Bioaccumulation of heavy metals and organochlorines in a lake ecosystem with special reference to bream (Abramis brama L.)

the Science of the Total Environment &. t m . . m e ~ m

ELSEVIER

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The Science of the Total Environment 155 (1994) 187-197

Bioaccumulation of heavy metals and organochlorines in a lake ecosystem with special reference to bream ( Abramis brama L.) W o l f g a n g S c h a r e n b e r g *a, P e t r a G r a m a n n a, W e r n e r H. P f e i f f e r b alnstitut flir Toxikologie, Brunswiker Str. 10, D-24105 Kdel, Germany blnstitut s~tirHydrobiologie und Fischereiwissenschaft, Olbersweg 24, D-22767 Hamburg, Germany Received 29 November 1993; accepted 23 March 1994

Abstract

Homogenates of bream from a lake in N. Germany, caught during 1989-1991, were analyzed for heavy metals and organochlorine residues. In general, fish from this lake are relatively low in contamination in comparison with fish from other sites, hence this lake can be regarded as a 'reference lake'. The animals showed no obvious damage caused by anthropogenic chemicals although negative effects for the whole population, especially for synergistic effects, could not be excluded. By comparison of residues with official limit values, bream can at present be used for food. Furthermore, we investigated some samples from pike, roach and plankton. No biomagnification could be detected for heavy metals (except mercury). In relation to lipid concentration there might be a biomagnification for organochlorines; this effect is not measurable on the basis of dry weight. Bream are good indicators for the contamination status of freshwater lakes.

Keywords: Bioaccumulation; Fish; Heavy metals; Organochlorines

1. Introduction One of the central questions in ecosystem research is: how do systems develop? This question cannot be answered without considering the flux of energy and materials. Environmental contaminants cannot be disregarded if we consider the flux of material because chemicals might strongly influence the development of ecosystems up to the elimination of species. There is considerable

* Corresponding author.

information concerning environmental chemicals in contaminated areas, but less is known about uncontaminated 'reference' areas. This investigation deals with the question of the contamination status and the flux of contaminants in a limnic food chain of a relatively uncontaminated area; we have investigated prey as a link between plankton and predatory fish. This area was chosen because of the relatively small anthropogenic influence. Lake Belau is an eutrophic lake; the pH value during 1989 was 7.6-9.3 (30 cm below the surface; Schernewski and Fr~inzle, 1991). In this area heavy metal con-

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W.. Scharenberg et al. / Sci. Total Enbiron. 155 (1994) 187-197

188

centrations in plants and air pollution are relatively low compared with other places in Schleswig-Holstein (Aul3enthal, 1990; Jensen-Hug and Gewerbeaufsichtsamt, 1990; Kirchhoff and Rudolph, 1990). Little is known about the pollution with chlorinated hydrocarbons in this environment. Instead of organochlorine pesticides, other pesticides such as triazines and pyrethroids have been used in this area for agricultural purposes. Bream are one of three species which together make up 80% of the fish population of Lake Belau. It is considered a representative fish indicator for this lake. Bream feed predominantly on zooplankton (with a smaller part of mosquito larvae), whereas pike feed on fish (Pfeiffer and P~c, 1990). It was the aim of this investigation to present the contamination with heavy metals and organochlorines of edible fish, in comparison with plankton and piscivores, and to document the seasonal variation. The question to be answered was whether bream are good bioindicators for this lake or not. 2. Materials

and

methods

We analyzed tissue samples of 95 bream

(Abramis brama; 1989-91), 7 pike (Esox lucius; March 1992), 4 European roach (Rutilus rutilus; March 1993) as well as zooplankton (250 /zm; March 1993) of the Lake Belau (Germany; 10.20 E, 54.10 N). The average weights of fish were: bream, 323 g; pike, 1118 g; European roach, 417 g. Until preparation fish were frozen at -25°C, weighed after defrosting and homogenized. Plankton were filtered immediately under low vacuum with washed paper filters and then weighed. All samples were dried at 90°C until constant weight.

2.1. Heavy metals For the analysis of cadmium (Cd), copper (Cu), lead (Pb), mercury (Hg) and zinc (Zn) 200 mg dry sample were dissolved in concentrated nitric acid (1 ml) using acid pressure decomposition in teflon vessels (190°C, 4 h). For the Hg analysis we added 8.5 ml distilled water and 0.4 ml K2Cr20 7. For

analysis of the other metals the samples were evaporated to dryness (90°C) and then dissolved in nitric acid. Samples were analyzed by atomic absorption spectrometry: ---

--

--

graphite furnace: Perkin Elmer Zeeman 3030 with background correction (Pb) graphite furnace: Perkin Elmer 5100 with background correction (Cd, Cu) flame: Perkin Elmer 2100 (Zn) cold vapour: Perkin Elmer 1100B equipped with FIAS 400 (Hg).

Recovery and accuracy was determined by measuring blanks and standard reference material (Fa. Promochem; mussel tissue CRM 278 BCR). Detection limits ranged from (/zg/g dry wt.): Cd 0.002, Cu 0.03, Hg 0.05, Pb 0.1 and Zn 2.

2.2. Organochlorines (OC) Lipid extraction was carried out by Soxhlet extraction (8 h) with 150 ml n-hexane. Extracts were evaporated to dryness, and the lipid weight was determined gravimetrically. The lipid was dissolved in cyclohexane/ethylacetate (1:1), separated by gel-permeation chromatography and cleaned up by silica-gel chromatography as described by Specht and Tillkes (1985) (see also Scharenberg, 1991). Samples were analyzed for: ct-, /3-, y-HCB, HCB, octachlorostyrene (OCS), 4,4'-DDE, 4,4'-DDT, 4,4'-DDD, and PCB (IUPAC no.) 28, 52, 101, 138, 153, 180. Identification and quantification was the same as described by Scharenberg (1991). Because PCB no. 28, 101, 138, 153 can co-elute with other PCB congeners on a GC-column similar to SE 54, PCBs were separated by multidimensional gaschromatography as described by Duinker et al. (1988) (Scharenberg and Gramann in prep.). For purposes of comparison in this investigation only we present the 6 PCBs mentioned. We cross checked our results with an external standard and participated in a 'ring analysis'. Recoveries were > 85% and detection limits were: HCB, OCS = 0.020 mg/kg (lipid weight); DDT and metabolites, PCB = 0.015 mg/kg; H C H isomers = 0.010 mg/kg. Statistical analysis was performed using SAS.

W.. Scharenberg et al. / Sci. Total Encgron. 155 (1994) 187-197

The samples showed no normal distribution (Shapiro-Wilk test; Kolmogoroff-Smirnoff test; P < 0.05). Sex differences were tested by the X 2-test (Kruskal-Wallis) for heavy metals and by the Mann-Whitney U-test for organochlorines. Regression analysis was used to define the relation of residues to weight. By rank-correlation-coefficient we compared the metal residues for different months. For mathematical analysis we used the median, for the figures we used the mean. For statistical analysis values below detection limits were taken into consideration with half the value of the detection limit. 3. Results

Since no significant differences (P < 0.01) were found between the sexes, the samples from males and females were combined.

3.1. Heavy metals Residues in bream are shown in Table 1 and Figs. 1 and 2. The averages of different months show great variability. Values for the months with highest concentrations of metals (March or July 1990) were significantly different from values for months with lowest concentrations (May or September 1991; December 1990 for Pb) by factors up to 4.3 for Cd, 3.7 for Hg, and around 1.9 for Cu, Pb and Zn. Minimum and maximum of essential metals are much closer (factor 2.8 for Cu and 1.9 for Zn) than values of the non-essential metals (factor 14.6 for pb, 18.2 for Hg and 21.3 for Cd). Regression analysis showed that the weight did not influence the residues, although the average

189

values of Cd and weights were rather similar (Fig. 1). Two characteristics for the residues in pike should be noted: Pb in muscle and Hg in ovary were below the detection limits (Table 2). Concentrations of metals in muscle of pike were, in comparison with concentrations in bream, slightly lower for Cd and Pb, almost the same for Cu and Zn and slightly higher for Hg. Metal residues in muscle and ovary of roach are comparable with those found in pike (Table 2). Pb values in muscle and Hg values in ovary were below detection limits. The residues found in plankton are at least ten times higher for Cd and Pb than residues in fish and slightly higher for Cu and Zn (Table 2). Only Hg was more concentrated in fish than in plankton.

3.2. Organochlorines Seasonal differences are also obvious for the OC residues (Figs. 3, 4 and 5). High concentrations were determined in March 1990, low concentrations in September 1991. The differences varied by factors of 3-10. Although the average monthly residues were in inverse proportion to the average monthly lipid content (Fig. 3), we could not find a significant relation between the single values of all animals. PCBs (Fig. 3) show a typical pattern for aquatic organisms with high concentrations of no. 138 > 153 > 180 > 101 and low concentrations of no. 28 and 52. The pattern DDE > DDD = DDT is typical for freshwater fish; we could not find a constant pattern for HCH, HCB and OCS during the 2 years of investigation (Fig. 5). The differ-

Table 1 Mean, standard deviation, median, minimum and maximum of metal concentrations in bream, caught during 1990/91 in Lake Belau

2 S.D. 2 Min Max

Cadmium

Copper

Zinc

Lead

Mercury

2.3 1.8 1.2 0.4 8.5

451 123 398 292 827

18545 2 620 18 100 13 700 25 800

29.7 15.4 23.8 8.2 119.9

19.2 11.2 14.4 2.6 47.4

Sample size n = 90; values are given in /xg/kg fresh weight of bream homogenate. 2, mean; S.D., standard deviation; £, median; Min, minimum; Max, maximum.

190

W. Scharenberg et al. / Sci. Total En eiron. 155 (1994) 187-197

500 -

O0

-- 4002

00

,~.~ 3 O0

trations we conclude that pike were slightly more contaminated than bream and bream obviously higher than plankton.

600

600

-

d -6 200 ~

F

4. Discussion

I

oo

'fl [[

O0

J

~OO

00

-

0 ""

._

I~

3

9

12 3 monlh

L E ~ copper

5

weight

9

~

11

cadmium I

Fig. 1. Cadmium and copper-concentrations in homogenate of bream from Lake Belau as fresh weight of these fish. Animals were caught at 8 different months during 1990 and 1991. Values are given as means; fw, fresh weight; g, grams.

f43 -35

40 35 i

~, 30

_

3o ~ o

"~20

20

o

# Io

o ~

5 0

. 3

. 7

. 9

.

.

,

12

3

5

:0 9

monlh [ [~]

leoc

~

...... y [~

zinc

]

Fig. 2. Lead, mercury and zinc concentrations in homogenate of bream from Lake Belau. See also Fig. 1. Zinc values have to be multiplied by the factor 1000.

ences between the highest and the lowest contaminated bream were significant (Table 3), which might be of interest for monitoring programs. Although we measured only one pooled plankton sample, muscle tissue of pike, and homogenate of bream (all samples were taken during March), we can tentatively draw the following conclusion (Table 4): regarding the dry weight concentrations, pike were less contaminated than bream, while some OCs were higher concentrated in bream than in plankton, some were lower concentrated. Regarding the lipid weight concen-

If fish are used as bioindicators for heavy metals one has to take into account the seasonal variations of residues. As shown by regression analysis significant differences in bream of Lake Belau could not be explained by weight variations during the year. Other factors seemed to markedly influence the accumulation of pollutants. Ashraf et al. (1992) tried to eliminate the influence of weight by catching fish of the same size (500 + 20 g) during 1 year. They measured higher metal concentrations in summer than in winter. On the other hand, Barak and Mason (1990a,b) attribute seasonal differences in trace metal residues of eels, roach and other fish species to length of fish. Also Wissmath and Klein (1981) found higher Hg residues in bream and dace with increasing weight. Hg could be present also as methylmercury and has to be considered as a lipophilic compound. Thompson (1990) demonstrated higher Hg values in fish with increasing weight, but for Pb he could find only little correlations and no correlations with weight for Cd, Cu and Zn. The accumulation of some metals in different fish species seems to be influenced by length or weight; but these results are not in accordance with other investigations. If the possible influence of length or weight is eliminated, seasonal variations are still detectable: other parameters, such as water temperature or pH value, might have influenced these findings (Dallinger, 1986; Wiener et al., 1990). In our study essential metals in bream did not show large differences during 1990-1991 probably due to effective regulatory mechanisms. Similar to the findings for heavy metals we found seasonal variations of the OC residues of bream. A regression analysis showed no influence of weight or extractable lipid to residues, although the average lipid content is in an inverse proportion to the average residues of all months (Figs. 3 and 4).

191

W. Scharenberg et al. / Sci. Total Environ. 155 (1994) 187-197

Table 2 Metal residues in different tissues of pike and roach and in plankton Organ

Cadmium

Pike Muscle S.D. Ovary S.D. Liver S.D. European roach Muscle S.D. Ovary S.D. Plankton

0.44 0.10 0.93 0.58 2.30 2.70 1.1 0.33 1.46 0.95 26.82

Copper

Zinc

Lead

Mercury

n

316.8 83.9 1360 96 1245 1481

32 500 13 300 62 000 9 000 51000 58 000

n.d.

100.5 41.9 n.d.

7

635 123 1900 183 2347

16 210 1620 57 320 4 780 14100

n.d.

16.0 6.0 18.9 28.5

21.7 2.1 293

3

33.0 64.0

2

104.1 19.1 n.d.

4

13.9

1

4

Values are given in /zg/kg fresh weight; S.D., standard deviation; n, sample size; n.d., not detectable. I n a g r e e m e n t with o t h e r a u t h o r s ( D u i n k e r et al., 1983; K 6 h l e r , 1989; S c h n e i d e r , 1982; Svobod o v a et al., 1983) w e assume, t h a t e x c h a n g e p r o c e s s e s b e t w e e n the lipid o f fish a n d t h e surr o u n d i n g w a t e r a r e r e s p o n s i b l e for the a c c u m u l a tion. O n t h e o t h e r h a n d , C o n n e l l (1987) c o u l d find a r e l a t i o n b e t w e e n O C c o n c e n t r a t i o n a n d fish length. S e a s o n a l v a r i a t i o n s m i g h t b e exp l a i n e d by t h e possibility o f fish e l i m i n a t i n g O C s via g o n a d p r o d u c t s d u r i n g s p a w n i n g ( L a r s s o n et

lipid 7000 6000

l 20

28~E~ 15 52

E 5000 4000

~

10

r~

;01

!400

158

1200

I

3000

;55

S'~2000

5

180

1000 0

al., 1992), a l t h o u g h o t h e r a u t h o r s d o n o t find O C r e d u c t i o n d u r i n g s p a w n i n g (Niimi a n d Oliver, 1989). R e s i d u e p a t t e r n s in b r e a m w e r e typical for f r e s h w a t e r fish, b u t it is astonishing, t h a t b r e a m still c o n t a i n a c e r t a i n a m o u n t o f D D T , a p e s t i c i d e b a n n e d in G e r m a n y since 1978. T h e v a r i a t i o n in the p a t t e r n o f H C H i s o m e r s d u r i n g the y e a r c a n n o t b e explained; in g e n e r a l , a - H C H is t h e d o m i n a n t i s o m e r in a q u a t i c organisms. F o r c o m p a r i s o n with o t h e r investigations we p o o l e d all s a m p l e s o f b r e a m ( T a b l e s 5 a n d 6). M o s t limnic fish f r o m G e r m a n y a n d o t h e r countries e x a m i n e d by v a r i o u s a u t h o r s have c o n s i d e r -

~0 0 0

S

80o

"B a, 600 11 3

7

9 12 3

5

9

11

"~ 400 200

Fig. 3. Concentrations of 6 different PCB congeners in homogenate of bream from Lake Belau. Animals were caught at 9 different months during 1989-1991. Curve: fat as percentage of dry weight. Values are given as means. Extr. lipid, extractable lipid; 28, 52, 101, 138, 153, 180 = IUPAC no. of PCB congeners.

9

12

3

5

9

11

Fig. 4. Concentrations of 4,4'-DDE, 4,4'-DDD and 4,4'-DDT in homogenate of bream from Lake Belau. See also Fig. 3.

192

W. Scharenberg et al. / Sci. Total En~ron. 155 (1994) 187-197

250 -HCH 200

6 -HCH

I

"0 °_ Q_

d

I

6 -HCH 150 HCB

x

OCS

100

3 50

0

I

11

I Fig. 5.

3

11

7

9

I

I

12

month

I

5

I

5

9

11

I 1991

Concentrations of a-HCH, /3-HCH, y-HCH, HCB and OCS in homogenate of bream from Lake Belau. See also Fig. 3.

ably higher metals and OC residues. Toxic metals in particular, are 10-100-fold more concentrated. Even the acceptable limit for metals in edible freshwater fish in Germany was not exceeded by the maximum values in bream. Only Zn - - an essential metal - - was relatively highly concentrated in bream. In comparison marine fish have 10-fold higher concentrations of toxic metals (Thompson 1990). We conclude: bream of the ecosystem Lake Belau were only slightly contaminated with the toxic metals Cd, Hg, and Pb; because of similar residues in pike and European roach we can draw the same conclusion for these species. In comparison with other bream from different places in Germany, bream from Lake Belau are also not very highly contaminated with OC de-

spite the higher chlorinated PCB no. 138, 153 and 180 (Table 6). To assess fish as food we compared limit values for edible flesh with the average residues in bream homogenate ( Tables 5 and 7). Except for the higher chlorinated PCB congeneres (no. 138, 153), most values were at least 10 times lower than present limit values. Because of the large variation within our sample, we assumed that single bream might have higher PCB concentration in muscle than limit values. This is of interest, because we generally consider Lake Belau a relatively uncontaminated freshwater lake. For the comparison p r e y / p r e d a t o r we analyzed tissue of pike and European roach. Both in muscle tissue and in the ovary, roach showed slightly

W. Scharenberg et al. / Sci. Total Environ. 155 (1994) 187-197

193

Table 3 Organochlorine residues in homogenate of bream

ot-HCH /3-HCH y-HCH HCB OCS 4,4'-DDE 4,4'-DDT 4,4'-DDD PCB 28 PCB 52 PCB 101 PCB 138 PCB 153 PCB 180

59.5 94.7 68.3 59.5 46.0 591 181 161 50.1 59.0 590 1894 2054 969

S.D.

x

Min

Max

117 274 200 184 127 1037 281 264 141 112 941 3388 3779 1648

38.6 16.8 27.5 11.8 20.9 223 95.3 72.2 1.4 1.6 429 714 1248 316

0.95 0.09 0.73 0.58 0.61 23 0.09 0.57 0.09 0.09 21 89 78 50

1001 1929 1 874 1 695 1 126 6 468 1638 2 073 1013 660 6 693 26 205 29 938 12 057

~, mean; S.D., standard deviation; x, median; Min, minimum; Max, maximum. Values are given in /zg/kg extractable lipid.

higher contamination with metals than pike (Table 2). With the exception of Hg this is also the case for bream. Biomagnification could not be found from prey to predator. Plankton concentrated metals more than fish. Only Hg was accumulated from plankton to fish. These findings are supported by other authors (Carpene et al., 1990; Dallinger, 1986; Janssen and Brune, 1984; Wissmath and Klein, 1981). Bioconcentration could be

detected from water to sediment and biomagnification from sediment to Tubifex species, but no magnifcation from Tubifex to carp (Yediler and Braun, 1980). Even in water with high metal concentrations in the sediment, no elevated residues were found in fish muscle (Wissmath and Klein, 1981). Karbe et al. (1987) carried out an experiment with cadmium in a food chain, algae-mussel-fish. The proportion for Cd was:

Table 4 Mean of organochlorine residues in pike (flesh; n = 7; March 1992), bream (homogenate; n = 24; March 1990 and 1991) and plankton (n = 1; March 1993) from Lake Belau Plankton a-HCH /3-HCH y-HCH HCB 4,4'-DDE 4,4'-DDT 4,4'-DDD PCB 28 PCB 52 PCB 101 PCB 138 PCB 153 PCB 180

12.3 36.7 59.6 21.7 73.3 38.6 37.4 18.4 5.4 57.6 117.6 133.1 52.6

Bream 2.6 7.8 12.7 4.6 15.6 8.2 7.9 3.9 1.1 12.4 25.1 28.4 11.2

53.1 126.3 64.1 67.8 1072 276 219 81.4 116 896 4070 4355 2062

Pike 2.3 4.2 2.4 3.2 52.2 12.7 11.2 2.5 5.1 43.0 179 185 90.3

Values are given in ~ g / k g lipid weight (left column) and ,~g/kg dry weight (right column).

107 389 391 184 2215 1228 1004 169 45 1947 6605 9441 5566

1.1 1.8 2.5 0.9 12.8 6.7 6.2 0.7 1.2 13.9 53.1 68.8 45.5

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W. Scharenberg et al. / Sci. Total Environ. 155 (1994) 187-197

Table 5 Concentrations of metals in different freshwater fish from different countries Species/country

Cadmium

Freshwater fish a Germany Abramis brama b Germany Diff. species c Pakistan Diff. species d Pakistan Diff. species e Germany Esox lucius f Perca fluviatilis f England Onchorhynchos mykiss g Italy Anguilla anguilla h Rutilus rutilus h England A. anguilla i England

Copper

Zinc

Lead

50

Mercury 500

2.3 S.D.1.8 25 - 1500

451 S.D.123 153

18545 S.D.2620 3361

- 7200

-50650

1000

29.7 S.D.15.4 80

19.2 S.D.11.2 32

- 45 320

- 30

-

20 50

- 26800

104

-

1310

80 90

210 180

50 30

80 70

370 180

800

470

590

700 S.D.300

9500 S.D.1700

S.D., standard deviation; - , m a x i m u m mean; values are given in / x g / k g fresh weight of muscle except aZEBS, 1979 (limit values for edible flesh; Germany). bThis investigation, CAshraf et al., 1992. djaffar et al., 1988. ~cited by Hiibner, 1990. fBarak and Mason, 1990a. gCarpene et al., 1990. hBarak and Mason, 1990b. ' M a s o n and Barak, 1990.

i

liver and b: homogenate.

Table 6 Residues of chlorinated hydrocarbons in the edible part of bream. n

HCB

Elbe, Gorleben Elbe, L a u e n b u r g Elbe, Glfickstadt Ems, M e p p e n Aller, Gifhorn Leine, Hannover Saar, b. VSlklingen Bodensee

12 6275 37 5848 14 4164 12 122 12 47 4 207 3 178 4 34

(~lper See D f i m m e r See Belauer See a

30 3 95

80 41 60

a-HCH

y - H C H OCS

DDE

DDD

PCB 28 P C B 5 2

PCB 101 PCB 138 PCB 153 PCB 180

448 233 468 84 22 69 120 112

658 553 696 372 147 144 684 247

1836 2185 1638 26 < 5 33 111 54

3063 3643 1921 585 918 794 837 6170

3981 2841 4054 180 232 126 426 2364

966 786 < 50 862 430 269 8600 1051

438 374 336 398 1695 580 8203 1375

890 740 900 604 1908 1043 3230 2785

1297 1468 1823 1363 2230 1383 1723 6773

1587 1747 1944 1358 2700 1505 1667 7713

407 762 783 555 990 674 569 3205

33 25 42

317 524 69

9 16 46

1461 400 591

505 133 162

156 -50

243 133 59

910 384 590

2126 448 1894

2595 900 2054

1267 408 969

From: Umweltbundesamt, 1989, revised. aBream h o m o g e n a t e (average, /~g/kg extractable fat.).

W. Scharenberg et al. / Sci. Total Environ. 155 (1994) 187-197 Table 7 Limit-values (SHmV 1988) a in edible flesh of fish in comparison to average-values in homogenate of bream from Lake Belau.

HCB a-HCH y-HCH E EDDT PCB 28 PCB 52 PCB 101 PCB 138 PCB 153 PCB 180

Average-value bream [ p.g/kg fw]

Limit-value fish [/zg/kg fw]

0.83 0.96 1.0 14.18 0.67 1.31 9.6 34.2 29.8 13.3

50 50 200 3500 200 200 200 200 200 200

fw, fresh weight. aSchadstoff-Hochstmengen-Verordnungvom 23.3.1988.

water 86%, algae 10.3%, mussel 2.4% and fish 1.3%. There was no accumulation detectable. Only Hg is a marked exception (Wissmath and Klein, 1981; Janssen and Brune, 1984). Bioconcentration from water to fish is well known (Jaffar et al., 1988; Pesch and Kraus, 1991; Yediler and Braun, 1980). Fish are good indicators for water contamination, but they do not necessarily magnify via the food chain. Different residues in various species of the same size might be the result of species-specific absorption and excretion. Novey et al. (1990) pointed out, that metal sensitive and insensitive fish accumulate Cd to quite a different extent. OC accumulate especially in the lipid. From the data in Table 4 one could assume that OC accumulate via the food chain. This is in contrast to the exchange hypothesis of OC via fish/water, because then contamination of fish would be independent of the food contamination, (Falkner and Simonis, 1982; Lacorte and Eggens, 1993). Hansen (1987) compared the accumulation of lindane in fish by magnification in relation to accumulation by concentration. His conclusion was: the bioconcentration is more important than the magnification. Various residues in species of the same environment may be the result of different processes: (a) biomagnification, (b) elimination, (c) metabolism and (d) concentration (van der Oost et al., 1988; DFG, 1987; Schneider, 1982;

195

Biessmann, 1981). In relation to the dry weight we cannot find any accumulation from plankton to bream and from bream to pike. Toxicity from low metal concentrations in fish from Lake Belau is not a hazard to adult fish. Cd, for example, skeleton deformation is caused at 100-fold higher concentrations (Pesch and Kraus, 1981). Adult fish were less sensitive to heavy metals than embryos, and embryos less sensitive than larvae (Dethlefsen, 1981). OC residues were below those values which obviously cause damage to fish. Less obvious harm to fish in natural ecosystems is hard to assess. For example, it is known that low PCB concentrations can cause lower hatching rates (Westernhagen et al., 1981; Hansen, 1985). Furthermore, we have to take synergistic effects into consideration and we have to expect more environmental contaminants such as dioxins or toxaphens. To assess fish as food we compared limit values for edible flesh with the average residues in bream homogenate (Tables 5 and 7). Except the higher chlorinated PCB congeners, most residues were at least 10 times lower than present limit values. Because of the great variation within our sample we assume that single bream might have higher PCB concentration in muscle flesh than the limit values. This is of interest, because in general we would draw the conclusion, that Lake Belau is a relatively uncontaminated freshwater lake.

Acknowledgements This investigation was funded by the BMFTproject: 'Okosystemforschung im Bereich der Bornh6veder Seenkette'. We thank Anke Schmidt and Sigrid Ulrich for analytical support, L. Theesen for his mathematical assistance and E. Ebeling for correction of the English version. The weight data of bream are part of the Ph.D thesis of W.H. Pfeiffer.

References Ashraf, M., M. Jaffar and J. Tario, 1992. Annual variation of selected trace metals in freshwater lake fish, Labeo rohita, as an index of environmental pollution. Toxicol. Environ. Chem., 35: 1-7.

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