Rapp. P.-v. Réun. Cons. int. Explor. Mer, 190: 198-205. 1989

Eutrophication and fishery resources in the Baltic

Dietwart Nehring, Sigurd Schulz, and Otto Rechlin

Nehring, Dietwart, Schulz, Sigurd, and Rechlin. Otto. 1989. Eutrophication and fishery resources in the Baltic. — Rapp. P.-v. Réun. Cons. int. Explor. Mer, 190: 198-205.

Winter concentrations o f phosphate and nitrate have significantly increased, on average, in the surface layer o f the Baltic Proper since the late 1960s. Increasing nutrient concentrations have also been observed in all other subregions of the Baltic Sea Area. Phosphate and nitrate concentrations are also increasing in oxic Baltic deep waters below the halocline. In the temporarily stagnant bottom water with alternating oxic and anoxic conditions, this long-term trend is masked by phosphate liberation from-the sediments and by denitrification. Positive trends for chlorophyll and primary productivity were obtained analysing summer values (1975-1983) of some pelagic biological determinants in several Subareas o f the Baltic. These trends were, however, not statistically significant in all cases. Zooplankton biomass only significantly increased in the Mecklenburg Bay, whereas the trend was indifferent in the other parts. From 1973 to 1984, fish species composition changed distinctly in favour of the development o f the predator cod, while the total biomass o f herring, sprat, and cod, estimated by means o f V PA for Subdivisions 22-29, decreased by 7-3%. A n augmentation o f total fish biomass might be estimated at about 5% for this period, taking into account the enlarged catches and the proportion o f fish eaten by the increased cod population. Herring stocks and the Baltic sprat showed a significant increase in growth at the same time.

D. Nehring, S. Schulz, and O. Rechlin: Institut fü r M eereskunde der A kadem ie der Wissenschaften der D D R , Seestraße 15, D D R -2 5 3 0 Rostock-W arnem ünde, and Institut für Hochseefischerei und Fischverarbeitung, A n der Jägerbäk 2, D D R -2510 R ostock 5, German Dem ocratic Republic.

Introduction

The development of fish stocks in the Baltic depends to a large extent on hydrographic conditions, eutrophication, and exploitation, whereas harmful substances have, up till now, played an insignificant part or are only of regional significance.

The most important hydrographical factors are water temperature, salinity and oxygen concentration. Eutrophication includes the whole biogeochemical cycle beginning with the increase of nutrients, followed by higher phyto- and zooplankton production and better food supply of fish stocks, and ending with the deterioration of oxygen conditions below the permanent Baltic halocline as a consequence of oxygen consumption by bacterial degradation of the increasing amount of organic matter. Whereas variations in salinity and water temperatures have only natural causes, eutrophication and therefore also oxygen conditions are influenced by man. The exploitation of fish stocks depends mainly on human activities, while sea-birds and seals are of lower significance.

In this paper relationships between eutrophication and fishery resources in the Baltic are studied. The

influence of salinity and water temperatures on the food chain and thus on the development of fish stocks is also taken into consideration. This contribution continues earlier investigations on the same subject (Nehring et al., 1984).

Long-term nutrient variations

Inorganic phosphorus and nitrogen compounds are the most important nutrients limiting algal growth in the Baltic. Phosphate and nitrate are the final products of aerobic microbial nutrient mineralization of organic matter. The concentrations of these nutrients were used in the following investigations.

Increasing nutrient concentrations have been observed in the surface layer of the Baltic Proper (Nehring et al., 1984; Nehring, 1984) and the Western Baltic Sea (Nehring and Francke, 1985) in winter, when biological productivity is restricted by low light intensity. An example is shown in Figure 1 for the Central Baltic, using phosphate data for the period 1958-1983 and nitrate data for the period 1965-1983 from the southeastern Gotland Sea (for a map of the station, see

198

• Sta t . 15 A

o S ta t . 11 B

a Stat. 10 B

0.6 -

0.5-

Sta t . BCS n i 10

S ta t . 8A0.4-

0.3-

0.2-

0. 1 - J a n . - Apr.

0.0-1978 198319731963 19681958

6.0-r

jjmol/l

5.0-

4.0-

3.0-

2 .0 -

J a n . - A p r .

0.0-1978 1983197319681965

Figure 1. Long-term variations in nutrients and salinity in the surface layer of the southeastern Gotland Sea in winter.

8.0-

7 .5 - 'J a n . - A p r.

• 6,9 7 )1958 1963 1978 19831968 1973

Nehring et al. (1984) and Nehring (1984)). As the Figure demonstrates, phosphate concentrations increased, on average, in the period under investigation, but the accumulation was more pronounced between 1969 and1975 than during the preceding period, when the trend was statistically insignificant. Nitrate concentration also increased continuously since 1969, the beginning of comparable nitrate measurements.

Overall trends of phosphate and nitrate concentrations in the winter surface layer are interrupted by a period with a decreasing or steady tendency between1976 and 1980, followed by renewed increase up to 1983. The salinity (and density) shows the same behaviour as the nutrients (Fig. 1). This connection is studied in more detail in Figure 2. The close correlations identified were restricted to the Baltic Proper.

Increasing winter concentrations of nutrients in the surface layer have also been observed in all other Subregions of the Baltic Sea Area (Figs. 3 ,4; Nehring, 1987a). The increase was highest in the Gulf of Finland and the

• Sta t . 15A

° Sta t . 11 B

» S ta t . 10 B

» S ta t . 9 A ■ S ta t . 8 A

• S ta t . BCS m 100.8 -

P 0 43‘ - pmol/l

0.6 -

0.4-

0. 2 -

0 . 0 —I-------------- 1---------------- 1--------------- 1---------------- o .o -

7.5 8.0 S%o

Figure 2. Correlations between nutrients and salinity in the surface layer of the southeastern Gotland Sea in winter.

8.0 s%

199

0.050x[

0,060

0,098

0.019“ ^ ^ ^

wvlZZZZZZZZZZZWinter s u r f a c e l a y e r

0,0260,034

ao2 t"V///////r,

0,015

0,022

0,005 V 7

0, 0 0 2 [

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P r o b a b i l i t y of e r r o r ( t - t e s t I: * « 5 % ; ’0 ‘« 1 % i 0,1%

Kattegat E 1 9 7 5 - 1984

The Sound 197 5 - 1 9 8 4

Great Belt 197 5 - 1984

Fehmarn Be l t 19 75 - 1 9 8 4

Mecklen burg Big ht 1971 - 1 9 8 4

Arko na Sea 19 6 4 - 1 9 8 3

Bornholm Sea 1 9 5 8 - 1 9 8 3

Gotland Sea SE 1 9 5 8 - 1983

Gdansk Deep 1961 - 1983

A l a n d S e a 21 1 9 6 3 - 1984 19 67 - 1984

B o t h n i a n S e a 21 1 9 6 9 - 1 9 8 4 1 9 6 7 - 1 9 8 4

B o t h n i a n Bay21 1 9 7 0 - 1984 1 9 6 9 - 1984

Gulf of F in land W 19 7 4 - 1 9 8 3

Gulf of F in lan d E 1 9 7 5 - 1983

Gulf of Riga 1978 - 1983

Figure 3. Mean year-to-year increase o f phosphate and total phosphorus in the surface layer of the Baltic Sea Area in winter.

W in te r s u r fa c e la ye r

I I N 0 30.2 pm ol • drr

r e JZL

P r o b a b i l i t y o f e r r o r ( t - t e s t ) : * « 5 % ; *x < 1 % ; xx

PO*

198319761973196819631958

Figure 5. Long-term variations o f nutrients at 100 m depth in1978 198319731968

Kattegat, and lowest in the Gulf of Bothnia (HELCOM , 1987). Positive phosphate and nitrate trends were also detected in Baltic deep waters below the halocline (Nehring etal. , 1984; Nehring, 1984; HELCOM , 1987), as shown in Figure 5 for the 100 m depth in the Gotland Deep. They are restricted, in general, to depths with permanent oxic conditions. While the phosphate trend is positive during the whole period under investigation, nitrate concentrations have decreased, on average, since 1978-1979 due to unfavourable oxygen conditions in the deep water and denitrification in the anoxic transition layer during this period.

Alternating oxic and anoxic conditions masked longterm trends in temporarily stagnant deep waters (Nehring, 1987b), because phosphate is liberated from the sediments, when hydrogen sulphide is formed, whereas nitrate is reduced by denitrification to nitrogen gas during the anoxic transition.

Plankton production

The positive nutrient trends also imply increasing activity of phytoplankton as well as of other plants, following Liebig’s Law, and might result in improved nutritional levels in other biological compartments of the ecosystem. Analysis of biological data from the Western Baltic and the Baltic Proper for the summers 1975-1983 (Schulz and Kaiser, 1986) showed, for phytoplankton biomass (Fig. 6, expressed as chlorophyll) for all areas, significant positive trends. The trend for phytoplankton productivity (Fig. 7) also increased for the parts of the Baltic investigated, but was statistically significant only for the Arkona Sea.

100-, Chl.-am gm

5 0 -

MB

5 0 -

AS

5 0 -

BS

5 0 -

GS

1975 76 77 78

Figure 6. Trends (linear regression) for chlorophyll. The points represent summer mean values for the years. The horizontal bars indicate the mean values for each 4-year-period with 5% confidence limits. MB = Mecklenburg Bight, A S = Arkona Sea, BS = Bornholm Sea, GS = Gotland Sea.

201

gC

MB

AS

BS

GS

1975 76 77 78

Figure 7. Trends and mean values o f primary production (for explanation see Fig. 6).

Results of zooplankton analysis (Fig. 8) were indifferent. The trend in the Bornholm Sea decreased but showed a slight increase in other areas. The changes, however, were not statistically significant.

Table 1. Developm ent of fish biomasses in Subdivisions 2 2 - 29S/32, t x 10“3.

Species

Year Herring Sprat Cod “F ood ”8 Total

1973 2 881-0 1719-4 792-9 792-9 6 186-21974 2 862-3 1 435-3 887-2 887-2 6072-0

1975 2 801-8 1 487-3 956-0 956-0 6201-11976 2 723-2 1 319-5 981-8 981-8 6006-3

1977 2 699-2 1 065-2 1007-7 1 007-7 5 779-8

1978 2 589-1 806-4 1 163-5 1163-5 5 722-51979 2 476-6 506-1 1 420-9 1 420-9 5 824-5

1980 2 631-2 487-7 1564-4 1 564-4 6 247-7

1981 2 691-7 455-3 1 601-6 1681-6 6 510-21982 2 445-2 638-0 1 667-6 1 667-6 6 410-41983 2 441-6 800-3 1 614-7 1614-7 6 471-31984 2 529-4 847-1 1 564-6 1 564-6 6 505-7

a Rough estimate o f sprat and herring biomass eaten by cod and not included in the biomasses of both species.

2-1 ZPBml m'

MB

AS

BS

GS

1975 76 77 78

Figure 8. Trends and mean values o f zooplankton biomass (for explanation see Fig. 6).

Fishery resources

The development of Baltic fish stocks has been investigated by ICES Working Groups on Assessment of Baltic Stocks since the early 1970s. Stock assessments (A non., 1986,1987a and b) and fishery statistics (Anon., 1982, 1983, 1987a and b) enable estimates of total biomasses of the main commercial fish species: herring, sprat, and cod, for an area comprising Subdivisions 22 to 29S and 32. The stock biomasses are assessed by Working Groups on the basis of catches and by means of Virtual Population Analysis. The assessment, however, only partly takes into consideration the predation of cod on herring and sprat.

In Table 1 and Figure 9, the stock biomasses and catches by species are summarized. In addition a biomass of “food” has been added to account for the amount of herring and sprat yearly consumed by the cod stock. This “food biomass” has been roughly estimated to be in the same order of magnitude as the cod stock itself as derived from results of multispecies assessments (Anon., 1987c). The biomasses and catches of herring taken for the Western Baltic do not include herring older than two years because adult herring from this area are known to migrate from the Baltic for feeding.

202

biomass catch

i r1960

Figure 9. Developm ent of fish stock biomass in the Baltic Proper and Western Baltic. TB: total biomass, PB: herring and sprat biomass, CB: cod biomass, SB: sprat biomass, TC: total catch, year before.

The “food biomass” is added once in TB, (Fig. 1) and twice in TB2 to stock biomasses and to catches by species as being estimated for fish yearly consumed by the cod stock.

In the period under investigation, total fish biomass showed an increasing trend that was interrupted from1976 to 1979. Cod biomass increased steadily over the whole period whereas pelagic fish biomass distinctly decreased, mainly by a continuous decrease of sprat biomass until 1979. Earlier investigations (Rechlin and G roth, 1979; Wrzesinski, 1983; Rechlin, 1984) showed a distinctly increasing growth rate of sprat and herring in several areas of the Baltic during the 1970s. The increased growth rates can partly be interpreted as a result of improved feeding conditions.

A possible positive relationship of eutrophication to fish growth should be traceable, especially in the growth of plankton feeding fish. Such a relationship, however, is complicated and might be masked by a variety of influences.

Discussion

The main reasons for eutrophication in the Baltic are human activities (Larsson etal. , 1985). In Danish marine waters, the winter concentrations of total inorganic nitrogen mainly resulting from agriculture has been shown to be proportional to drainage from Denmark and Sweden (HELCOM , 1987). Mass balances including land-based discharges and airborne decompositions, ship-generated wastes and fishery, nitrogen fixation and exchange between bottom and sea water, yield a significant gross input and net supply (difference between input and output) into the Baltic. Mass balances of different origins revealed an annual gross input of

62 000-77 0001 phosphorus, 800 000-1 200 0001 nitrogen, and an annual net supply of 26 000-55 0001 phosphorus and about 530 0001 nitrogen into the Baltic (HELCOM , 1987). These figures are only rough estimates because quantitative statements are uncertain for some sources and sinks.

Microbial denitrification probably balances the supply of nitrogen compounds into the Baltic Proper (Gun- dersen, 1981; Shaffer and Rönner, 1984; Christensen et al., 1987), whereas no comparable process exists for phosphate other than transient deposition in the sediments.

Although human activities are the main reason for eutrophication in the Baltic, natural changes cannot be excluded. As already shown by Nehring et al. (1984), anthropogenic influences on the salinity (density) can now be excluded. Therefore, the close correlations between increasing nutrient concentrations and the salinity found in the winter surface layer of the Baltic Proper indicate the increasing exchange across the halocline by hydrographic processes. In connection with naturally caused eutrophication, the so-called Seen- alterung (Thienemann, 1955) should also be considered as an important process for this brackish sea.

From our results and other sources (Wulff et al., 1986), biological effects of eutrophication are obvious in the pelagic ecosystem of the Baltic. The trends, however, are diversely pronounced in different compartments and areas. This is mainly due to methodological uncertainties, e.g. to low measuring frequency, but it also depends on the ecological function of the compartment considered. The clearest results are therefore obtained in phytoplankton. Here, the increasing winter nutrient pool acts directly and causes higher phytoplankton productivity and consequently its biomass. An increase in biomass parallel to that of productivity could not be expected, because phytoplankton biomass is kept down by zooplankton.

In ecological experiments with enclosed plankton communities, it could be stated that higher nutrient concentrations resulted in higher phytoplankton production and biomass (Schulz et al. , 1985). Besides these findings, a prolongation of the productive season was observed. This result seems ecologically important for the plankton community, especially in spring by lowering the gap in the time scales of phyto- and zooplankton and retaining more energy in the pelagic system towards the summer.

The trends measured in zooplankton are weaker and did not significantly support the idea of eutrophication in the system. This might simply be due to the fact that zooplankton development depends on a higher number of environmental factors; for example, low summer temperatures alone may lead to a low zooplankton stock, although enough phytoplankton as a food source is present (Kostrichkina, 1977). On the other hand, a higher stock of pelagic fish (herring and sprat), as shown

203

for this period (Schulz, 1985), could suppress the zooplankton biomass. Although this seems to be likely, the speculation cannot be proved, because no real production data are available for mesozooplankton. Besides fish, jellyfish (Aurelia aurita) might also decrease the zooplankton biomass (Möller, 1980; Shus- kina, 1984, pers. comm.).

Eutrophication effects are not only proved for the pelagic system but also for the benthic community in the Baltic. Revisiting HESSLE-stations from the 20s, Cederwall and Elmgren (1980) could show that the biomass above the permanent halocline increased by a factor of 4, whereas below the halocline a clear decrease was visible. A n increase was also reported by Gosselck (1985) for the benthos of the Arkona Sea. In both cases, eutrophication is mentioned as the cause for the changes.

The fish biomasses assessed species by species for the Western Baltic and the Baltic Proper show, in total, a decrease for the period 1973-1984. Analysis of the results of ICES Assessment Working Groups shows distinct changes of species composition of total fish biomass for this period. A constant increase of the biomass of the ; predator species cod has been accompanied by a remarkable decrease of one main prey species. The increasing predation of cod on sprat and herring has not, until now, been fully considered in Working G roups’ assessments of sprat and herring biomasses.

For the time being it is impossible to estimate with more precision the amount of fish consumed by cod. But according to results of the Multispecies Working Group, it might be estimated at about twice the cod biomass. If predation by cod has been partly taken into account in the assessment of sprat and herring biomasses, it might be appropriate to take the total fish stock biomass with index 1 (Fig. 9) as an estimate. This results in an increase of total fish biomass by about 5% for the period 1973-1984.

The development of fish biomass might be interpreted as a hitherto positively acting effect of the eutrophication on fish biomasses, also taking into account increasing growth rates observed for sprat and herring during these years. Further results of multispecies assessment of fish stocks may perhaps give more information on these connections than the rough estimates based on single species assessments. Recent observations of decreasing growth rates of herring show, however, that eutrophication is not the single factor influencing changes in fish stock biomasses. There is some evidence that the low water temperature in winter and also in summer has been the dominating factor during the years 1985-1987, acting directly on migration, spawning behaviour, and the metabolism of fish species, or indirectly via the food chain.

Matthäus (1979, 1983) studied long-term trends of oxygen in Central Baltic deep waters. The oxygen con

centrations beneath the permanent halocline have significantly decreased since the beginning of this century. This decrease became more pronounced in recent decades, since microbial decomposition of organic material, which is increasingly produced in the euphotic layer and sediments across the halocline or is of anthropogenic origin, consumes oxygen in the stagnant deep water. Deterioration of oxygen conditions and the more frequent occurrence of hydrogen sulphide restricts tem porarily the living space of fish and other aerobic organisms including the benthic community.

Regardless of whether human activities or natural causes predominate, eutrophication is becoming a serious problem in the Baltic Sea Area. Consequences expected by the increasing nutrient pool are higher biological production and deterioration of oxygen conditions. The Baltic is very sensitive to eutrophication because of its morphology and resulting oceanological features. Therefore the countries bordering this semienclosed basin should make great efforts to protect the Baltic ecosystem against irreversible damage.

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