l l u t i o n p r e c i p i t a t i o n i n e a s t e r n North
America e n t e r e d t h e s o i l w i t h a pH probably i n t h e
r a n g e 5 . 5 t o 6.0.
M i c r o b i a l a c t i v i t y i n t h e org a n i c l i t t e r produces molecular CO2 which can r e duce t h e pH c o n s i d e r a b l y (as low a s 4.5 t o 5 . 0 ) .
The p r o d u c t i o n of v a r i o u s o r g a n i c weak a c i d s (e.g.
f u l v i c and humic) may d e p r e s s t h e pH f u r t h e r t o
4 . 0 t o 4.5.
To m a i n t a i n e l e c t r i c a l n e u t r a l i t y i n
t h e s e s o l u t i o n s t o o f f s e t t h e ff4' p r o d u c t i o n , e i t h e r a n i o n s must be gained ( H C Oa~c t i v i t y i s r e duced by t h e l o w e r i n g of pH; o r g a n i c a n i o n s may
be produced) o r c a t i o n s must be l o s t from t h e sol u t i o n . T h i s i s most e f f e c t i v e l y accomplished i n
t h e l i t t e r and a t r o o t s u r f a c e s where H+ i s exchanged f o r p e r c o l a t i n g c a t i o n s . Thus n u t r i e n t s
a r e gained by t h e s o i l .
I f t h e p r e c i p i t a t i o n h a s a pH o f 4.0 due t o t h e
s t r o n g a c i d s H2S04 and HN03, t h e exchange r e a c t i o n
i s forced strongly t o the right, stripping cations
( p a r t i c u l a r l y Ca and Mg) from t h e l i t t e r , r e d u c i n g
p e r c e n t of b a s e s a t u r a t i o n . Continued p r o d u c t i o n
of CO2, o r g a n i c a c i d s , by m i c r o b i a l a c t i v i t y ass u r e s t h a t t h e c a t i o n s a r e l o s t from t h e system.
No l o n g term s t u d i e s of s o i l s e x i s t t o demons t r a t e t h e s w i t c h from accumulation t o l o s s o f
c a t i o n i c n u t r i e n t s a s a r e s u l t of low pH p r e c i p i tation.
I n d i r e c t evidence f o r t h e switch c o n s i s t s
of l o n g term changes i n s u r f a c e w a t e r q u a l i t y s u c h
a s c o n d u c t i v i t y (Malmer 1976 [ i n wede en] ) , a l k a l i
and a l k a l i e a r t h c o n c e n t r a t i o n s (Malmer 1976), and
t h e commonly observed r e l a t i o n s h i p between nond y s t r o p h i c low pH w a t e r s and e l e v a t e d Ca, Mg, Al,
Mn, and o t h e r m e t a l s .
U l r i c h (1980) and Linzon
and Temple (1980) have shown l o s s o f b a s e s a t u r a t i o n i n s o i l s o v e r 8 and 1 6 y e a r s , r e s p e c t i v e l y .
Abundant e x p e r i m e n t a l e v i d e n c e ( e . g . , Hutchinson
1980; Abrahamsen and S t u a n e s 1980) and s t u d i e s o f
s o i l s adjacent t o l a r g e point source e m i t t e r s of
SOx and NOx s u g g e s t what l o n g t e r m r e s u l t s might
look l i k e .
Ca, Mg, K, Zn, Cd, and Mn a r e r e a d i l y
l e a c h e d from l i t t e r . However, t h e pH l e v e l s employed f o r e x p e r i m e n t a l work a r e commonly w e l l below what we might e x p e c t on a r e g i o n a l b a s i s .
P r o c e s s e s o p e r a t i n g a t a pH <4.0 may n o t b e e f f e c t i v e even o v e r l o n g p e r i o d s of time a t pH>5.0.
A t r a n s e c t of " e q u i v a l e n t " s o i l s i t e s a c r o s s a
r e g i o n a l pH g r a d i e n t ( f i g . 2 ) , i n e f f e c t , i s a
t i m e s t u d y of t h e e f f e c t s of low pH p r e c i p i t a t i o n .
Table 1 i n d i c a t e s a p r o g r e s s i v e d e c r e a s e i n
and -i n a s o u t h w e s t e r l y d i r e c t i o n , toA1203
A1203
ward lower pH p r e c i p i t a t i o n . We i n t e r p r e t t h i s
a s a p r e f e r e n t i a l l e a c h i n g ( d e s o r p t i o n ) of Ca and
Mn from t h e l i t t e r .
Organic m a t t e r i n l a k e sediment i s d e r i v e d from
b o t h t h e w a t e r s h e d and t h e l a k e w a t e r column. O r ganic-rich l a k e sediments i n a c i d i f i e d l a k e s i n
New England (Williams 1980) a r e d e p l e t e d o f CaO,
F i g u r e 2--Location of s o i l l o c a l i t i e s r e p o r t e d i n
Table 1 (Hanson 1980) and l o c a t i o n of l a k e s f o r
which F i g u r e 4 i s developed. pH i s o p l e t h s a r e
f o r 197516 (Likens e t a l . 1979).
a l s o suggesting t h a t f o r e s t l i t t e r is being leached
before i t i s transported t o t h e l a k e and/or t h a t
l e a c h i n g c o n t i n u e s w h i l e t h e sediment i s i n cont a c t with a c i d i c lake water.
S o l u t i o n o f Metals from M i n e r a l s o r C o l l o i d s
The r e l e a s e of c a t i o n s from m i n e r a l s due t o
a c i d i c s o i l w a t e r w e a t h e r i n g may b e r e p r e s e n t e d
Table I ~ C h e m i s t r yo f f o r e s t l i t t e r from h i g h
a l t i t u d e f i r f o r e s t s . Sample s i t e s a r e shown
on f i g u r e 2. Note: S i t e 6 i s anomalous f o r a l l
p a r a m e t e r s . S i t e 1 2 had abundant admixed m i n e r a l
s o i l and t h e bedrock i s v e r y low i n MnO. D e t a i l s
of c o l l e c t i o n and a n a l y s i s a r e i n Hanson (1980).
Site
1
CaO
MnO
A ~ ~ O ?~ 1 . 7 0 3
7.62
0.21
Pb ( P P ~ )
189
Zn ( P P ~ )
72
Overall, acidic precipitation accelerates pod- solization, expanding depths of upper soil hori- zons, and depleting thenutrient pool in the upper soil. Changes in Speciation of Dissolved Metals Biological uptake of nutrients and toxicants is via molecular diffusion through semi-permeable membranes which are somewhat specific to the spe- cies involved. Biological response is highly specific to the dissolved form ("biological avail- ability") of the element. Decreasing pH affects speciation in two ways. Elements which complex with OH groups will be preferentially partitioned into a less hydroxylated form. For example: Lowered pH also results in protonation of weak acid radicals (e-g. HC03, fulvic , humic ) causing
a decrease in ligands for the cation of interest. For example: PH Figure 3--Eh-pH diagram for the system Al-Te-Mn- 0-H20 at 2 5 " ~
and 1 atmosphere. Solubility in moles/I. where R is a metal, n is the valence of the metal, Xxl is the stoichiometric formula for the mineral (minus R), H+ is a proton, and aq refers to aqueous species. Reactions of this type are forced by elevated H+ levels. MII+~-, ~e+2-, F~-^-Y and ~ l + ~ bearing minerals are somewhat unique in that the respective metal's mobility is a function of [@I2
or even [&I3
activity and the transition from geo- chemical immobility to mobility under oxidizing conditions takes place at a pH between 4 and 6
(fig. 3). Increased mobilization of A1 from inorganic soils has been documented by Cronan and Schofield (1979). Both experimental and field studies (Table 1) suggest that Mn is leached from upper levels of soils. Data are sparse but indicate that acidic non-dystrophic surface waters have elevated Mn levels. A1 and Mn have toxic effects
on plants via root effects, and on aquatic animals. Fe, although leached somewhat by acidic percolating solutions (Cronan and Schofield 1979), is relatively enriched in the litter in leaching experiments and in the field (Hanson 1980). Downstream reduction in concentration of metals may be accomplished by dilution (Ca, Mg, K), precipitation (Fe, Al, Mn), and adsorption (P, Zn)
.
Because toxicity is generally greater for the uncomplexed metal (Hg is an exception), acidifi- cation of soil waters should result in greater direct toxicity to roots and micro-organisms or foliage after uptake, and on "downstream" ecosys- tems. INCREASING TRACE METAL MOBILITY/AVAILABILITY Trace metal levels, particularly heavy metals, are
intimately affected by acidic precipitation in that their flux to the forest ecosystem is greatly increased over pre-pollution values and their mobility is in some cases greatly altered. Historic data to evaluate the changing atmos- phericflux of heavy metals in North America is absent. Changes in fluxes with time in remote regions have been evaluated from snowlice cores (Cragin et al. 1975). Anthropogenic emission rates may be compared with natural emission rates to obtain an estimated percentage increase or mobilization factor. For Cd, Mn, Pb, and Zn the factors are 13, 0.48, 180, and 13 respectively. However, these factors may not relate closely to deposition values (Galloway et al. 1980), because of spatially non-homogeneous emission, dispersion, and deposition. Limits on pre-pollution metal concentrations in precipitation may be established using modern data for precipitation chemistry from modern re- mote sites. Metal levels for eastern North Ameri- can must have been between the remote (Antarctica) MRRRNR
SPECK
GRRNITE
UNNRMED
Zn u
PPÃ ICN.
PPU IGN.
P M IGN.
e s t l i t t e r (Table 1 ) a l t h o u g h b i o l o g i c a l l y a v a i l a b l e B-horizon accumulation o c c u r s (Conrad, p e r s .
corn.).
This appears t o be r e l a t e d t o increased
l e a c h i n g of Zn from l i t t e r due t o d e c r e a s e d p H o f
p r e c i p i t a t i o n ; l a k e sediments from New England
e x h i b i t s i m i l a r behavior ( f i g . 4 ) . A c i d i f i e d
watersheds have l a k e sediments b e i n g d e p o s i t e d
which have l e s s Zn t h a n sediments d e p o s i t e d p r i o r
t o their acidification.
SUMMARY
PPH IGh.
PPU IGN.
CONC.
VS.
PPK IGh.
DEPTH (CM)
F i g u r e A--Pb and Zn p r o f i l e s f o r sediment from a
circum-neutral (Maranacook), s l i g h t l y a c i d i c (pH,
5-6) ( G r a n i t e ) , and a c i d i c (pIK5) (Speck) l a k e
w i t h s u r f a c e i n l e t s and o u t l e t s and an a c i d i c (pH
< 5 ) k e t t l e pond ("unnamed" Pond). Ponds a r e l o c a t e d on F i g u r e 2.
and North A t l a n t i c v a l u e s .
(We know t h a t Greenl a n d [ c r a g i n e t a l . 19751 h a s been r e c e i v i n g
p o l l u t e d p r e c i p i t a t i o n f o r approximately 200
y e a r s . ) Modern d e p o s i t i o n r a t e s f o r Pb and Zn a r e
roughly known ( f i g . 1 ) . However, o u r poor knowl e d g e of p r e - p o l l u t i o n d e p o s i t i o n r a t e s d o e s n ' t
permit a good assessment of t h e i n c r e a s e s t h a t
have o c c u r r e d f o r e a s t e r n North America.
Lake sediments r e c o r d changes i n atmospheric
d e p o s i t i o n b u t normally n o t i n a s t r a i g h t forward
manner because of watershed e f f e c t s , sediment foc u s i n g , and d i a g e n e s i s (Norton e t a l . 1980). . F i g u r e 4 (unnamed Pond p r o f i l e ) s u g g e s t s a minimum
of a n 800% and 200% i n c r e a s e f o r t h e d e p o s i t i o n
r a t e f o r Pb and Zn, r e s p e c t i v e l y , o v e r t h e l a s t
100 y e a r s . These f i g u r e s assume t h a t t h e background l e v e l s (below 15-30 cm, depending on t h e
l a k e ) a r e due t o atmospheric d e p o s i t i o n . However,
most of t h e background c o n c e n t r a t i o n s a r e probably
bedrock c o n t r i b u t i o n s . Based on i n c r e a s e s f o r Pb
and Zn c o n c e n t r a t i o n s i n sediments from remote
l a k e s i n New England, we e s t i m a t e t h a t atmospheric
d e p o s i t i o n r a t e s i n New England have i n c r e a s e d a t
l e a s t by a f a c t o r of 30X. P r e - p o l l u t i o n concentrat i o n s of heavy m e t a l s i n s o i l s a r e unknown. S h o r t
term s t u d i e s (Siccama e t a l . 1980) of heavy m e t a l s
i n s o i l s i n d i c a t e t h a t concentrations a r e increasi n g w i t h time. Our d a t a ( f i g . 2, Table 1 ) shows
a s t r o n g r e l a t i o n s h i p between t h e pH g r a d i e n t and
Pb accumulation, c o n s i s t e n t w i t h t h e Pb d e p o s i t i o n
g r a d i e n t ( f i g . 1 ) . These c o n c e n t r a t i o n s a r e of
concern w i t h r e s p e c t t o t o x i c e f f e c t s f o r s o i l
m i c r o b i a l a c t i v i t y and n u t r i e n t c y c l i n g . High Pb
concentrations i n l i t t e r a r e a l s o considered a s
p o s s i b l e c o n t r o l s on t h e b i o l o g i c a l a v a i l a b i l i t y
of phosphorus i n s o i l s (Cox and Raisons 1972).
However, even w i t h e l e v a t e d d e p o s i t i o n r a t e s
i t a p p e a r s t h a t Zn (and o t h e r elements w i t h s i m i l a r chemical b e h a v i o r ) i s n o t accumulating i n f o r -
Acidic p r e c i p i t a t i o n , e x p e r i m e n t a l l y and emp i r i c a l l y , a c c e l e r a t e s p o d s o l i z a t i o n and d e p l e t e s
n u t r i e n t p o o l s i n f o r e s t l i t t e r and s h a l l o w i n o r g a n i c s o i l s . Some heavy m e t a l s (e.g. Pb) accumul a t e t o c o n c e n t r a t i o n s which may impede b i o l o g i c a l
s o i l p r o c e s s e s and immobilize phosphorus. Other
m e t a l s a r e mobilized (Al, Mn, Zn) and may a f f e c t
ecosystems "downstream".
Paleolirnnological and
s o i l s d a t a from New England i n d i c a t e t h a t a c i d i f i c a t i o n of d r a i n a g e b a s i n s ( i n c l u d i n g s o i l s ) h a s
o c c u r r e d and n u t r i e n t d e p l e t i o n i s underway.
P a r t i a l s u p p o r t f o r t h i s work came from t h e
U.S. N a t i o n a l Science Foundation g r a n t //DEB-7810641 t o S.A. Norton; t h e U.S. Dept. of t h e Int e r i o r g r a n t #14-31-001-4240,
and t h e U.S. Dept.
of t h e I n t e r i o r g r a n t #AO26-W ( O f f i c e of Water
Resources and Research).
LITERATURE CITED Abrahamsen, Gunnar, and A.O. Stuanes. 1980. Effects of simulated rain on the effluent from lysimeters with acid, shallow soil, rich in organic matter (abs.).
Int. Conf. on The Ecological Impact of Acid Precipitation, The SNSF Project. 1:27. Art, Henry W., F.H. Bormann, G.K. Voigt, and G.M. Woodwell. 1974. Barrier island forest ecosystem: Role of meteorologic nutrient inputs. Science. 184: 60-62. Bunzl, K. 1974. Kinetics of ion exchange in soil organic matter. I1 Ion exchange during continuous addition of ~bZ+-ions to humic acid and peat. Jour. Soil. Sc. 25: (3). Cox, W.J., and D.W. Raisons. 1972. Effect of lime and lead uptake by five plant species. Jour. Envir. Qual. 1:167-169. Cragin, J.H., M.M. Herron, and C.C. Langway, Jr. 1975. The chemistry of 700 years of precipita- tion at Dye 3, Greenland: CREEL Research Report 341, 18 p. Cronan, Christopher S., and C.S. Schofield. 1979. Aluminum leaching response to acid pre- cipitation: Effects on high-elevation water- sheds in the northeast. Science. 204:304-305. Davis, Anthony O., and J.N. Galloway. 1980. Atmospheric trace metal deposition into lakes of the eastern United States. In Input of Atmospheric Pollutants to Natural Waters. Steven J. Eisenreich, ed. Ann Arbor Science Publishers, Michigan. Environment Canada. 1979. CANSAP data summary. 15 p. Galloway, James N., H.L. Volchok, D. Thornton, S. A. Norton, and R. McLean. 1980. Trace metals: a review and assessment. In Toxic substances in atmospheric deposition. James N. Galloway, S.J. Eisenreich, and B. Scott, eds. National Atmospheric Deposition Program, Fort Collins, Col. Hanson, Denis W. 1980. Acidic precipitation-induced changes in sub-alpine fir forest organic soil layers. M. S. unpublished thesis, University of Maine. Hutchinson, Thomas C. 1980. Effects of acid leaching on cation loss from soils. In Effects of acid precipitation on terrestrial ecosystems. T.C. Hutchinson and M. Havas, eds. p. 481-497. Plenum Press, New York. Hutchinson, Thomas C., and M. Havas, eds. 1980. Effects of Acid Precipitation on Terres- trial Ecosystems. 654 p. Plenum Publishers, New York. Lazrus, A.L., E. Lorange, and J.P. Lodge, Jr. 1970. Lead and other metal ions in United States precipitation. Envir. Sci. Tech. 4:55- 58. Likens, Gene E., R.F. Wright, J.N. Galloway, and T.J. Butler. 1979. Acid Rain. Scient. Amer. 241:43-51. Linzon, S.N., and P.J. Temple. 1980. Soil resampling and pH measurements after an 18-year period in Ontario (abs). Int. Conf. on The Ecological Impact of Acid Precipitation, The SNSF Project. 1:42. Malmer, Nils. 1976, Acid precipitation: chemical changes in the soil. Ambio. 5:231-234. National Atmospheric Deposition Program. 1979. NADP first data report, July 1978 through February 1979. Fort Collins, Col. Natural Resource Ecol. Lab., Col. State University. National Atmospheric Deposition Program. 1980a,b. NADP Data Report. Fort Collins, Col. Natural Resource Ecol. Lab., Col. State Uni- versity, I and 11: (1)&(2). Norton, Stephen A., C.T. Hess, and R.B. Davis. 1980. Rates of accumulation of heavy metals in pre- and post-European sediments in New Eng- land lakes. 2 Input of Atmospheric Pollutants
to Natural Waters. Steven J. Eisenreich, ed. Ann Arbor Science Publishers, Michigan. Siccama, T.G., W.H. Smith, and D.L. Mader. 1980. Changes in lead, zinc, copper, dry weight, and organic matter content of the forest floor of white pine stands in central Massachusetts over 16 years. Envir. Sci. Tech. 14:54-56. Ulrich, B. 1980. Deposition, production and consumption of hydrogen ions in a beech and spruce ecosystem in the Soiling District (abs.).
Int. Conf. on the Ecological Impact of Acid Precipitation, The SNSF Project. 1:50. Voigt, Garth K. 1979. Acid precipitation forest ecosystems and intensive harvesting. In Impact of Intensive Harvesting on Forest ~utrientCycling. p. 33- 48. College of Environmental Science and Forestry, School of Forestry, Syracuse, N.Y. Williams, John S. 1980. The relative contributions of local and regional atmospheric pollutants to lake sedi- ments in northern New England. M.S. unpub- lished thesis, University of Maine at Orono. Impact of Heavy Metals on Terrestrial
and Aquatic Ecosystems1
Tom C. Hutchinson
2
Abstract:
The high t o x i c i t y of many m e t a l s and m e t a l l o i d s
t o a wide range of b i o t a , coupled w i t h t h e i r long r e s i d e n c e
times i n t h e s o i l s , i n sediments and i n t h e oceans has l e d
t o r e a l concern about t h e i r r o l e i n environmental d e t e r i o r a t i o n . Residence times i n watersheds a r e commonly measured
i n hundreds of y e a r s , w h i l e r e s i d e n c e of m e t a l s i n a i r i s
r a r e l y a s long a s s e v e r a l days. O v e r a l l i n d u s t r i a l a c t i v i t y
and t r a n s p o r t a t i o n l e a d s t o widespread m e t a l d i s p e r s i o n .
Major e l e v a t i o n s i n many m e t a l s occur around mines and
s m e l t e r s and f o r l e a d e s p e c i a l l y , alongside highways. Coalburning and a p p l i c a t i o n s of f e r t i l i s e r s and p e s t i c i d e s add
m e t a l s t o a g r i c u l t u r a l s o i l s and t o n a t u r a l ecosystems. The
s u r f a c e o r g a n i c l a y e r s of b o t h s o i l s and sediments a c t a s
a d s o r p t i o n and exchange s i t e s s o t h a t major accumulations
may occur. Yet, t h i s shallow o r g a n i c l a y e r is t h e c r i t i c a l
s i t e f o r many m i c r o b i a l a c t i v i t i e s , i n c l u d i n g those e s s e n t i a l
f o r n u t r i e n t c y c l i n g , n i t r o g e n f i x a t i o n and f o r pathogens.
Genetic and p h y s i o l o g i c a l t o l e r a n c e s a r e shown i n a wide
a r r a y of d i f f e r e n t organisms which have s u r v i v e d i n metals t r e s s e d h a b i t a t s . 'Most r e c e n t l y , a c i d p r e c i p i t a t i o n has
mobilised A l , Mn, Fe and Zn from t h e s o i l and sediment.
These a r e now producing p a r t i c u l a r s t r e s s e s f o r a q u a t i c b i o t a .
The c o n s i d e r a b l e t o x i c i t y of many m e t a l s t o
b i o t a i s w e l l known e.g. l e a d , mercury, cadmium,
a r s e n i c . The q u a n t i t i e s mined and smelted f o r
innumerable uses c o n t i n u e s t o r i s e each y e a r . It
is a p p a r e n t t h a t b o t h n a t u r a l and man-made eco-
p r e s e n t e d a t t h e Symposium on E f f e c t s of Air
P o l l u t a n t s on Mediterranean and Temperate F o r e s t
Ecosystems,
June 22-27? 1980,
Riverside,
C a l i f o r n i a , U.S.A.
~ r o f e s s o rof Botany and F o r e s t r y , Chairman,
Dept. of Botany, U n i v e r s i t y of Toronto, Toronto,
Ontario, Canada. M5S 1 A 1 .
systems a r e t h e r e c i p i e n t s of i n c r e a s i n g quantit i e s of t h e s e elements and t h a t we have t o be
continuously v i g i l a n t t o ensure t h a t we do n o t
e i t h e r poison o u r s e l v e s , our f e l l o w b i o t a , o r our
a g r i c u l t u r a l and n a t u r a l ecosystems. Major human
poisonings have occurred such a s t h a t by mercury
i n Japan and i n I r a q , and t h a t of cadmium i n
Japan. The beer-deaths i n Birmingham, England a t
t h e t u r n of t h e century were a l s o b e l i e v e d t o b e
metal-related, being variously ascribed t o arsenic
and/or selenium. Concern h a s a l s o been expressed
about n o t only t h e p o t e n t i a l f o r food c h a i n contamination l e a d i n g t o man b u t a l s o t o d i r e c t a e r i a l
i n p u t s t o man v i a t h e r e s p i r a t o r y t r a c t . Lead
from automobile emissions where i t i s used a s an
anti-knock i n g a s o l i n e , and from primary and
secondary s m e l t e r s , a s w e l l a s many o t h e r s m e l t e r -
emitted metals such as a r s e n i c , copper, n i c k e l ,
zinc, cadmium, antimony and selenium, a l l have
p o t e n t i a l l y harmful consequences due t o t h e i r
p e r s i s t e n c e i n t h e body, and t h e i r a b i l i t y t o
i n t e r f e r e with s p e c i f i c enzyme systems. It
should a l s o be noted t h a t many instances a r e
known of s y n e r g i s t i c and a n t a g o n i s t i c i n t e r a c t i o n s
between metals, both i n a q u a t i c and t e r r e s t r i a l
systems and i n t h e human body. Notable amongst
these a e t h e a m e l i o r a t i v e e f f e c t s of selenium
and a r s e n i c on both mercury and cadmium t o x i c i t y
i n mammals and t h e r e c e n t l y described s y n e r g i s t i c
e f f e c t s of ozone on cadmium and n i c k e l t o x i c i t y
i n c e r t a i n crop p l a n t s , e.g. Parizek 1978,
Levander 1977, Groth and o t h e r s 1973, Czuba and
Onnrod 1974. Nickel and copper synergisms Rave
been described f o r a v a r i e t y of b i o t a , including
freshwater u n i c e l l u l a r a l g a e , f l o a t i n g a q u a t i c
p l a n t s , and t r e e s e e d l i n g s , Hutchinson 1973,
Hutchinson and Stokes 1975, Hutchinson and Czyrska
1972, and Hutchinson and Whitby 1974.
f
Despite what sometimes seems t o be a gloomy
p i c t u r e with r e s p e c t t o metal accumulations i n
the environment, i t ought t o be borne i n mind
t h a t concentration of a i r b o r n e p a r t i c u l a t e s i n
urban and i n d u s t r i a l a r e a s of Europe and North
America have o f t e n been much worse i n t h e p a s t .
Cohen and Ruston (1925) r e p o r t e d very high
a r s e n i c l e v e l s i n t h e a i r of Leeds, England i n
1902-1910 due t o c o a l burning, w h i l e t h e o v e r a l l
l e v e l s of SO2 and a c i d i c a e r o s o l s were much
h i g h e r than p r e s e n t l y occur.
aluminium, manganese, z i n c and f e r r i c i r o n e n t e r
drainage water i n t h i s way. Sediments can
s i m i l a r l y l o s e these same elements t o t h e water
bodies of l a k e s under a c i d i f y i n g conditions
(Schindler and o t h e r s 1980). The damaging e f f e c t
of t h e r e s u l t a n t aluminium concentrations t o f i s h
have been described by various a u t h o r s , including
Schofleld 1976, Baker and Schofield 1980, a t l e v e l s
as low a s 0.1-0.2 mg/1. Increased t r a n s p o r t of
aluminium I n t o a q u a t i c systems can a l s o a f f e c t
phosphorus a v a i l a b i l i t y (Cronan and Schofield 1979).
The residence time of metals i n the a i r i s
always very much s h o r t e r than t h a t i n s o i l , w a t e r ,
sediments o r oceans. This i s I l l u s t r a t e d by Table
1, which emphasises t h e r a t h e r l a r g e r residence
time of l e a d i n a i r than t h a t of a wide range of
o t h e r m e t a l s . This p a r t l y explains t h e e l e v a t i o n s
of l e a d noted a t remote l o c a t i o n s , such a s i n t h e
a r c t i c , i n g l a c i a l i c e i n Greenland and a t mount a i n tops i n C a l i f o r n i a (see National Academy of
Sciences Lead Review 1980). It i s a l s o a f u n c t i o n
of p a r t i c u l a t e s i z e and p a r t i a l vapour pressure.
Table 1. Residence times1 of metals i n t h e atmosphere a t La J o l l a and Ensenada, from Hodge,
Johnson and Goldberg, 1978.
days
L a Jolla
'1'
'
7
Rather l i t t l e a t t e n t i o n has been paid t o the
residence times of metals i n components of t h e
biosphere. The s t r o n g r e t e n t i o n of metals on
the organic s u r f a c e l a y e r s of the s o i l and of t h e
sediments is of g r e a t importance, i n t h a t i t
causes long residence times a s w e l l a s i n allowi n g accumulation of metals t o p o t e n t i a l l y t o x i c
concentrations. Since i t i s p r e c i s e l y i n these
s u r f a c e zones t h a t t h e major populations of
microbes a r e l o c a t e d , and where the e s s e n t i a l
processes take p l a c e of decomposition of organic
m a t t e r , of n i t r o g e n f i x a t i o n and of elemental
cycling e t c . Thus, t h e p e r s i s t e n c e and
accumulations of t o x i c elements i s of r e a l
concern. L i t t e r i n urban a r e a s contains e l e v a t e d
l e a d concentrations.
The r e t e n t i o n of t h e metals themselves i s an
exchange process, with the elements behaving i n
a reasonably p r e d i c t i v e way, based on such
p r o p e r t i e s a s i o n i c r a d i u s and e l e c t r o n e g a t i v i t y .
Acid leaching can cause a downward movement of
heavy metals through t h e p r o f i l e , s o t h a t they
may e n t e r ground water o r watershed streams.
(e.g. Abrahamsen, Stuanes and Bjor 1979, Cronan
and Schofield 1979, Hutchinson 1980, Bacon and
Maas 1979 .) Both rock s u r f a c e s and s o i l s can
c o n t r i b u t e and such c l a y -mineral c o n s t i t u e n t s a s
8
0.5
0.1
0.3
1
0.8
0.2
0.4
0.2
0.4
0.6
0.2
0.7
0.2
0.4
0.5
3
1.2
1.0
0.8
0.8
Residence Times and Watershed Loss
Ensenada
1.0
-----
5
1
Standing rop of metals on p a r t i c u l a t e s i n
1,000 m X 1 an column of a i r f i l t e r data--Table
1 ) divided by the f l u x t o 1 cm of ground s u r f a c e
("bucket d a t a --Table 2 ) . F i l t e r d a t a averaged
over period during which buckets exposed. F i l t e r
concentrations of Co, Fe, Mn, C r and A l h a v e been
m u l t i p l i e d by 2 i n order t o account f o r t h e
d i s c r i m i n a t i o n a g a i n s t l a r g e p a r t i c l e s by t h e H i Vol sampler.
3
$
The c o n t r a s t i n g d a t a f o r watershed s o i l s a r e
i l l u s t r a t e d by Table 2, taken from Bowen 1975, and
from which i t is apparent t h a t soil-watershed
residence t i m e s a r e measured i n hundreds of y e a r s .
z
Table 2 . Inputs and outputs i n mg X/m y r , and
residence times i n years, f o r nine elements i n
s o i l s of the Upper Thames b a s i n .
X
Rain
input
Fertilizer
input
Rock
input
Clearly t h e p o t e n t i a l f o r accumulation t o t o x i c
l e v e l s i s much g r e a t e r . The excess of l e a d and
chromium i n i n p u t over output i s a f e a t u r e of
systems s u b j e c t e d t o i n d u s t r i a l deposition. The
v o l a t i l i z a t i o n of some of these heavy metals
from t h e f o l i a g e of v e g e t a t i o n e.g. zinc, mercury,
and selenium (Beauford and o t h e r s 1975) may
re-mobilise s m a l l q u a n t i t i e s of t h e s e metals and
i n c r e a s e atmospheric r e s i d e n c e times b u t i t w i l l
n o t i n f l u e n c e l o s s i n t o drainage waters. Allen
and Steinnes (1979) determined t h e r e g i o n a l
d i s t r i b u t i o n of l e a d , z i n c , cadmium, copper,
a r s e n i c , antimony and selenium i n Norwegian
s u r f a c e s o i l s , u t i l i s i n g 500 humus samples. Lead
l e v e l s were 10-fold h i g h e r i n t h e south than i n
t h e a r c t i c a r e a s and a l s o h i g h e r along t h e c o a s t
than i n l a n d . Cadmium, a r s e n i c , antimony and
selenium showed a s i m i l a r north-south trend. A l l
of these elements a r e h i g h l y v o l a t i l e , low b o i l i n g
p o i n t components of t h e atmospheric load from
i n d u s t r i a l and urban c e n t r e s .
Residence times i n s o i l water a r e a f f e c t e d t o
a g r e a t e x t e n t by pore s i z e . The water i n t h e
l a r g e pores i n f i l t r a t e s i n t o lower l a y e r s and a i r
e n t e r s a g a i n behind i t . The residence t i m e i n
these pores i s not more than hours. Meanwhile,
t h e w a t e r i n t h e narrow pores i s displaced only
centimetres o r m i l l i m e t r e s . An example i s shown
i n Table 3 from t h e work of F r i s s e l (19781, w i t h
residence times a s high a s 5000 y e a r s . The
i m p l i c a t i o n s f o r ground water contamination a r e
apparent.
+ and
Targets f o r H
Drainage
output
Cropping
output
Residence time/
years
a g r i c u l t u r a l ones, the s u r f a c e of the l e a f i s one
such i n t e r f a c e . Higher p l a n t s a r e covered by a
r a t h e r impermeable waxy c u t i c l e , which reduces
g a s and water flow t o a minimum b u t i s p e r f o r a t e d
by numerous stomata, o f t e n on t h e underside of t h e
l e a f e s p e c i a l l y . While p a r t i c u l a t e s can accumulate
on such a s u r f a c e , they a r e a l s o r a t h e r e a s i l y
washed off by r a i n o r blown o f f by wind.
Frequently, however, small p a r t i c l e s can b e
incorporated i n t o the c u t i c l e o r e n t e r the stomata.
The l e a f s u r f a c e of many p l a n t s a l s o a r e covered
By numerous f i n e branched h a i r s o r glands. These
can a c t as t r a p s f o r p a r t i c u l a t e s s o t h a t very
d i r t y l e a f s u r f a c e s can occur i n a r e a s of h i g h
d u s t f a l l . Nevertheless, the d i r e c t l y t o x i c e f f e c t s
of p a r t i c u l a t e metal c o n s t i t u e n t s a r e l i m i t e d a s
they a r e k e p t away from m e t a b o l i c a l l y a c t i v e s i t e s
.
I n the mosses, l i v e r w o r t s and l i c h e n s , the
c u t i c l e i s e f f e c t i v e l y absent. The exposed c e l l
w a l l s u r f a c e i s a t t h e a i r i n t e r f a c e and i t
c o n s i s t s of charged s i t e s , which can exchange both
c a t i o n s and anions. The metals a r e s e l e c t i v e l y
exchanged onto t h i s s u r f a c e and a r e h e l d t h e r e .
Large accumulations can take p l a c e . Lichens i n
p o l l u t e d regions a t t e s t t o t h i s , a s do mosses
The use of
Ce.g. RUhling and Tyler 1970)
Sphagnum moss bags a s a i r monitors is based on
t h i s c a t i o n exchange capacity. The s p e c i a l s e n s i t i v i t y of -many l i c h e n s t o a i r p o l l u t a n t s is due
t o t h e ready e n t r y of t h e p o l l u t a n t t o metabolically
active s i t e s
.
.
S o i l Surface Layers a s Target Areas a t Risk
Heavy Metals
I should l i k e t o emphasize t h a t one very
u s e f u l way of considering t h e p o t e n t i a l t h r e a t s
t o ecosystems i s through a c o n s i d e r a t i o n of t a r g e t s i n the ecosystem. Clearly, a l l s u r f a c e
i n t e r f a c e s f a l l i n t o t h i s category. Surfaces
p r e s e n t a r e a s of p o t e n t i a l accumulation o r
residence. I n t e r r e s t r i a l ecosystems including
,
The s u r f a c e of t h e s o i l a s a c r i t i c a l s i t e f o r
accumulation of a i r b o r n e m e t a l l i c contaminants has
already Been r e f e r r e d t o h e r e . It is i n t h i s
upper few centimetres of t h e s o i l t h a t n u t r i e n t
uptake i n t o p l a n t r o o t s takes p l a c e and i n which
new r o o t h a i r s a r e developed. Seeds germinate i n
t h i s l a y e r amongst t h e f o r e s t l i t t e r and s e e d l i n g s
e s t a b l i s h t h e r e . The m i c r o b i a l sequences e s s e n t i a l
Table 3. Residence times of water i n t h e s a t u r a t e d
zone of t h e s o i l .
Discharge
per year
bud
System
and pathways
Estimated r e s i d e n c e time
depending on t h e p l a c e of
i n f i l t r a t i o n and on p o r o s i t y
of t h e s o i l o r rock
About c o n s t a n t
slow discharge 17
p a r t l y 12
slow discharge 1 2
f a s t discharge 10
p a r t l y 12
100-500 y e a r s
1-500
years
1 dayÑ1 y e a r s
10Ñ100 y e a r s
1 d a y ~ 1 0y e a r s
1 h o u r ~ dl ay
f o r e f f e c t i v e l i t t e r decomposition t a k e p l a c e , i n
t h i s zone pathogens s t r i v e t o i n f e c t s e e d l i n g s o r
r o o t systems, and t h e mycorrhizal f u n g i e s s e n t i a l
f o r e f f e c t i v e n u t r i t i o n of many f o r e s t t r e e s ,
e s p e c i a l l y i n t h e b o r e a l f o r e s t , develop h e r e .
The r h i z o b i a l b a c t e r i a which a c t a s n i t r o g e n
f i x e r s i n legumes and t h e actinomycetes and bluegreens which f u l f i l l t h i s r o l e i n o t h e r shrubs
and g r a s s e s a l s o have t o i n f e c t r o o t s i n t h e s e
upper few c e n t i m e t r e s of t h e s o i l . Yet, onto t h i s
s u r f a c e is b e i n g d e p o s i t e d i n c r e a s i n g loads of
t o x i c heavy m e t a l s , of a c i d i f y i n g substances and
a l s o of gaseous p o l l u t a n t s . The t h r e a t t o t h e
s a f e f u n c t i o n i n g of such ecosystems and t o t h e
well-being of man a r e focused on t h i s zone.
Indeed, we can c o n s i d e r t h a t t h e r e d u c t i o n o r
e l i m i n a t i o n of j u s t a few key processes could p u t
t h e whole system a t r i s k . The enzyme a r y l
s u l p h a t a s e which produces t h e p l a n t - a v a i l a b l e
s u l p h a t e from t h e non-available o r g a n i c s u l p h u r
i n s o i l s i s one such s t e p , and i t i s known t h a t
soil-extracted a r y l sulphatases a r e susceptible
t o a wide range of heavy metals i n c l u d i n g
aluminium. The a b i l i t y of r h i z o b i a l b a c t e r i a l
t o i n f e c t legume r o o t s i s a l s o known t o be a c i d
s e n s i t i v e and heavy metal s u s c e p t i b l e . The condit i o n s f o r s e e d l i n g e s t a b l i s h m e n t might be a f f e c t e d
i f t h e atmospheric i n p u t s of wet and dry d e p o s i t i o n
were t o a c i d i f y s u r f a c e s o i l s s o a s t o favour f u n g i
a t t h e expense of b a c t e r i a .
The t h r e a t t o l i t t e r decomposition may b e a
long time i n developing i n most a r e a s b u t i n those
where i n t e n s e heavy metal accumulations have
occurred from s m e l t e r emissions, examples of t h i s
have a l r e a d y been demonstrated. I n both t h e
remnant f o r e s t i n t h e major Sudbury s m e l t i n g a r e a ,
where n i c k e l and copper c o n c e n t r a t i o n s have reached
up t o 2000 ppm i n t h e p a s t and i n t h e New Lead
B e l t of Missouri, where l e a d , z i n c , cadmium and
Watershed
Merkenf r i t z b a c h
( F e d e r a l Republic of
Germany, Land
Hessen, Main a r e a )
1600 h a
Okkenbroek
(The Netherlands,
I J s s e l area)
443 h a
copper a r e now h i g h , abnormal accumulations of
l i t t e r on t h e f o r e s t a r e r e p o r t e d (Freedman and
Hutchinson 1980, Watson and o t h e r s 1976). A t
t h e z i n c s m e l t e r of Palmerton i n Pennsylvania,
S t r o j a n (1978) r e p o r t e d reduced decomposition of
t h e f o l i a g e of a number of s p e c i e s and a s c r i b e d
i t t o e l e v a t e d z i n c and cadmium c o n c e n t r a t i o n s .
The r e p o r t s have a l s o included d e t r i m e n t a l
e f f e c t s on a number of s o i l enzyme a c t i v i t i e s , on
o v e r a l l m i c r o b i a l r e s p i r a t o r y a c t i v i t y and on
micro-arthropod and earthworm a c t i o n . Indeed,
many r e p o r t s a r e now a v a i l a b l e which show s e n s i t i v i t y of earthworms t o heavy metal accumulations.
Sediments a s S i t e s of Risk
I t should b e noted t h a t a r a t h e r s i m i l a r b u t
p a r a l l e l c a s e can b e made f o r e f f e c t s on s u r f a c e
sediments. Again, i n t h i s zone, much of t h e
microbial a c t i v i t y takes place, the aquatic p l a n t s
have t o r o o t , t h e b e n t h i c organisms l i v e and
reproduce and t h e important gas exchanges t a k e
p l a c e w i t h t h e w a t e r column. This i s t h e zone of
d e p o s i t i o n of t h e dead p l a n k t o n i c organisms, of
incoming p a r t i c u l a t e m a t t e r and of p o l l u t a n t
m a t e r i a l e q u a l l y . The sediments a r e o f t e n h i g h l y
organic and have a l a r g e c a t i o n exchange c a p a c i t y .
Toxic l e v e l s can develop (mercury i n p o l l u t e d
sediments of t h e D e t r o i t River and Lake S t . C l a i r ) ,
and benthic-feeding f i s h , clams, c r a y f i s h e t c .
come i n c o n t a c t w i t h p e s t i c i d e s , PCB's and heavy
m e t a l s which have been i n i t i a l l y t r a n s p o r t e d by
a i r . The methylation t r a n s f o r m a t i o n which c r e a t e s
o r g a n i c mercury compounds n i n e t o t e n times more
t o x i c than i n o r g a n i c e q u i v a l e n t s take p l a c e h e r e
i n t h e sediment s u r f a c e l a y e r s .
Reproduction: c r u c i a l s t e p s a t r i s k
A number of e s s e n t i a l s e q u e n t i a l s t e p s can b e
i d e n t i f i e d , t h e i n t e r f e r e n c e with which places t h e
whole reproductive process a t r i s k . These include
a ) the h e a l t h of p o l l i n a t o r s , e s p e c i a l l y the
i n s e c t s e s s e n t i a l f o r those p l a n t s with s p e c i a l i s e d
mechanisms and a p p r o p r i a t e f l o r a l guides, t r i p
mechanisms, n e c t a r production, p o l l e n p o s i t i o n i n g
e t c . , b) t h e a b i l i t y of the p o l l e n t o germinate
s u c c e s s f u l l y on t h e stigma and t o then successf u l l y produce a p o l l e n tube which can reach t h e
u n f e r t i l i s e d ovules c ) t h e a b i l i t y of sperm t o
s u c c e s s f u l l y reach and f e r t i l i s e t h e egg d) the
a b i l i t y of p l a n t s and animals t o s u c c e s s f u l l y
d i s p e r s e t h e i r progeny t o s u i t a b l e new h a b i t a t s .
I n t e r e s t i n g l y , i t i s known t h a t s t e p s a-c
can a l l b e a f f e c t e d by e l e v a t e d l e v e l s of heavy
metals. For example, bees a r e known t o b e very
s u s c e p t i b l e t o a i r b o r n e a r s e n i c . I n the region
of t h e Novatny power s t a t i o n i n Czechoslovakia
bee h i v e s have been wiped out. The power-station
burns c o a l with high l e v e l s of a r s e n i c i . e .
s e v e r a l hundred ppm, and consequent elevated a i r
l e v e l s of As203 occur. Bees a l s o w i l l pick up
and accumulate selenium when p o l l i n a t i n g high
selenium p l a n t s , such a s some of t h e loco-weeds
(As t r a g a l u s s p e c i e s )
.
The s u c c e s s f u l growth of t h e p o l l e n tube i s
r e p o r t e d t o b e pH-dependent and a l s o t o b e
inÂluenced by t h e presence of t o x i c heavy metals
such a s z i n c and copper. The f e r t i l i z a t i o n of
f e r n archegonia has been r e p o r t e d t o b e i n h i b i t e d
by acid s o l u t i o n s and a c i d r a i n , L. Evans,
(personal communication)
L. S c h l i c t e r , a
graduate s t u d e n t i n botany a t University of Toronto,
has r e c e n t l y shown t h a t t h e s u c c e s s f u l postf e r t i l i z a t i o n s t e p s i n embryo development of f r o g
eggs i s i n h i b i t e d by even q u i t e minor decreases
i n pH below 6.0 and t h a t m u l t i p l e - f e r t i l i z a t i o n of
an egg, which normally a r e precluded, can occur
under t h e s e a c i d i c conditions. The consequence
i s an e a r l y aborted embryo. E f f e c t s on t r o u t
eggs, on New J e r s e y f r o g s and on reproductive
success i n planktonic crustaceans have been
reported by a number of workers, e.g. Krishna
(1953), Gosner and Black (1957), Havas (1980).
.
Other Factors which Inf h e n c e t h e
Outcome of Metal Impacts
I t does seem t o b e t h e c a s e t h a t the damage
t o ecosystems from a i r b o r n e metals, a s w e l l a s
from gaseous p o l l u t a n t s , i s g r e a t e s t when t h e
i n d i v i d u a l s i n t h e ecosys tem a r e metabolically
most a c t i v e . Thus, t h e damage t o f o r e s t ecosystems i n temperate zones i s c l e a r l y g r e a t e s t
i n t h e summer growing season. Damage during t h e
day when stomata a r e open Is g r e a t e r than a t n i g h t
when they a r e closed. The l i c h e n s and mosses a r e
most s u s c e p t i b l e when they a r e moist and photos y n t h e s i z i n g a c t i v e l y . I n dry o r a r i d h a b i t a t s ,
such a s t h e d e s e r t s of Arizona, t h e damage from
l a r g e smelter-emitted SO2 and copper p a r t i c u l a t e s
i s minimal i n c o n t r a s t t o t h a t of w e t t e r a r e a s
such a s Palmer ton Pennsylvania, Ducktown
Tennessee o r Sudbury, Ontario. This probably
p a r t l y r e l a t e s t o metabolic a c t i v i t y including
t h e percentage of time stomata remain open, b u t
a l s o t o the a c t i v i t y of the r o o t systems. I f one
follows through with t h i s g e n e r a l i z a t i o n , then we
can p r e d i c t t h a t a r c t i c regions w i t h very s h o r t
growing seasons and very long dormant periods
w i l l b e l e s s a f f e c t e d by e q u i v a l e n t metal pollut a n t inputs ( o r SO?, O3 o r F l i n p u t s ) than would
Temperate o r e s p e c i a l l y Tropic Rain Forest
systems. Equally, one can p r e d i c t t h a t the more
a r i d an a r e a , t h e l e s s s u s c e p t i b l e t h i s a r e a ' s
vegetation w i l l b e t o t o x i c damage.
The importance of t h e longevity of t h e
i n d i v i d u a l a l s o needs t o be s t r e s s e d . Damage t o
long-lived trees- may t a k e a long time t o become
apparent and f i n a l l y perhaps only through t h e i r
i n a b i l i t y t o reproduce. Equivalent reproductive
f a i l u r e i n a n annual w i l l obviously be very
r a p i d l y apparent. Metal s t r e s s e s o f t e n seem t o
favour p e r e n n i a l p l a n t s w i t h l a r g e l y v e g e t a t i v e
reproduction, such a s grasses and sedges. The
equivalent a q u a t i c examples would be t h a t of
f i s h compared with planktonic a l g a e o r crustaceans.
F i n a l l y , i t must b e emphasised t h a t t h e
i n i t i a l s e n s i t i v i t i e s of t h e s p e c i e s , populations
and i n d i v i d u a l s of an a r e a when f i r s t s u b j e c t t o
m e t a l s t r e s s , a r e n o t t h e f i n a l response. While
pre-adaptation o r p r e - s e n s i t i v i t y may allow a n
i n i t i a l s e l e c t i o n and s o r t i n g , t h e s t r e s s e d
environment r e p r e s e n t s a changing h a b i t a t i n
which evolutionary change occurs. The occurrence
of m e t a l - t o l e r a n t g r a s s e s on mine waste s i t e s i s
w e l l known. The a b i l i t y of some of these g r a s s e s
t o evolve multiple-metal tolerances and cotolerances is now a l s o r e c e i v i n g a t t e n t i o n
(Tatsuyama- and o t h e r s 1975, Cox and Hutchinson
1980J. Even i n h a b i t a t s such a s t h e Smoking H i l l s
of a r c t i c Canada, where pond water pH's may reach
a s low a s 1.8 and s o i l pH's t o < 3.0 from t h e
i n i t i a l v a l u e s of pH > 7.0, some organisms do
survive. These even have the a b i l i t y t o t o l e r a t e
t h e high a c i d i t y , low n i t r o g e n and phosphorus
a v a i l a b i l i t y and t h e extremely e l e v a t e d l e v e l s of
normally t o x i c metals such a s aluminium, manganese,
f e r r i c i o n and zinc, Hutchinson and o t h e r s (1978).
A t t h e extreme, some organisms seem t o have
evolved s p e c i f i c a l l y on a r e a s of very elevated
metal l e v e l s . The occurrence of a legume Becium
homblei on copper m i n e r a l i z a t i o n s i s an example
of t h i s , i n which the p l a n t accumulates enormous
l e v e l s of copper and a l s o r e q u i r e s concentrations
which would b e l e t h a l t o o t h e r p l a n t s ( R e i l l y
1967).
While t h i s allows a c e r t a i n re-assurance about
t h e a b i l i t y of l i f e to t h r i v e under even most
adverse conditions, i t does n o t a t a l l i n f l u e n c e
t h e f a c t t h a t we must b e extremely concerned w i t h
t h e a c c e l e r a t i n g l i b e r a t i o n of heavy metals t o o u r
a g r i c u l t u r a l and n a t u r a l environments. Economic
and s o c i a l p r e s s u r e s do compel us t o t a k e s t o c k .
Proposals f o r d i s p o s a l of metal-contaminated
sewage s l u d g e on farm o r f o r e s t l a n d s have a l r e a d y
caused agencies and s c i e n t i s t s t o t h i n k i n a much
longer time frame than we a r e used t o do.
Acknowledgments: I wish t o thank D r . A 1 Page
of t h e S o i l Science Dep., U n i v e r s i t y of C a l i f o r n i a ,
R i v e r s i d e , f o r most u s e f u l d i s c u s s i o n s i n connect i o n w i t h t h i s paper.
Czuba, M. and D.P. Ormrod. 1974. E f f e c t s of
cadmium and z i n c on ozone-induced phytot o x i c i t y i n c r e s s and l e t t u c e . Can. J. Bot.
52:645-650.
Freedman, B. and T.C. Hutchinson. 1980.
Smelter p o l l u t i o n near Sudbury, O n t a r i o and
e f f e c t s on f o r e s t l i t t e r decomposition.
I n . E f f e c t s of Acid P r e c i p i t a t i o n on
T e r r e s t r i a l Ecosystems. NATO Eco-Sciences
Conf. Ecology S e r i e s 4. Ed. T.C. Hutchinson
395-434.
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1978. Cycling of Minerals
F r i s s e l , M.J.
Pub.
N u t r i e n t s i n A g r i c u l t u r a l Ecosys tems
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.
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.
Effects of Acidic Precipitation on Health
and the Productivity of Forests1
Ellis B. Cowling and Leon S. Dochinger2 Abstract: Acid precipitation has become a dominant feature of man-induced change in the chemical climate of the earth. But acid precipitation is only one special feature of the changing chemistry of atmospheric deposition in developed and developing regions throughout the world. In recent decades, human activities (mainly increased combustion of fossil fuels and decomposition or combustion of waste products) have greatly increased the total emissions and deposition of beneficial nutrients and injurious substances (such as strong mineral acids) from the atmosphere. Pro-
jected increases in the use of fossil fuels, and especially in the use of coal, will add still further to the total burden of beneficial and injurious substances deposited on forest and rangeland ecosystems from the atmosphere. The purpose of this brief paper is to summarize certain impor- tant principles concerning the phenomena of acid precipi- tation and atmospheric deposition and their beneficial and detrimental effects on the health and productivity of forests
.
The supply of both beneficial nutrientelements
and injurious substances in the atmosphere influence the health and welfare of forests. Plant life as we know it would be impossible without
atmospheric sources of carbon dioxide for photosynthesis, nitrogen for biological fixation and
proteins synthesis, oxygen for respiration and
synthesis of carbohydrates, and water for
presented at the Symposium on Effects of Air
Pollutants on Mediterranean and Temperate Forest
Ecosystems, June 22-27, 1980, Riverside,
California, U.S.A.
~ssociateDean for Research, School of
Forest Resources, North Carolina State University, Raleigh. N. C.; and Project Leader,
Northeastern Forest Experiment Station, Forest
Service, U. S. Department of Agriculture,
Delaware, Ohio.
transpiration and the maintenance of cell turgor. Some epiphytic plants, such as orchids, spanish moss, and certain lichens, obtain essen- tially all their nutrients and water from the atmosphere. Although these plants represent an extreme case of dependence on atmospheric resources, many forest trees and some herbaceous plants also derive a significant portion of their nutrients from the atmosphere. Plants suffer when the concentrations of inju- rious substances in the atmosphere exceed the amounts they can tolerate. Injurious gases can enter through the stomata of leaf tissues and poison the photosynthetic system of living cells. Toxic particles can accumulate on plant surfaces and injure plant cells. Strong acids can dissolve in rain drops or adsorb to snowflakes and then be deposited in precipitation. Dissolved substances can accumulate in snow where they may be concen- trated and released with the first meltwater. In all these ways substances transferred from the atmosphere to the biosphere can influence plant growth, beneficially (as in the case of beneficial nutrient elements) or harmfully (as in the case of toxic gases, aerosols, dry particulate matter, and injurious substances dissolved in precipitation). The growth and productivity of forests are determined by the availability of sixteen elements that are essential for growth and a few that are toxic to plants. The essential elements include nine major elements: carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, sulfur, calcium, and magnesium and seven minor elements: iron, copper, zinc, manganese, molybdenum, boron, and chlorine. Some elements are both essential and injurious to plants. For example, sulfur and nitrogen are needed for synthesis of protein, nucleic acids, and other substances; hut gaseous sulfur and nitrogen oxides and sulfuric and nitric acid aerosols are also injurious to plants at very low concentrations. Similarly, excess amounts of the minor nutrient elements also can injure plants. Atmospheric fluoride is toxic to plants at 25-50 ppm. Aluminum is the most abundant potentially toxic element in soils. Its avail- ability (and thus its toxicity) is influenced greatly by the acidity of soils, which in turn is influenced by the abundance of acid precipi- tation. Uptake of nutrients from atmospheric sources is especially important in natural ecosystems such as lakes, estuaries, wetlands, forests, and rangelands where nutrients from other sources are scarce and where fertilization is not a normal management procedure. But this capacity also increases the vulnerability of terrestrial and aquatic organisms to injury by acid precipitation and toxic aerosols and gases (Galloway and others 1978). ACID PRECIPITATION AS PART OF A GENERAL PHENOMENON 0F.ATMOSPHERICDEPOSITION cause very sudden increases in acidity of surface soils, vegetation, and surface waters. Thus a given plant may be subject to beneficial atmos- pheric influences at one time and to negative influences at another time within a given day, month, growing season, or the years of its development in the case of perennial plants and animals. Even a given substance, such as sulfur or nitrogen dioxides, may be absorbed and utilized as a beneficial nutrient at one concen- tration in the atmosphere. At another, higher concentration, even on the same day, however, the same substance may be absorbed and found to be toxic or even injurious to the very same plant. Forests and rangelands cover a larger fraction of the total land area of the United States than all other uses of land combined. For this reason, terrestrial vegetation, soils, and surface waters are the primary deposition sites for precipitation and airborne particulate matter of all types. Trees develop very large canopies of leaves and branches that extend high into the air. Thus, forests and range plants provide an extremely large surface for deposition and assimilation of both beneficial nutrient elements and injurious substances dispersed in the atmosphere. Direct injury to vegetation is most likely when a particularly vulnerable life form is exposed at a particularly vulnerable life stage, and is growing in a poorly buffered environment during a season of the year when acid precipita- tion is most likely. For example, a tender young plant, at the earliest stage of reproduction, growing on a poorly buffered sandy soil, during a heavy spring rain is especially vulnerable to acid rain. Both herbaceous and perennial plants are subject to changes in atmospheric deposition within a given growing season. In addition, perennial shrubs and trees live in the same environment for many years or even decades. As a result, they are subject to very long-term changes in the chemistry of the air and precipi- tation. Air-borne substances that influence terrestrial plants include sea spray from oceans and large lakes; dust resulting from wind erosion of soil as well as from volcanic and cosmic sources; gases such as CO2, NH3, SOy, H2S, CH4, released from decomposing organic matter and volcanoes; biogenic particles such as spores, hyphal frag- ments, bacteria, and pollen; particulate matter, aerosols, and gases produced by wild fires and controlled burning of agricultural, forest and urban wastes as well as from industrial, agricul- tural residential and commercial heating, and transportation operations (Tam 1958). The effect of acid precipitation on plants is only one facet of the much larger subject of atmospheric/plant/soil interactions. Acidity in precipitation should be understood as a reflection not only of the amounts of substances yielding hydrogen ions (such as sulfuric, nitric, hydrochloric, and organic acids) but also of the total balance between all the cations and anions dissolved in precipitation. These major anions and cations include W-, NH4+, NO3-,
SO4= and many others including K+, Na+, Ca*, Mg*,
C03-, Cl-, and PO^. Rain and snow change in chemical composition within, as well as between, precipitation events. In cold climates, acid substances accumulate in the snowpack where they are released in concen- trated form with the first melt water and thus For all of the above reasons, it is difficult to assess the effects of acids in rain or snow in isolation from the general chemistry of precipitation and atmospheric deposition. Also, the effects of a given "acid rain" or a prevailing condition of "acid rains" are very complex, variable in time, and involve significant inter- actions that are only partially understood. POSSIBLE DETRIMENTAL EFFECTS OF ACID PRECIPITATION ON VEGETATION A partial list of theoretical effects of acid precipitation on vegetation was developed earlier by T a m and Cowling (1977) and is reproduced in Table 1. The effects are classified as either direct or indirect, although most direct effects will have many indirect consequences as well.. A decreased rate of growth is the expected conse- quence of most of the effects postulated in Table 1 but unequivocal evidence of significant growth effects have yet to be demonstrated in forest or range ecosystems. Specific biological effects that have been proven to occur in at least one experimental plant are marked with an asterisk (*) in Table 1.
Many factors (i.e., genetic composition, biotic and abiotic stress factors, dose of pollutant, and pollutant combinations) affect the impact of acid precipitation and other pollutants on terrestrial plants and animals. Variation fn any one factor can result in variation in the nature and magni- tude of pollutant effects. This is shown simplis- tically in Figure 1. Previously, it was believed that the essential and potentially toxic elements listed above were taken up by plants almost entirely from the soil solution. Now, it is recognized that airborne gases, particulate matter, and aerosols signifi- cantly augment the supply of both essential and injurious elements. All of the substances -I
POLLUTANT
CONCENTRATION
NUMBER OF
EXPOSURES
DURATION OF
DOSE -*ÑÑ'à EACH EXPOSURE
listed above can be taken up readily through foliar organs as well as by absorption from the soil solution (Tam 1958; Wittwer and Bukovac 1969). Much larger amounts of essential nutrients are required for sustained-yield agriculture than for sustained-yield range management or hardwood or softwood forestry. This is true in rangelands because biomass yields are very low and in forests because the parts of trees that usually are harvested(thewood and bark of treestems) contain much less of most essential elements than the seeds and fruits that are commonly harvested in agriculture. This is a major reason why fertilization is so common in agriculture and so rare in forestry and range management. In some forested regions, atmos- pheric deposition alone is more than adequate to permit harvesting of crop after crop of trees without fertilizing the forest. This is much less likely to remain so, as more and more of the nutrient-rich branches, foliage, and roots of trees are harvested in so-called "whole-tree chipping" 2nd other modern harvesting practices. Some scientists believe that acid rain and snow are deposited directly onto soils where acid substances can be neutralized in well- buffered soils or by applications of lime. This is true on some agricultural lands, especially after harvest, but is not true in forests, rangelands, or even on most agricultural lands during the growing season. Most raindrops are intercepted by the foliage of plants where sub- stances dissolved in rain can induce various physiological changes before reaching the soil (see Table 1). In a mature forest, for example, rain will wash over at least three tiers of foliage before reaching the soil. SOURCES, AMOUNTS, AND DISTANCES OF TRANSPORT OF BENEFICIAL AND INJURIOUS SUBSTANCES IN THE ATMOSPHERE Forest, range, and aquatic biologists are becoming increasingly concerned about atmospheric transport and deposition of both nutritionally beneficial and potentially injurious substances for three major reasons:
EDAPH IC FACTORS Ñ^
(1) vegetation, soils, and surface waters are the primary deposition sites for precipi- tation and airborne particulate matter; MECHANISM OF ACTION
7
ACUTE
CHRONIC
(2) atmospheric deposition constitutes an important source of nutrients and poten- tially injurious substances that affect the productivity and stability of agricul- tural, forest, and aquatic ecosystems; and SUBTLE
Figure 1. Conceptual model of factors involved in air pollution effects on vegetation (Heck and others 1977). (3) human activities are steadily increasing the amounts and variety of substances dispersed in the atmosphere (Oden 1968; Bolin and others 1972). Table I - - P o t e n t i a l E f f e c t s of Acid
P r e c i p i t a t i o n on T e r r e s t r i a l Vegetation
DIRECT EFFECTS
*1. Damage t o p r o t e c t i v e s u r f a c e s t r u c t u r e s such a s c u t i c l e . Damage t o s u r f a c e s t r u c t u r e s
may occur due t o a c c e l e r a t e d e r o s i o n of t h e c u t i c u l a r l a y e r t h a t p r o t e c t s most f o l i a r organs.
It a l s o could r e s u l t from d i r e c t i n j u r y t o s u r f a c e c e l l s by high c o n c e n t r a t i o n s of s u l f u r i c
a c i d and o t h e r harmful substances t h a t a r e concentrated by evaporation o r adherence of s o o t
p a r t i c l e s on p l a n t s u r f a c e s .
*2.
I n t e r f e r e n c e w i t h normal f u n c t i o n s of guard c e l l s . Malfunction of guard c e l l s w i l l l e a d
t o l o s s of c o n t r o l of stomata and t h u s a l t e r e d r a t e s of t r a n s p i r a t i o n and gas-exchange p r o c e s s e s
and p o s s i b l y i n c r e a s e d s u s c e p t i b i l i t y t o p e n e t r a t i o n by l e a f - a t t a c k i n g p l a n t pathogens.
*3. Poisoning of p l a n t c e l l s a f t e r d i f f u s i o n of a c i d i c substances through stomata o r c u t i c l e .
This could l e a d t o development of n e c r o t i c o r senescent s p o t s on f o l i a r organs i n c l u d i n g
l e a v e s , f l o w e r s , t w i g s , and branches.
*4. Disturbance of normal metabolism o r growth processes without n e c r o s i s of p l a n t c e l l s .
Such d i s t u r b a n c e may l e a d t o decreased p h o t o s y n t h e t i c e f f i c i e n c y , a l t e r e d r e s p i r a t o r y p a t t e r n s
and i n t e r m e d i a r y m e t a b o l i s m , a s w e l l a s abnormal development o r premature senescence of l e a v e s
o r o t h e r organs.
*5. A l t e r a t i o n of l e a f - and root-exudation processes. Such a l t e r a t i o n s may l e a d t o changes
i n p o p u l a t i o n s of l e a f - s u r f a c e and root-surface microorganisms, i n c l u d i n g n i t r o g e n - f i x i n g
organisms.
*6.
I n t e r f e r e n c e w i t h reproduction processes.
Such i n t e r f e r e n c e may be achieved by d e c r e a s i n g
t h e v i a b i l i t y of p o l l e n , i n t e r f e r e n c e w i t h f e r t i l i z a t i o n , decreased f r u i t o r seed production,
decreased g e r m i n a b i l i t y of s e e d s , e t c .
7. S y n e r g i s t i c i n t e r a c t i o n w i t h o t h e r environmental s t r e s s f a c t o r s . Such r e i n f o r c i n g i n t e r a c t i o n s may occur w i t h gaseous s u l f u r d i o x i d e , ozone, f l u o r i d e , s o o t p a r t i c l e s , and o t h e r a i r
p o l l u t a n t s a s w e l l a s drought, f l o o d i n g , e t c .
INDIRECT EFFECTS
*l. Accelerated l e a c h i n g of substances from f o l i a r organs. Damage t o c u t i c l e and s u r f a c e
c e l l s may l e a d t o a c c e l e r a t e d l e a c h i n g of m i n e r a l elements and o r g a n i c substances from l e a v e s ,
twigs, branches, and s t e m s .
2. I n c r e a s e d s u s c e p t i b i l i t y t o drought and o t h e r environmental s t r e s s f a c t o r s . Erosion of
c u t i c l e , i n t e r f e r e n c e w i t h normal f u n c t i o n i n g of guard c e l l s , and d i r e c t i n j u r y t o s u r f a c e
c e l l s may l e a d t o i n c r e a s e d e v a p o t r a n s p i r a t i o n from f o l i a r organs and v u l n e r a b i l i t y t o drought,
a i r p o l l u t a n t s , and o t h e r environmental s t r e s s f a c t o r s .
* 3 . A l t e r a t i o n of symbiotic a s s o c i a t i o n s . Changes i n l e a f - and root-exudation processes and
a c c e l e r a t e d l e a c h i n g of o r g a n i c and i n o r g a n i c substances from p l a n t s may a f f e c t t h e formation,
development, balance, and f u n c t i o n of symbiotic a s s o c i a t i o n s , such a s mycorrhizae, nitrogenf i x i n g organisms, l i c h e n s , e t c .
*4. A l t e r a t i o n of h o s t - p a r a s i t e i n t e r a c t i o n s . Resistance and/or s u s c e p t i b i l i t y t o b i o t i c
pathogens, p a r a s i t e s , and i n s e c t s may be a l t e r e d by s u b j e c t i n g p l a n t s t o any environmental
stress. Acid p r e c i p i t a t i o n may i n c r e a s e t h e s u s c e p t i b i l i t y of p l a n t s t o t h e s e i n j u r i o u s
a g e n t s , a l t e r t h e i r c a p a c i t y t o t o l e r a t e d i s e a s e o r i n j u r y , o r a l t e r t h e v i r u l e n c e of pathogens.
The e f f e c t s of a c i d i c p r e c i p i t a t i o n may vary w i t h t h e following: t h e n a t u r e of t h e pathogen
involved (whether a fungus, bacterium, mycoplasma, v i r u s , nematode, p a r a s i t i c seed p l a n t ,
i n s e c t , o r multiple-pathogen complex); t h e s p e c i e s , age and p h y s i o l o g i c a l s t a t u s of t h e h o s t ;
and t h e s t a g e i n t h e d i s e a s e c y c l e i n which t h e a c i d i c s t r e s s i s appled, f o r example, a c i d i c
r a i n might d e c r e a s e t h e i n f e c t i v e c a p a c i t y of b a c t e r i a b e f o r e i n f e c t i o n and i n c r e a s e t h e
s u s c e p t i b i l i t y of t h e h o s t t o d i s e a s e development a f t e r i n f e c t i o n .
Source:
Tamm and Cowling, 1977
Recent increases in the deposition of sub- stances on terrestrial vegetation are due mainly to increases in combustion of fossil fuels in industrial enterprises, residential heating, transportation, and agricultural operations. Previously, it was believed that most of these materials fell out of the atmosphere near the site of emission. Now it is recognized, particu- larly with increased use of tall stacks at power plants, that atmospheric processes can lead to extensive mixing and both chemical and physical interactions and transformations of atmospheric particles, aerosols, and gases. Furthermore, these substances and their reaction products are dispersed by meteorological processes and finally are deposited on vegetation or soils as much as several hundreds of kilometers from the original sources of emission. The recent fallout of radioactive materials in the eastern United States as the result of atomic explosions in the Peoples Republic of China provides a dramatic reminder of the long-distance transport and deposition of pollutants. The amounts of substances introduced delib- erately or inadvertently by man are becoming so large that man is becoming a major force in the biogeochemistry of the earth (Kovda 1975). This is shown in table 2 which contains a tabulation of data on annual outpuG of fertilizers, indus- trial dusts, garbage and other urban wastes and by-products, mine refuse, and discharges of aerosols and gases mainly from the combustion of fossil fuels. All these categories of matter are becoming comparable in magnitude to the discharges of dissolved and suspended substances in all the rivers of the world, the annual yield of photosynthetic products, or the cycling of inorganic elements in the earth as a whole. Man-made emissions into the atmosphere are also Table 2--Biogeochemical and Technological Forces in the Biosphere of the Earth Biosphere Components
Biogeochemical processes: Global yield of photosynthesis Cycle of inorganic elements River discharges: Dissolved substances Suspended substances Tons/Year:.
very large, as shown in table 3. Most gases, carbon oxides, and aerosols result from the com- bustion of fossil fuels. A very large part of these global emissions are produced in the United States. If the United States continues to add to the amount of substances dispersed i n the atmosphere and deposited into the biosphere of the earth, it is essential that we measure the amount and chemical form of the deposited matter and understand the biological consequences of that deposition. Regret-
tably our understanding of these processes in the United States is very fragmentary. Fortunately, however, more extensive measurements of atmospheric deposition and its biological consequences have been made in Europe, where an atmospheric-deposition network has been maintained since the late 1940s (Oden 1968). The European Air Chemistry Network began in Sweden and has gradually spread to include most of western Europe and parts of eastern Europe, including Poland and the Soviet Union. Since the mid 19501s,a network of about 100 stations has made monthly measurements of changes in the chemistry of precipitation. The substances analyzed at most of these stations include the following major cations and anions: NH4, Na, Ca, K, Mg, 804, NO3, PO4, Cl as well as pH, conductivity, and titratable acidity and akla- Unity. These data have shown various long-term trends. for example, the amount of nitrate nitrogen in precipitation (an important fertilizer element) increased markedly in many parts of Europe during the fifteen years between 1955 and 1970. Nitrate nitrogen helps plants grow. Thus, the nitrogen added in precipitation probably increased yields of agricultural and forest crops. But not all the substances detected in precipi- tation were beneficial. Long-term trends of in- jurious sulfate and hydrogen ions also were detected from 1955-1970. The latter changes were attributed to strong acids formed in the atmosphere, mainly from oxides of sulfur and nitrogen produced during combustion of fossil fuels. More recent data show that thesetrends of Increasing acidity are Table 3--Anthropogenic Emissions into the Atmosphere
Types of Emissions
I~onsf~ear
I
Anthropogenic sources: Output of fertilizers Industrial dust Garbage, urban wastes and byproducts Mine refuse Aerosols and gas discharges Dust
Gases (mainly .SOg,HC and N O )
Carbon oxides (CO + C02)
Aerosols
Source: Kovda, 1975 Source: Kovda, 1975 Note:
2.5 x lo8
6.5 x lo8
2.0
lo9
1.0 x 109
Doubling about every 7-10 years continuing although t h e r e l a t i v e c o n t r i b u t i o n of
s u l f u r i c and n i t r i c a c i d s i s changing (Likens
1976).
CHANGES I N THE CHEMISTRY OF
PRECIPITATION I N THE UNITED STATES
Some monitoring of t h e chemistry of p r e c i p i t a t i o n has been c a r r i e d on i n t h e United S t a t e s
(Feth and o t h e r s 1964; Lodge and o t h e r s 1968).
Many of t h e s e s t u d i e s provide e x c e l l e n t and
r e l i a b l e information about t h e a c i d i t y of p r e c i p i t a t i o n . But most s t u d i e s i n t h i s country have
s u f f e r e d from t h r e e major shortcomings:
1.
The d a t a were c o l l e c t e d f o r a l i m i t e d land
a r e a ~ t y p i c a l l yonly a s i n g l e p o i n t o r a
few p o i n t s i n one o r two s t a t e s (Gambell
and F i s h e r 1966);
2.
The d a t a were c o l l e c t e d f o r very l i m i t e d
p e r i o d s of t i m e ~ t y p i c a l l yonly one o r
two y e a r s ;
3.
Very few d i r e c t measurements of a c i d i t y
have been made.
There i s only one l o c a t i o n i n t h e United
States~aH
t ubbard Brook Experiment F o r e s t i n
New H a m p s h i r e ~ w h e r et h e a c i d i t y of r a i n has been
measured d i r e c t l y and c o n s i s t e n t l y f o r a s long a s
10 y e a r s . The longest-term n a t i o n a l monitoring
program was o p e r a t e d by t h e U.S. P u b l i c Health
S e r v i c e f o r 6 y e a r s , from 1960 t o 1966 (Lodge
and o t h e r s 1968). These d a t a showed t h a t
p r e c i p i t a t i o n g e n e r a l l y i s a c i d i c e a s t and
g e n e r a l l y a l k a l i n e west of t h e M i s s i s s i p p i River,
t h e l a t t e r because of a l k a l i n e d u s t i n t h e a i r .
Using fragmentary b i t s of information,
o b t a i n e d i n d i r e c t l y and i n l i m i t e d a r e a s and
p e r i o d s of time, C o g b i l l and Likens (1974)
managed t o c a l c u l a t e t h e probable changes i n
average a c i d i t y of r a i n f a l l i n v a r i o u s p a r t s of
t h e e a s t e r n United S t a t e s from 1955-1973. ' P r e c i p i t a t i o n i n a l a r g e p o r t i o n of t h e e a s t e r n United
S t a t e s was l e s s than pH 5.6 i n 1955-56; t h e zone
of g r e a t e s t a c i d i t y (lowest pH) was g e n e r a l l y
c o n s i s t e n t w i t h t h e zone where s u l f u r emissions
were high--parts of Ohio, Pennsylvania, West
V i r g i n i a , New York, and New England. By 1973,
however, t h e a r e a w i t h an average pH of r a i n
below 4.5 had extended t o i n c l u d e p a r t s of
Missouri, Arkansas, M i s s i s s i p p i , Alabama,
Georgia, South C a r o l i n a , V i r g i n i a , Kentucky,
I l l i n o i s , Michigan, and f u r t h e r n o r t h i n t o New
England and Canada. E s s e n t i a l l y , i t embraces
most of t h e a r e a e a s t of t h e M i s s i s s i p p i River.
I n d i v i d u a l r a i n s t o r m s w i t h pH v a l u e s between 2.1
and 3.6 have been r e p o r t e d i n New York, I l l i n o i s ,
Indiana, New Hampshire, Massachusetts and North
C a r o l i n a ~ i nsome c a s e s many hundreds of k i l o meters from major s o u r c e s of a i r p o l l u t i o n
(Likens 1976).
The r e l a t i v e c o n t r i b u t i o n of s u l f a t e and
n i t r a t e t o t h e t o t a l a c i d i t y of p r e c i p i t a t i o n
a p p a r e n t l y changed markedly during t h e y e a r s s i n c e
A t Hubbard Brook, New Hampshire, t h e
1964-65.
r a t i o of s u l f a t e t o n i t r a t e changed from 83:15
i n 1964 t o 66:30 i n 1974. During t h i s same
decade, t h e t o t a l i n p u t of hydrogen i o n s i n c r e a s e d
by 36 p e t . Thus, most of t h i s i n c r e a s e appears
t o be due t o i n c r e a s e d d e p o s i t i o n of n i t r i c a c i d .
EFFECTS OF ACID PRECIPITATION ON
TERRESTRIAL ECOSYSTEMS
Cowling (1980a, 1980b) has r e c e n t l y completed
2 h i s t o r i c a l a n a l y s e s of p r o g r e s s i n s c i e n t i f i c
and p u b l i c understanding of a c i d p r e c i p i t a t i o n
S e v e r a l pub1i.and i t s b i o l o g i c a l consequences.
c a t i o n s a r e worthy of s p e c i a l n o t i c e i n t h i s
connection. The p i o n e e r i n g r e s e a r c h e s by Robert
Smith. E v i l l e Gorham and Svante Oden d e a l t w i t h
e f f e c t s on l a k e w a t e r s , a q u a t i c v e g e t a t i o n ,
t e r r e s t r i a l v e g e t a t i o n , andhuman h e a l t h (Smith
1872: Gorham 1958, 1976: Oden 1968). I n 1971.
Bolin and h i s co-workers completed t h e Swedish
Case Study Contribution t o t h e United Nations'
Conference on t h e Human Environment (Bolin and
o t h e r s 1972). I n 1972, t h r e e Norwegian r e s e a r c h
organizations established a s p e c i a l research
p r o j e c t c a l l e d Acid P r e c i p i t a t i o n : E f f e c t s on
F o r e s t s and Fish, w i t h an annual budget of
10,000,000 Norwegian kroner (U. S. $2,000,000).
The f i r s t I n t e r n a t i o n a l Conference on Acid
P r e c i p i t a t i o n and t h e F o r e s t Ecosystem was h e l d
a t Ohio S t a t e U n i v e r s i t y a t Columbus i n May of
1975 (Dochinger-and S e l i g a 1976a). I n June of
1976, an I n t e r n a t i o n a l Conference on E f f e c t s of
Acid P r e c i p i t a t i o n was h e l d a t Telemark, Norway,
and t h e major papers assembled f o r t h i s meeting
published by Braekke (1976) and i n a s p e c i a l
i s s u e of Ambio (1976). I n November, 1976, Gene
Likens published h i s summary r e p o r t i n Chemical
and Engineering News (Likens 1976). I n May. 1978,
a NATO Advanced Research I n s t i t u t e on E c o l o g i c a l
E f f e c t s of Acid P r e c i p i t a t i o n was held a t
Toronto, Canada (Hutchinson and Havas 1980). In
September, 1978, t h e C e n t r a l E l e c t r i c i t y Generating
Board i n England and t h e E l e c t r i c Power Research
I n s t i t u t e i n t h e United S t a t e s sponsored an
i n t e r n a t i o n a l symposium on t h e b i o l o g i c a l e f f e c t s
of a c i d p r e c i p i t a t i o n (Howells 1979). I n March,
1980, t h e Norwegian s p e c i a l p r o j e c t on a c i d
p r e c i p i t a t i o n sponsored an I n t e r n a t i o n a l Conference
on E f f e c t s of Acid P r e c i p i t a t i o n i n Sandefjord,
Norway (SNSF 1980).
The e f f e c t s of a c i d p r e c i p i t a t i o n on t e r r e s t r i a l
ecosystems g e n e r a l l y have been l e s s w e l l documented
than t h o s e on populations of f r e s h w a t e r f i s h and
o t h e r a q u a t i c organisms (Ambio 1976; Braekke 1976).
Nevertheless, c e r t a i n d e f i n i t e e f f e c t s have been
r e p o r t e d . The most s t r i k i n g of t h e s e e f f e c t s was
t h e development of p e a t moss (Sphagnum s p . ) a s a
submarine, r a t h e r than a t e r r e s t r i a l p l a n t i n
a c i d i f i e d l a k e s and streams i n Sweden. Dense
mats of Sphagnum and heavy f e l t s of a l g a e develop
on t h e bottom of t h e s e l a k e s i n water a s deep a s
1 8 m. This growth i s r e p o r t e d by Grahn and
o t h e r s (1974) t o induce o l i g o t r o p h i c a t i o n
(opposite of eutrophication)--a s e l f - a c c e l e r a t i n g
process that leads to a substantial nutrient impoverishment of lake waters. Analyses of forest growth in southern Sweden from 1896 to 1965 showed a 2 to 7 percent decrease in growth between 1950 and 1965. Johnsson and Sundberg (1972) "found no good reason for attrib- uting [this] reduction in growth to any cause other than acidification." Similar attempts to quantify possible effects on growth of forests in the United States have been inconclusive. Both direct and indirect damage to crops and forests have been reported by various investiga- tors in laboratory, greenhouse, and field experi- ments in which synthetic rain equivalent in chemical composition and rate of deposition to natural rains has been applied. The biological effects recorded in these experiments include the fol3.owing (Cowling 1980c) :
--Induction of necrotic lesions on foliage; --Loss of nutrients due to leaching from leaves and other foliar organs; --Predisposition of plants to infection by bacterial and fungal pathogens; --Accelerated erosion of waxes on leaf surfaces; --Inhibition of nodulation of legumes leading to decreased fixation of nitrogen by symbiotic bacteria; and --Reduced rates of decomposition of leaf litter leading to decreased mineralization of organically-bound nutrients. Abrahamsen (1980) has recently summarized many years of research showing both positive and negative effects of acid precipitation on forest growth. He concludes with the following general statements: "Apart from possible direct effects of acid precipitation on forest trees, the effects on forest growth can be considered a nutrition increased deposition of N and S can
problem
be regarded as a
fertilization effect, and
the increased leaching of nutrient cations
as an oligotrophication or acidification effect the general hypothesis that acid precipitation significantly will decrease forest production over large areas must be revalued. The deposition is likely to
of N and to some extent S
increase forest production. Reduced growth may be expected where or when nutrients like Mg and possibly K are the growth limiting elements." ...
...
...
...
...
RECENT INITIATIVES DEALING WITH ACID PRECIPITATION AND ITS BIOLOGICAL EFFECTS In 1975, the National Academy of Sciences' Committee on Atmospheric Sciences published its report on Atmospheric Chemistry: Problems and Scope (NAS 1975). Growing awareness of important influences of acid precipitation on fish popula- tions and potential effects on forest and crop plants led the U. S. Forest Service to sponsor the First International Symposium on Acid Precipi- tation and the Forest Ecosystem at Columbus, Ohio in May, 1975. The proceedings of this Symposium and the Associated Workshop Report were published by Dochinger and Seliga (1976a, 1976b). At Congressional hearings in July, 1975, Cowling (1976) testified on the inadequacy of research in the United States on Acid Precipitation and its biological consequences. Specifically, the lack of a coordinated program of research on ecological effects and lack of a stable monitoring network were recognized as primary causes of our profound ignorance of acid precipitation. In the spring of 1976, however, a cadre of scientists in various institutions and agencies throughout the United States began the process of creating the National Atmospheric Deposition Program (NADP) to meet these two critical needs (Kennedy 1977; Galloway and Cowling 1978). In the fall of 1977; the President's Council on Environmental Quality contracted with the NADP for the drafting of "A National Program for Assessing the Problem of Atmospheric Deposition (Acid Rain)." This publication (Galloway and others 1978) provided the basis for a Presidential Initiative on acid precipitation which President Carter announced on August 2, 1979 in his Second Environmental Message (Carter 1979). This initiative calls for a 10-year long, $10,000,000 per year program of research on the causes and consequences of acid precipitation. A standing Acid Rain Coordinating Committee was established by the President to plan and manage the program. Leadership for the Committee is provided by co-chairmen from the Department of Agriculture and the Environmental Protection Agency. At the present time, the Acid Rain Coordinating Committee is drafting a coherent program of research on atmospheric chemistry and transport, chemical and biological monitoring, ecological and materials-damage effects, economic assessments, and public-policy options for control of acid precipitation and/or amelioration of its ecological effects. Wetstone (1980) has recently summarized the biological and materials-damage effects of acid precipitation in relation to the pollution- control laws in North America. In conclusion, the Presidential Initiative on Acid Precipitation, coupled with growing Congres- sional, public, and private-industrial interest in acid precipitation research, provide a basis for increasing hope that the United States will do its part, together with Sweden, Norway, England, Canada and other nations, to meet the challenge of continuing economic development with adequate safeguards for the quality of life and the long- term productivity of ecosystems on which our good life critically depends. LITERATURE CITED Abrahamsen, G. 1980. Acid precipitation, plant nutrients and forest growth. In Proc. Int. Conf. on Ecological impacts of Acid Precipitation [Sandefjord, Norway]. Ambio.
1976. Report of t h e I n t e r n a t i o n a l Conference on
t h e e f f e c t s of a c i d p r e c i p i t a t i o n i n Telemark,
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