4. Water Chemistry – Dissolved Oxygen, Phosphorus and Trophic State
Draft Guiding Question: comedian Johnny Carson described Lake Erie as “the place where fish
go to die”. In 1969 an NBC documentary declared the entire lake dead. The scientific
community indicted phosphorus discharges, aided and abetted by the thermal stratification
process, as the perpetrators of the crime. Describe the interaction between nutrient loads, the
mixing regime and the needs of aquatic organisms that forms the basis for this indictment.
A comprehensive examination of topics relating to Great Lakes water chemistry could
easily occupy hundreds of pages.
These systems exhibit seasonally and spatially diverse
chemistries, attracting the attention of environmental scientists for decades and yielding a
body of knowledge supporting a dedicated outlet, the Journal of Great Lakes Research.
Management of the lakes with respect to water chemistry, however, focuses on two topic
areas: the persistent bioaccumulative toxins treated in Section 9 and the phosphorus – algae –
oxygen dynamic examined here.
Among the general rules established under the Clean Water Act (formally, the 1972
amendments to the Federal Water Pollution Control Act; see Section 10), is that
‘fishable/swimmable’ conditions are to be achieved for all of the Nation’s waters. Fundamental
to such conditions is the maintenance of adequate concentrations of dissolved oxygen. Absent
consumptive chemical or biological processes, levels of oxygen in lakes remain in equilibrium
with the essentially constant partial pressure of the gas in the atmosphere and concentrations
are said to be at saturation. This relationship is described by Henry’s Law where dissolved
oxygen concentrations increase as water temperature decreases due to the impact of
temperature on the Henry’s Law coefficient (equilibrium constant).
For the range of
temperatures typically observed in Great Lakes waters (0-25 C), saturation oxygen
concentrations range from approximately 8-14 mg·L-1. Biological and chemical consumption of
1
oxygen in lakes is balanced by reaeration, i.e. transfer of the gas across the air-water interface.
The rate of reaeration increases with temperature, turbulence at the air-water interface and
the degree of departure from saturation. In the spring and fall, when the water column in the
Great Lakes is well mixed, oxygen concentrations are near-saturation (~13 mg·L-1 at 4 C) from
surface to bottom. During summer stratification (see Section 3), mixing between well-aerated
surface waters (the epilimnion) and bottom waters (the hypolimnion) becomes limited.
Microbial consumption and respiration of particulate organic matter (largely algae) settling to
the lake bottom exerts a sediment oxygen demand (SOD; see Section 7), the magnitude of
which is proportional to the degree to which the lake receives fertilizing nutrients. In Lake
Superior, where nutrient levels are low and consumptive processes are negligible, oxygen levels
remain near saturation and the summer depth profile tracks that of temperature (Figure 4.1a).
More highly enriched systems such as Lake Erie and Green Bay in Lake Michigan harbor organicrich sediments which exert a significant demand on oxygen resources. In this case, bottom
water oxygen resources are consumed over the summer with little replenishment from surface
waters (limited vertical mixing) and hypoxia (oxygen < 2 ppm) or anoxia (absence of oxygen)
may result (Figure 4.1b). Hypoxia in Lake Erie covers much of the system’s central basin (Figure
4.2), an area roughly equivalent to the combined size of Delaware and Rhode Island. Re-supply
of oxygen to bottom waters occurs at turnover. Oxygen depletion under the ice in winter, a
common feature of ponds and shallow, inland lakes is not an important phenomenon in the
Great Lakes.
The Great Lakes Water Quality Agreement (see Section 10) states that oxygen in
hypolimnetic waters should not be less that that necessary for the support of fish life and
2
specifically calls for restoration of the required levels of oxygen to the bottom waters of the
central basin of Lake Erie. However, the engineered approaches used to address the problem
in ponds and small inland lakes, increasing oxygen input through mechanical mixing or
hypolimnetic aeration, are not feasible for systems the size of the Great Lakes. Thus, attention
must turn to reducing the degree of anthropogenic impact on ecosystem processes resulting in
oxygen consumption. In his classic text on Surface Water Quality Modeling, Dr. Steven C.
Chapra introduces the concept of slow and fast response times (eigenvalues) for the water and
sediment components of lakes. Supported by rates of flushing and reaction typically much
faster than those of the sediments, the water column will respond to changes in the input of
fertilizing materials more rapidly. Sediments, containing a substantial fraction of the materials
discharged to and produced within lakes over decades, will require an extended period to reach
equilibrium with the ‘remediated’ water column lying above them. The first step in initiating
that response is to implement controls on sources of the fertilizing nutrients stimulating algal
growth and attendant accumulation of organic matter in the sediment.
Like all plants, algae require light, a temperature consistent with their growth optimum
and a suite of nutrients to support growth. The requirement for some nutrients is a significant
fraction of the total plant biomass (macronutrients, e.g. N, P and K) and for others much less so
(micronutrients, e.g. Fe, Mn, Zn). Leibig’s Law of the Minimum (Figure 4.3), formulated and
popularized in the 19th Century, states that plant growth will be is limited by the nutrient
present in the smallest amount relative its requirement.
In the Great Lakes, this growth-
limiting nutrient is phosphorus. As phosphorus is added to the lakes, algal growth (water level
in Figure 4.3) increases until the next nutrient, present in the least amount relative to the needs
3
of the algae, becomes limiting. Phosphorus-rich waters will exhibit a drawdown in nutrients
such as silicon and nitrate over the growing (Figure 4.4), however this does not result in a
limitation on production of total algal biomass. Rather, shifts in the algal species contributing
to the phytoplankton community occur, with diatoms declining in abundance as silica is
depleted and nitrogen-fixing cyanobacteria increasing in abundance as fixed inorganic nitrogen
( NH 3 , NO3 ) concentrations fall.
The degree to which a lake is supplied with nutrients is described by its trophic state
(from the Greek trophe or nourishment).
Trophic state categorized based on the TP
concentration and lakes of a particular trophic state share certain characteristics: oligotrophic poorly nourished, TP <10 ppb,
low levels of algae, high water clarity and abundant
hypolimnetic oxygen reserves; mesotrophic - moderately nourished, TP 10-20 ppb; and
eutrophic - well nourished, high levels of algae, poor water clarity and oxygen-depleted
hypolimnia, TP >20. It is generally assumed that all of the Great Lakes were oligotrophic
following the retreat of the glaciers (see Section 1) and that they are moving along a natural
continuum of nutrient enrichment (eutrophication) that will transform them into dry land in a
period of thousands of years. The rate of enrichment has, however, been vastly accelerated
through anthropogenic activity (cultural eutrophication), leading to a contemporary interest in
reducing nutrient levels and returning the lakes to a trophic state condition consistent with the
geology of their watersheds (see Section 1).
An extensive body of scientific study, including the dramatic divided lake experiment of
Dr. David W. Schindler (Figure 4.5), has clearly identified phosphorus as the driving force behind
excessive algal growth leading to oxygen depletion, nuisance growth of the attached alga
4
Cladophora and proliferation of harmful algal blooms (HABs; see Section 5). Phosphorus is thus
the appropriate focus for management. Work pursuant to the Great Lakes Water Quality
Agreement (see Section 10) established guidelines for in-lake total phosphorus concentrations
and trophic state (Table 4.1) and called for the allocation of total phosphorus loads to meet
these guidelines. A suite of mathematical models (e.g. Figure 4.6) were used to calculate the
target load which would yield the lake response called for in the guidelines. The major point
and nonpoint sources of phosphorus to the Great Lakes were identified (Table 4.1) and loading
reductions were sought by: (1) requiring all municipal waste treatment facilities discharging
more than one million gallons per day to the Great Lakes or waters tributary to the Great Lakes
to achieve an effluent TP concentration of 1 ppm, (2) placing restrictions on the phosphorus
content of detergents and (3) implementing phosphorus management plans for urban and
agricultural nonpoint sources.
The response to implementation of load reductions has, in some cases, been dramatic
(Table 4.2). For example, TP levels in Lake Ontario declined to one-third of their 1979s
concentrations, moving the trophic state from eutrophy to oligotrophy. TP in Lake Erie declined
by about one-half, a monumental achievement in this heavily loaded system, with trophic state
conditions in the eastern and central basins approaching mesotrophy. It is only in the shallow
water of Lake Erie’s western basin that eutrophic conditions persist.
Total phosphorus
concentrations in Lakes Superior, Michigan and Huron have declined slightly, maintaining the
target trophic state of oligotrophy.
The Great Lakes phosphorus story is, however, not yet fully told. For example, rather
than declining in response to changes in phosphorus loading, the areal extent of hypoxia in the
5
central basin of Lake Erie has recently expanded. And, the oligotrophication of Lake Ontario
has been decried by the sport fishing industry due to suspicion that attendant changes in food
web dynamics are unfavorable for the support of target recreational species, e.g. salmonids.
However, perhaps the most challenging issue lying before the Great Lakes scientific community
is to understand and incorporate in management plans the dramatic changes in patterns of
phosphorus cycling initiated through colonization by invasive zebra and quagga mussels. The
total phosphorus analyte consists of three fractions: soluble reactive P which is fully and
immediately available to algae and dissolved organic particulate phosphorus, fractions of which
may eventually be made available to algae through enzymatic hydrolysis. The particulate
fraction yields it phosphorus slowly and thus is transported out of the Great Lakes nearshore
and deposited in deep waters before it can be made available to algae (Figure 4.7a). Attached
algae (Cladophora) and phytoplankton (including cyanobacteria, HABs) then compete for the
remainder, i.e. the dissolved fraction. Filter-feeding mussels impact this cycle in two ways: first
by capturing particulate-P and releasing it to nearshore waters as dissolved-P and second by
consuming phytoplankton and recycling the P contained therein (Figure 4.7b). The effect of this
phosphorus shunt (as conceptualized by Dr. Robert E. Hecky) is to re-structure phosphorus
cycling in the Great Lakes nearshore in a manner that favors organisms which are not grazed
(e.g. HABs species and Cladophora). While an appropriate management response to this
change in the function of the Great Lakes has not yet been formulated, scientists and engineers
laying the groundwork for a proposed revision of the Great Lakes Water Quality Agreement
have recognized that nuisance conditions may be experienced in nearshore areas even as water
quality objectives are met in offshore waters.
6
Table 4.1. Top-ranked point and nonpoint source total phosphorus loads to the Great Lakes
(metric tons·yr-1) . Provided courtesy of D. M. Dolan, University of Wisconsin – Green Bay.
a. Nonpoint Sources
River
Load
Discharging To
1. Maumee River
2947
Lake Erie
2. Sandusky River
741
Lake Erie
3. Fox River
630
Lake Michigan
4. Cuyahoga River
586
Lake Erie
5. Thames River
533
Lake Erie
Discharger
Load
Discharging To
Detroit WWTP
662
Lake Erie
Toronto Metro WPCP
269
Lake Ontario
b. Point Sources
Birds Island (Buffalo) WWTP
Cleveland Metro WWTP
Rochester WWTP
161
157
145
Lake Ontario
Lake Erie
Lake Ontario
7
Table 4.2. Great Lakes phosphorus and trophic state management: targets and status.
Lake
TP Load (metric tons·yr-1)
TP Concentration (gP·L-1)
Trophic State (TP-based)
19761
Target1,2
20063
1970s4
Target1
2000s4
1970s5
Target1
2000s5
Superior
4212
3400
6512
3.8
5
2.8
O
O
O
Michigan
6700
5600
3037
5.9
7
3.9
O
O
O
Huron
5050
4360
3018
4.6
5
4.1
O
O
O
52.9
15
28.3
E
M
E
20.2
10
12.6
E
O-M
M
19.8
10
12.3
E
O-M
M
21.5
10
7.3
E
O-M
O
Erie
western
basin
Erie
central basin
Erie
eastern basin
20000
Ontario
11000
11000
7000
8924
5287
Trophic state abbreviations: O, oligotrophic; O-M, oligomesotrophic, M, mesotrophic; E, eutrophic
1Phosphorus
Management Strategies Task Force. 1980. Phosphorus Management for the Great Lakes. Final Report to the
International Joint Commission. Great Lakes Water Quality Board and Great Lakes Science Advisory Board, Windsor, ON. 125 pp.
2Great
Lakes Water quality Agreement, Annex 3
3Courtesy
4State
of D.M. Dolan, University of Wisconsin – Green Bay.
of the Lakes 2009.
5Based
on the trophic state classification of Chapra 1997.
8
Figure 4.1. Depth profiles of dissolved oxygen and temperature in (a) Lake Superior and (b) Lake
Erie during summer stratification. Bottom waters in Lake Superior have higher oxygen
concentrations than surface waters due to lower temperatures. Bottom waters in Lake Erie
have lower oxygen concentration than surface waters due to consumption of oxygen by the
sediments.
Figure 4.2. An illustration of the areal extent of hypoxia and anoxia in Lake Erie in 2005 (NOAA
GLERL).
12 10
8
6
4
2
0
Dissolved Oxygen (mg·L-1)
9
Figure 4.3. Liebig’s Law of the Minimum. Algal growth
(level of water in the barrel) rises only as high as the
shortest stave (limiting nutrient).
Increasing the
concentration of that nutrient (raising the stave height)
allows algal growth to increase to the level where it is
limited by the next nutrient least in supply with respect to
its growth requirement.
Figure 4.4. Elevated levels of phosphorus can cause concentrations of other nutrients to be
drawn down leading, in the case of silica to reductions in the number of silica-demanding
diatoms and in the case of nitrate to proliferation of nitrogen-fixing cyanobacteria. These data
from Lake Ontario were collected by Dr. J.C. Makarewicz of SUNY Brockport.
0.6
Lake Ontario
0.6
0.4
silica
(2004)
0.4
0.2
nitrate
(2006)
0.2
0.0
Silica (mgSiO2·L-1)
Nitrate (mgN·L-1)
0.8
0.0
M
J
J
A
S
O
N
Figure 4.5.
Lake 226, the divided lake, in Canada’s
Experimental Lake Area was separated into two parts with a
curtain. Carbon and nitrogen were added to one side (top)
and carbon, nitrogen and phosphorus to the other (bottom).
The algal bloom that developed in the side receiving
phosphorus helped confirm that this nutrient was limiting in
freshwaters.
10
Figure 4.6. Mathematical models for total phosphorus were used to set target loads for the Great Lakes. This output from a mass
balance or budget model developed by Steven C. Chapra and William C. Sonzogni provides an example of such a tool.
11
Figure 4.7. The filtering activity of mussels served to modify phosphorus cycling in the nearshore waters of the Great Lakes.
Particulate phosphorus originating from the watershed and delivered largely unutilized to offshore waters and dissolved phosphorus
sequestered by phytoplankton was captured by mussels and released in soluble form through this nearshore phosphorus shunt.
Pre-mussels
phytoplankton
transport and
deepwater
deposition
Post-mussels
phytoplankton
Cladophora
watershed
dissolved P
mussels
Cladophora
watershed
particulate P
watershed
dissolved P
watershed
particulate P
12