,
\
leES e.M. 1991
M.E.Q.emttee
Paper E: 17, Ref. I
On eutrophication problems in the Baltic Sea causes and effects
•
Sigurd Schulz and Dietwart Nehring
Institute of Marine Research, 0-2530
Warn~münde,
Germany
1. Abstract
Eutrophication has
developed to a
global
problem which in-
creasingly affects coastal and semi-enclosed seas.
In recent decades oxygen concentrations, salinity, and temperature have decreased
in the deepwater of
Baltic Proper.
The
winter nutrient concentrations in the surface layer, which are
characterized by strong fluctuations
4t
in recent
times
show an
the
nutrient
increase in the long run.
As
expected
levels, also
from
the
long-term
increase
in
increasing tendencies could be observed for some
pelagic biological variables. ehanges have been detected even in
other compartments of the ecosystem.
The results are discussed from the ecological point of view.
The assessment of the present state of the Baltic Sea reflects
the complicated combination of natural changes and man induced
effects in a sensitive brackish environment.
1
,
,
2. Introduction
The BaI tic
Sea is
a semi -encl osed basin wi th a
restricted
horizontal water exchange. Due to its positive water balance
which is caused by the large drainage area, the Baltic belongs
to the greatest brackish seas in the world. One of its most
important features is the permanent halocline restricting the
vertical water exchange and dividing the water column in a surface layer with the lower salinity and the more saline deepwater. Below this discontinuity layer, stagnant conditions prevail in central BaI tic deep waters causing .. oxygen.consumption
and the formation of hydrogen sulphide.
•
Episodic.influxes of
highly saline water from theNorth Sea renew.the.stagnant deep
water and improve the oxygen
condition~. These m~jor inflows are
connected with certain meteorological and hydrographie conditions,
e.g.
longer periods of strong westerly winds or gales
generating a sea level difference between the Skagerrak and the
western BalticSea.
Since
surface
the Baltic Sea lies in the humid climatic zone,
layer is characterized by seasonal
hydrographie
conditions,
the
nutrient
variations
distribution,
its
in the
and
the
development of the biota .. Due to the heating in spring and summer a thermoclinc develops dividing the water column into thc
warm surface layer and the cold intermediate water. This discontinuity layer disappears in the cold season.
The Baltic Sea represents a unique brackish water environment
wi th a long residence time of wa ter cal cul a ted wi th about 30
years.
It is a geological young sea and exists in the present
state only ab out 5000 years. Therefore, SEGERSTRALE (1957) has
called it not yet "matured" what might partly explain the high
variability in the system . The fauna and flora is composed.~by
euryhal ine organismens wi th a high ecological capaci ty. Nev:~'r­
theless they live under physiological stress conditions which
2
,;.,<"
•
result e.g. in lower diversity and generally in smaller size of
the organismns compared with oceanic areas. All this
features
make the system very sensitive against natural changes and pollution.
On the
Research,
basis
of
data
obtained by the
Rostock-Warnemünde,
changes
which are related to the eutrophication
the present paper.
Effects
on
Insti tute
in
the
proce~s
the other
of
Marine
pelagic
system
are discussed in
compartments of
the
ecosystem are compiled from the literature. In Fig. 1 the different areas of the Baltic Sea are shown .
•
3. Long-term nutrient variations
Monitoring data are available for phosphate covering a thirty
year's period and for nitrate covering a twenty year's period in
the Baltic Sea. With respect to eutrophication, nutrient trends
in the surface layer are of high priority. Trend studies in this
layer are restricted to the period of low biological activity,
that me ans in winter and early spring, as long as light limits
the phytoplankton development.
Fig. 2 shows thatthe winter
ni trate are
•
concentration~
increasing on average
in the
of phosphate and
whol e period
under
investigation. The sub-trends indicate, however, that the positi ve
overall-trends
period 1969 -
1977.
mainl y
resul t
In recent
from
times,
the
increase
in
the
nutrient concentrations
show strong variations, but do no longer increase on average.
Theresults explained for stations in the eastern Gotland Sea
are also valid for other sub-regions of
the Baltic Sea Area
(NEHRING, MATTHÄUS, 1990, HELCOM, 1990).
Long-term trends for both phosphate and ni trate were al so
identified in central Baltic deep waters below the permanent
halocline, as long
as conditions are oxic. Whereas the nitrate
3
sub-trends do not differ very much from the positive overalltrend shown in Fig. 3, the phosphate accumul~tion rate is decreasing in recent times.
The situation is more complicated in the near-bottom water
layer characteri:ed by alternating oxic and anoxie conditions.
Due to denitrification nitrate concentrations decrease to zero
during the transition from oxic to anoxie conditions. On the
other hand phosphate is remobilized from the sediments in the
prescnce of hydrogen sulphide.
The rer:!ent stagnation period beginning in 1977 ü; the most
serious one that has been observed in the Gotland Deep up to
now. As in Fig. 4 shown, this period is characterized by the
strong increase of phosphate and hydrogen sulphide concentrations.The increase of the last aneis reflected in the high
negative oxygen equivalents. The recent salinity in central
Baltic deep water is the lowest one that has been measured since
the beginning of regul ar hydrographi c ohserva ti ons ab out 100
years ago.
•
4. Long term changes in pelagic biological variables
Although the available biological data cover a shorter time span
than the chemical data, even remarkable tendencies might be discerned. For this assessment only the summer data from four different areas in the Baltic Sea have been used. In this season
the system approaches astate of balance and shows therefore the
lowest variabilty compared to other seasons (SCHULZ, 1985). In
Fig. 5 the chlorophyll content is depicted as an equivalent for
the phytoplankton biomass. Although the median values show a
high degree of variabilty, in all areas an increasing tendency
is visible. Without the last three years, which exhibit a clear
decline, the increase would be statistical1y significant for all
areas. Fig. 6 contains the data on in situ primary production.
4
4t
In this ease the interannual variability is also very high, but
the rising tendeney is even present. Cal eul ating the 1inear
regression, the inerease was signifieant for all areas. The
zooplankton biomass (Fig. 7) shows only for the last years a
pronouneed inerease, whereas in the years before onl y minor
ehanges eould be diseerned.
5. Diseussion
•
•
The above mentioned ehanges in the pelagie~yst.em are obvi ous 1y cl osel y rel at ed. As eoul d be expeeted, "an inerease of
nutrients should improve the growth of at least phytoplankton
(SCHULZ, et al., 1985). But in most eases the nutrional eonditions improved also for the following biologieal eommunities in
the food ehain. Phosphorus and nitrogen eompounds originating
from natural and anthropogenie sources are the basis for this
process and the reason for an increasing biologieal produetion,
whieh regard to higher fish production, a positive effeet. The
deterioration of the oxygen eonditions in stagnant deep waters
eonneeting also wi th eutrophieation is however a serious and
negative effeet of this proeess.
The nutrients originate mainly from munieipal, agrieultural,
and industrial sourees. They reaeh the sea by direet waste water
input, by ri ver discharge and by air-borne transport. Mass
balanees for the Baltie Sea also ineluding the wet and dry deposition from the atmosphere yield a gross input of 60 000-80 000
t phosphorus and 800 000 - 1 200 000 t nitrogen eompounds per
year (MAXIMOVA, 1982, NEHRING, 1982, LARSON et al., 1985). These
balanees are very rough beeause the quantitative statements are
uneertain for some sources and sinks.
o
5
".
~
The air-borne deposition is roughly ealeulated to be 500 000
t inorganie nitrogen eompounds, whereas the atmospheric phosphate deposition is insignifieant. Generally the air-borne input
of nitrogen eompounds seems to beeom~ more, iMportant in reeent
deeades. Time seri~s have shown a ste~dy inerease of nitrate and
ammonium eoneentrations in atmospherie ~'1et deposi tions over
eentral Sweden (SEPA, 1990).
After the period with the strong inerease between 1969 and
1977, the winter eoneentrations of phosphate and nitrate remain,
on average, at their high levels in the surfaee layer of the
Baltie Proper, but are eharaeterized by stronger fluetuations in
recent times. Onere~son for the sometimec lower winter eoncentrations might be that, due to the earlier start of the spring
bloom, the nutrient reserves were already eon~umed to a eertain
extent by the phytoplankton before the m~asurements have been
performed. Especiall y in the Bornholm Basin and the southern
Gotland Sea, the bloom started very early after the last mild
winters.
If 'the inerease at least for the phytoplankton variables is
still ongoing, several reasons should be responsible for this.
At'first the nutrient input aets along the whole year and eannot
,
only be eonsidered from the winter values alone. The winter pool
should mainly influenee the spring production, whieh for problems in timing of the observations and variability in the eommeneement of the bloom eould not be ineluded in this assessment.
In this paper the summer values are used. It is weIl known that
the rivers not in winter but in early'summer transport the largest fright of water and subsequent.ly nutrients to the sea.
These nutrients should then enhanee the summer produetion.
During this time the utilization of the nutrients is mueh more
intensi ve than in spring beeause the pel agie eeosystem ap!
proaehes the sum~erly stage with higher turnover rates of the
nutrients compared with the other seasons of the.year (LEPPÄNEN,
1988). Also the air-borne input of nitrogen eompounds should
cumulate in the warm season.
6
..
4t
•
•
In the discussion on eutrophication in the BaI tic Sea, the
summerly blooms of the bluegreens and their contribution to the
nutrient cycle by fixing molecular nitrogen play an irnportant
part. Although their functioning especially with respect to the
nutrient remineralization is not yet fully understood, one can
expect that they are decayed under summer condi tions in the
surface layer (HORSTMANN, 1975, SCHULZ, 1985). In every case the
amount of phosphate left in the pelagic system after the spring
bloom is decisive .
other environmcntal signals might'~ause effects on the biological productivity and thenutrient ,distribution as weIl. It
is weIl known that an increase in temperature activates metabolic processes and can thus enhance the productivity and the
nutrient cycles.
Additionally, periodic cycles of 3, 6 - 7 and 9 - 12 years and
even longer are reported by KALEIS and OJAVEER (1990) and
TRZOSINSKA (1990) for the Baltic Sea. These cycles are attributed to changes in the atmospheric circulation creating variations in the temperature regime as weIl as in the freshwater
discharge. These variations are also reflected in nutrient
cycl es producing periods of decreasing and increasing winter
concentrations.
One of the natural responses of the ecosystem to get rid of
increasing nitrate concentrations is denitrifikation. This process counteracts eutrophication in the Baltic Sea. Budget studies account for a nitrogen 10ss of 470 000 t/a (RöNNER, 1985).
This is roughly 90 % of the total input from the catchment arca
and SO %, when the atmospheric deposition is also included.
The'present analysis for the three biological variables in the
peri6d 1976 - 1990 reveal beside the high degree of variability
also some tendencies in relation to the progressing time. The
course of the variables confirms in principle the increasing
7
tendency of previous analysis for the years 1975-1983 (SCHULZ et
al., 1985, SCHULZ, KAISER, 1986). This increase is tor the three
parameters differently pronounced and not over the whole time
in~rease
apparent. In the case of chlorphyll the
1987,
wheras
in cas e
of
primary product i on
reversed after
the
t endency
was
clearly supported by the steaper rise after the low values of
the years 1986/1987. The zooplankton biomass curve with the high
values in the last years compensated the deep just before.
In a previous paper (SCHULZ, KAISER, 1986) and the 3ubsequent
assessment
1987),
(HELCOM,
primarily
the
nutrient supply and possibly oceanological
increase
in
the
pecularities of the
•
Baltic ecosystem as e.g. the natural leaning towards stagnation,
the nitrogen input by bluegreens etc.
were
considered as pos-
sible causes for the rising trends of the biological variables.
The positive effect of nutrients to the phytoplankton productivity could be proofed experimentally in mesocosms also for the
Baltic (SCHULZ
1985).
et a1.,
In the
experiments
it. could be
documented that aft.er a nutrient input not only the produetivity
of the algae but also the duration of the bloom could be prolonged. This has, however, an important ecological implication,
because it diminished the lap in the time scaJes between phytoand
zooplankton
development
in
spring.
This
me ans
energy is stored in the surface layer and can
that
i~prove
more
the pro-
duction eonditions also in summer.
Beside our results from the pelagic system,
which are con-
firmed by other authors for different areas of the Baltic (HELCOM, 1990),
signs
for
the
eutrophi ation
process
are
reported for other compartments of the Baltic ecosystem.
the
permanent
haI oel ine
an
increase
of
benthos
also
Above
hiomass
was
reported and related to the elevated pelagic productivity (HELCOM, 1985, 1990). Below the halocline a clear decrease was documented because of the oxygen decline.
Another
eutrophication is
of some benthic red and
the
disappearance
8
consequence of
•
brown algae in shallow coastal
areas and a shift to floating
..
filamenteous plants. Partly the nutrient load but also the in-
creasing turbidity due to the higher phytoplankton production is
responsibl e for this devel opment. In this way, spawning substrate and nursery areas· for fish larvae and young fish are
lost.
Eutrophication might also be one reason for the appearance of
"exceptional phytoplankton blooms". An example is the Chrysochromulina polylepis bloom observed in the Kattegat, Skagerrak,
and northern North Sea~ in May 1988 (NIELSEN, RICHARDSON, 1990,
HORSTMANN , JOCHEM, 1990) causing among others fish losses in
mari
cul tures
(BORN' et
al .,
1990).
oceanographic condi tions. (doubl e
Because
layering in
of
the
di f ferent
summer,
thermo-
cline is not nutricline) in the Baltic, the probability of such
bloom events canbe excluded for the open Baltic waters. A feat ure of the BaI ti c, however, is the mass occurance of bl uegreens. They might also form toxic events under certain circumstances (HELCOM, 1990).
Remarkable changes have also taken place in the fish
stock~
of
the ecosystem. Total catches of the most important commercial
fish species herring, sprat, and cod increased in the Baltic Sea
from 450 000 t in 1965 to 900 000 t
in 1980 according to ICES
statistics. This is partly due to the increasing fishing effort
4t
but also induced by improved nutritional conditions for young
and adult fish and subsequently higher growth rates. This could
be proved for young herring (RECHLIN, 1984). The mentioned fish
stocks are ,however,in nutrional relation e.g. the clupeids are
food for cod. Variations in one stocks change automatically the
balance. The decrease in cod, as observed in recent times, influences the clupeid stocks generally positively. On the other
hand,' the environmental conditions do not only influence the
. stock size
but also
the recruitment success.
9
The decrease in
this respect especiaIIy for cod is not only caused by the unfavourable oxygen condition but also by the extremely low salinity
in Baltic deep waters injuring the fertilization of the cod eggs
(WESTIN, NISSLING, 1991).
The recent nutrient pool in the Baltic Sea and the resulting
organie produetion and sedimentation of organic matter burden
the oxygen conditions near the bottom. In late summer· and
autumn, this process is sometimes strenghened by the low oxygen
eoneentrations of the inf 1 owing deep/.water. Hydrogen sulphide
eoneentrations are inereasing in eentral BaI tic deep waten~.
Fig. 8 shows the areas which arevery often affeeted by lOH'
oxygen eoncentrations and anoxie conditions in the near- bottom
water layer.
Oeeasionally, anoxie eonditions are.also observed in the relatively shallow Mecklenburg, Lübeck, and Kiel Bays as weIl as in
the southern Kattegat. Even when the deep Hater eontains oxygen,
the transition zone between water and sediment is often anoxie
in late summer
and early autumn.
The deterioration of the
oxygen eonditions restriets the living space of benthie organisms including demersal fish.
•
5. Conelusions
The present status of the Baltic Sea indieates that eutrophieation and its consequenees are proofable for all compartments
of the ecosystem. The reaction of the biota to this process is
an increase in the productivity of th~ organisms living in the
weIl aereted surtace water. No clear ehanges have been identified in the species eomposition up to now. The opposite development must be stated tor the communities living in the deeper
parts·of the Baltic Sea. Below the halocline serious impoverishment at the speeies level and deerease in biomass has taken
place due to the deterioration of the'oxygen eonditions; ·This is
valid expecially for the benthie community.
10
..
•
The causes for the eutrophication are primarily the continuation of nutrient inputs into the system. However, the special
oceanological condi tions in the BaI tic characterized by the
restricted horizontal and vertical water exchange should not be
disregarded in this respect. Furtheron the climatological changes taking place, especially the global heating, must· be considered.
Thus, the changes observed in the Baltic Se3 are the combination of different causes. This makes the decisions finding very
difficult and uncertain. It is, however, a weIl proofed fact
that the man induced nutrient input into the sea creates at
least a considerable part of the problems. The decision of the
Baltic States to reduce the input of pollutants to 50 % until
1995 is therefore a good message for the Baltic ecosystem.
6. Literature
•
BOKN, T., BERGE, J.A., GREEN, N., RYGG, G., 1990. Invasions of
the planktic algae Chrysochromulina polylepis along south
Norway in May - June 1988: Acute effects on biological
communities along the coast. - Commission of the European
Communities. Water PoIl. Res. Rep. 12, 183 - 193 .
HELCOM, 1987: First periodic assessment of the state of the
marine environment of the Baltic Sea Area, 1980-1985.
Background Document. - Baltic Sea Environ. Proceed. 17 B:
1 - 351.
HELCOM, 1987. First Baltic Sea pollution load
Baltic Sea Environ. Proceed. 20, 1 - 53.
compilation.
HELCOM, 1990. Second periodic assessment of the state of the
marine envi ronment of the BaI ti c Sea Area, 1984 - 1988.
Background Document. - Baltic Sea Environ. Proceed. 35 B,
1 - 428.
11
HORSTMANN, U., 1975: Eutrophication and mass production of bluegreen algae in the Baltic. - Havsforskningsinst. Skr. 239:
83 - 90.
HORSTMANN,
first
bloom
Water
U., JOCHEM, F., 1990. Report of the activities and
results of the investigations on the Chrysochromulina
in FRG. - Commission of the European Communi ties.
PoIl. Res. Rep. 12, 75 - 92.
LARSSON, U., ELMGREN, R., WULFF, F., 1985. Eutrophication and
the Baltic Sea - causes and consequences. - Ambio 14, 9-14.
LEPPÄNEN, J. -M., 1988: Carbon and ni trogen cyc1 es during the
vernal growth period in the open northern Baltic proper. Meri 16: 1 - 118.
•
MAXIMOVA, R.M., 1982. The balance of biogenie elements and
organic matter in
the Baltic
Sea during
intensive
anthropogenie effects. - Okeanologija ~2, 751 - 756 (in
Russian) .
SCHULZ, S., 1985: Ergebnisse ökologischer
pelagischen ökosystem der Ostsee.
Rostock-Warnemünde: 1 - 185.
Untersuchungen im
Prornot. B-Schr.,
SCHULZ, S. and W. KAISER, 1986: Increasing trends in plankton
variables in the Baltic Sea - A further sign of eutrophication? - Ophelia, Suppl. 4: 249 - 257.
SEGERSTRALE, S., 1957:
67: 751 - 800.
Baltic
Sea.- Mem.
Geol.
Soc.
America
•
12
•
Fi 1· /(
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Fig. 2
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Fig. 4
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76 77 7S 00 81 82 83 84 85 86 87 88 89 90
SOTLAHD SEA
(X l00)r----r--r-......-...,.--.--.....--....--,.-,r--r--r--.----.--....-":;(X l00).----....-r--r-~...,.-.....-.....--....--,........,,...-.,---r--.--.---..
..
I
'U
M
-
.g
29
25
-
28
r1 B Ii -
15
• 16N
~ 12
u•
8
1I
4
E
- B
9
~
r t1 B~
t
-
9
10
5
76 77 78 B8 81 82 83 84 85 86 87 S8 89 90
YEM
The long-term variability of summer primary production values
-2 -1)
(mg C.m.d
in different areas of the Baltic Sea
..
~
•
ARKlHl SEA
/'ECKl..EHIllJl6 BllifT
21-
2
1.6 -
1.6
l-
1.2 -
1.2
~
n
I
e
e
I
...e
9.8 9.4
I-
91-
E1~gB~allBagQg~+
9.8
-
9.4
-
9
76 77 7S 80 81 82 83 84 85 86 87 88 89 90
n
76
BOOHHOUI SEA
2
2
1.6
1.6
-~
e 1.2
I
•
8.8
9.4
QeDB$~-e~
0
9
89 99
-
1.2 I8.8
f-
9.4
I-
B
-
~
9
76 Tl 7S Be 81 82 83 84 85 86 87 88 89 90
YEAR
es
OOMlll SEA
t'I
I
...E
7S 00 81 82 83 84 85 86 87
76
n
78 Be 81 82 83 84 85 86 87 88 89 99
YEAR
+:'s· +
The variability of mesozooplankton biomass (ml.m- J ) for the
surface layer (0 - 25 m) in the different areas of the Baltic
Sea
59
59·
H 2 S:
Mai 1982
mm3
Mai u. Ok I.INov. 1982
1111111
Ok1.lNov.1982
2cm 3 /dm 3 :
M~u.Okt/No~1982
0'
OkUNov.1982
S7"
Fig. 8
Areas with low oxygen concentrations and hydrogen sulphide
in the near-bottom water layer
57"
e
•