2.5. The Role of Lake-Specific Abiotic and Biotic Factors for the Transfer of Radiocaesium Fallout to Fish

79

2.5. THE ROLE OF LAKE-SPECIFIC ABIOTIC AND BIOTIC FACTORS FOR THE TRANSFER OF RADIOCAESIUM FALLOUT TO FISH

TORD ANDERSSONl & MARKUS MEILI2 1Dept. of Physical Geography, University of UmeB, 901 87 Umel, Sweden 2Inst. of Earth Sciences, Uppsala University, 752 36 Uppsala, Sweden

SUMMARY Lake-specific factors are used as a tool to explain the variation in transfer of 137Cs to fish at a certain level of fallout, and to describe and evaluate the temporal development of this transfer. The effect of biotic interrelationships and environmental variables influencing the tmphic structure of the lakes is briefly reviewed and discussed. The study is based on a large amount of existing Nordic post-Chernobyl data on 137Cs in different species of freshwater fish. The maximum transfer was reached within the first three years for all species in most lakes and normally in the order, small perch - trout and charr - larger perch - pike, a sequence that seems to reflect the trophic level each species occupies. Thus, the fish-eating pike is the species with the most extended temporal development and is also the species with the highest values of total time-integrated transfer. The transfer to fish differed over an order of magnitude between lakes, and lakes with a high total transfer to small perch also show a high total transfer to pike. Inter-lake differences in this respect could not be explained by the difference in growth rate of pike between lakes. Information is provided about the ranges for the total transfer to some common fish species in Nordic waters at different ranges of the theoretical residence time of 137Cs in lake waters (T&. Tcsis determined from the mean hydraulic residence time and the scavenging capacity of the lakes. The amount and nature of scavenging agents (possibly clay minerals) were well indicated, but not determined, by the natural concentration of base cations in lake water.

INTRODUCTION After the Chernobyl accident, high levels of radiocaesium became a major environmental problem in Sweden. 137Cs has been the major concern in most studies on the fate and effect of the Chernobyl fallout in Nordic lakes, partly due to its long physical half-life, partly because it is readily accumulated by organisms because of its chemical similarity to potassium, which is a major component in cell metabolism. In freshwater fish, which are an important factor in the Scandinavian life-style, high levels of radiocaesium occurred in the flesh and this led to various governmental actions. An estimated 14000 Swedish lakes contained fish with concentrations of 137Cs above 1500

Bq kg-1 wet weight during the autumn of 1987 (Hkanson et al., 1992). In this study, lake-specific abiotic and biotic factors are used as a tool to explain the variation in transfer of 137Cs to fish at a certain level of fallout, and to describe and evaluate the temporal development of this transfer. The study is based on a large amount of existing Nordic post-Chernobyl data on 137Cs in different species of freshwater fish. Much of the data on perch and pike originates from a comprehensive study initiated in 1986 on

80 "Liming-Mercury-Cesium", which was originally intended to evaluate different measures to decrease the mercury content in fish (HAkanson et al., 1990). However, in April 1986 the Chernobyl accident occurred and 41 lakes included in the project were affected by a major fallout of 137Cs in the

range of 3 to 70 kBq m-2 . High levels of 137Cs in small perch from these lakes were recorded

shortly after the fallout (Andersson et al., 1990; HAkanson et al., 1992), and the ongoing sampling schedule and planned remedial measures intended for mercury seemed well fitted to be supplemented with analyses of 137Cs and remedial treatments also directed towards reducing the levels of 137Cs in fish. Results from the project up to 1989 have been presented earlier, dealing, for example, with the relationships between drainage area characteristics and lake water quality (Nilsson and Hflkanson 1992), and effects of remedial measures on the content of radiocaesium in fish (Andersson et al., 1991; Hflkanson and Andersson 1992). In the present paper, data reported earlier are supplemented by new, unpublished data from the period 1990 to 1992 and by data on perch and pike (Andersson et al., 1990; Broberg and Andersson 1991; SaxCn 1992; Sundblad et al., 1991) and brown trout and Arctic charr (Hammar et al., 1991). Drainage lakes and their surroundings are complex systems where many chemical, physical and biological factors interact and potentially influence the distribution of different elements and their ecological impact. This paper concentrates on factors and processes which in the Nordic countries have turned out to be empirically significant, and which may affect the transport and fluxes of radiocaesium in the lakes. Beside the influence of abiotic factors, the effect of biotic interrelationships on the transfer and environmental variables influencing the trophic structure of the lakes are discussed. The basic transport route to fish is from atmospheric deposition to lake water with a certain retention within the catchment and the lake itself, and finally an uptake primarily via the food web. 137Cs is rather strongly adsorbed onto suspended matter, which implies that the flux of particles within lakes must be accounted for in lake budgets (Stumm and Morgan 1981). Radiocaesium has been widely used for sediment dating (Jaakola et al., 1983; Pennington et al., 1973) and more generally as a tracer of particle transport to and within aquatic environments (Campbell et al., 1982; Santschi et al., 1988; Walling 1989). Several papers have shown that great attention must be. paid to environmental factors influencing the distribution and behaviour of radiocaesium in lakes, such as the size fractionation of radiocaesium-carrying agents (Salbu et al., 1992) and the influence on the sorption of 137Cs to particles of the content and type of clay minerals (Cremers et al., 1988; Evans et al., 1983; Heit and Miller 1987), the organic content (Longmore et al., 1983) and the concentration of competing cations such as

m+or K+ (Comans et al., 1989; Evans et al., 1983).

As regards the radiocaesium content in fish, several water-quality variables have been reported to correlate significantly negatively, i.e., ionic strength and intercorrelated parameters such as hardness and potassium concentration (Andersson et al., 1990; HAkanson et al., 1992). It is also well documented that the relationship between the concentration of radiocaesium in organisms and in ambient water should be inversely proportional to the potassium concentration in lake water. Lake-specific parameters describing hydrology and lake morphology (e.g., the hydraulic residence time and lake depth) have also been identified and confirmed as correlates in several works

81 (Andersson et al., 1991; Hammar et al., 1991; HAkanson et al., 1989; HAkanson et al., 1992). The interpretation of these abiotic factors has mostly been focused on the retention of 137Cs in the lakes, but it has also been suggested that they could indicate inter-lake differences in bioproduction and biotic interrelationships, as the magnitude of the transfer to a given species or size group of fish appears to be a function of diet and feeding rate (Meili 1991; Forseth et al. 1992). The remedial measures tested in the "Liming-Mercury-Caesium" project included the addition of lime and potash. This made it possible to study if and how the distribution and retention of 137Cs were changed by the altered chemical conditions, and to what degree the retention was related to water chemistry as compared to lake characteristics being more or less unaffected by the remedial measures, such as hydraulic residence time, lake morphology and fundamental particle properties. The results of that study (Andersson, 1993) are used here to evaluate the role of the physical and chemical lake properties in the time-dependent transfer to fish. The Chernobyl fallout coincided with the snowmelt and a high runoff in the study region, which gave rise to a marked pulse of 137Cs traceable in different lake compartments and in different biota. This situation provided an occasion to quantify the decline and the time needed to reach conditions closer to steady state, and the results of this study might give some insight into these matters too.

MATERIAL AND METHODS As the results and data used in this work originate from several sources and projects, we will just give a brief description of the most important sources of error in the study. A more detailed description of the analytical methods and sampling routines as well as information concerning the temporal and spatial representativity and statistical variability of the data is given elsewhere (Andersson 1993; Andersson et al., 1991; Hammar et al., 1991; HAkanson and Andersson 1992; HAkanson et al., 1992; Hfianson et al., 1990). The overall strategy was to assign to each lake a representative mean value for each variable during a defined period of time. Annual fish samples, usually 10 to 15 individuals of each species, were used to give each lake a value of 137Cs for a specific period. The intra-lake variability for the same species and size was very high during 1986 when, for example, the coefficent of variation (CV) for small perch had an average of 0.60. The individual variation has gradually decreased and the possible error (10.95)in each yearly lake mean value is now mostly <20 %. Fallout on the lakes and their catchments was generally calculated from fallout maps based on aerial surveys of soil radiation, which were performed in 1986 at regular flight path distances of usually 5 to 10 km and calibrated against soil sample measurements. Considering that wide local variations occur, that numerous soil and vegetation factors could affect the measured radiation and that certain interpolation errors are inevitable, it seems obvious that these fallout values give a rather crude estimate of true loading, and the potential error in each lake mean value is probably higher than the calculated error for the content in fish. The relatively large possible errors in both radiocaesium content in fish and estimated fallout must be accounted for when dealing with transfer coefficients. Certain outlier lakes may be explained

82 by an erroneous estimate of, for instance, fallout, but because of the large number of lakes investigated it is unlikely that the generality of the results is significantly affected.

RESULTS AND DISCUSSION The effect of radioactive contamination is a function of the input of isotopes to ecosystems, given as a rate or as an accumulated amount. In the case of an atmospheric input of short duration as from the Chernobyl accident, concentrations and burdens of radioisotopes in different ecosystem compartments are most suitably normalized by means of the total areal deposition during April and May 1986 and expressed as transfer coefficients or factors (e.g. Bq kg-1 wet weight in fish per Bq m-2 deposited = m2 kg-1). Other commonly used ratios are concentration factors with respect to the activity in sediments or in ambient water (e.g. Bq kg-1 wet weight in fish per Bq m-3 in the ambient water). Transfer and concentration factors vary significantly between organisms, ecosystems and situations, and they become an intricate function of time when ecological turnover times exceed the duration of the input. Consequently, the transfer needs to be defined in relation to the time elapsed since contamination. In addition, the transfer from abiotic to biotic compartments is determined by a large number of physical, chemical, physiological and ecological processes that alter in importance as isotopes are redistributed within the ecosystem. As a result of the pulse-like type of emission, the transfer of Chernobyl caesium to fish showed strong temporal variation. At any time (t) the content of radiocaesium in fish within a lake (Cs-fish(t)) can be related to a load parameter, for example the fallout (Cssoil) as: TC(t)=Cs-fish(t)/Cssil (m2 kg-1)

UI

The magnitude of this lake specific ratio or transfer coefficent (TC) was largely dependant on the actual fish species and time after fallout. This is illustrated in Figure 2.5.1 where the variation of the transfer coefficient with time is shown for small perch (Percafluviufilis),pike @sox lucius), brown trout (Sulmo b u m ) and Arctic charr (Salvelinus alpinus) as compiled from data from a large number of Nordic lakes. The most striking feature of Figure 2.5.1 is perhaps the wide range of transfer coefficients in different lakes for the same species and size category at a particular time. This large inter-lake variation, of course, gives rise to a wide variety of possible lake specific curves for a certain species. The different temporal development of the transfer in different lakes is illustrated with curves for pike in lakes Lovsjon and Hamstasjon and for trout in Storsjouten and Grundvattnet respectively. Another measure of the spread around the mean TC(t) is given (Figure 2.5.1 A) by the yearly quartile values for small perch and pike in the most well-documented population of lakes (n=4 1), Another conclusion which can be derived from Figure 2.5.1 is that the maximum transfer was reached within the first three years for all species in most lakes and normally in the order, small perch - trout and cham - larger perch - pike. This sequence seems to reflect the trophic level each species occupies. Thus, the fish-eating pike is the species with the most extended temporal development and

83 is also the species with the highest values of total time-integrated transfer (F, Table 2.5.1). In this context it should also be noted that the types of lakes occupied by pike and perch on the one hand and trout and charr on the other are normally quite different.

A.

Jan-86

Jan-88

Jan-90

Jan-92

Date

1

r A

ir

E

0,1

B.

." - --

I

,

A LI

P

\

0,Ol

0,001 Jan-86

I

I

Jan-88

Jan-90

Jan-92

Date

Figure 2.5.1. Ranges (within arrows), inter-quartile ranges (vertical bars) and mean values (lines) for the time-dependent transfer coefficient TC(t) between the Chernobyl fallout and the lake mean content of 137Cs in fish, for (A) small perch, 4 0 g (unfilled symbols) and pike (filed) and, (B) Arctic charr (unfilled, dashed line) and brown trout (filled, full line), based on data from 230 Nordic lakes. Examples of lakes with a low and high total transfer to pike (A) and to trout (B), respectively. After the maximum value on the transfer (TC(*o))was reached, the decline can be fairly well described by an exponentially decreasing function: Cs-fish(t) = Cs-fish(t=O)*e-kt, from which the apparent or ecological half-life (TE) for each lake can be derived: TE = In 2 / k. This makes it possible to extrapolate future transfer coefficients and to estimate the total, time-integrated value of

84 the total expected transfer given by the transfer factor F F = F(t0) + TC(a)*TE/ln 2 where F(t0) is a linear approximation of the time-integrated transfer up until the maximum transfer coefficient was reached. Planktivores, like small perch, reached their maximum transfer coefficient within a few months after the fallout and consequently F(tO) becomes small compared to the total transfer for these fish categories. From Eqn. 2 it is also obvious that the magnitude of F is determined both by the magnitude of the initial transfer and the rate of decrease as expressed in TE. Table 2.5.1. Mean, maximum and minimum values of the the total expected transfer (F, m2 kg-1 yr> of Chernobyl fallout to pike, small perch ( 4 0 g), brown trout and Arctic charr. Fpi(6):F,i and Fpe(3):Fpegives the fraction (in %) of F transferred after 6 years for pike and 3 years for small perch.

MGXl

Mm Max

Fpike

Fpi(6):Fpi

Fperch

Fpe(3):Fpe

FTrout

FCharr

0.95 0.11 5.0

74 50 85

0.50 0.04 1.34

84 36 99

0.55 0.13

0.46 0.18 0.90

1.o

Table 2.5.1 gives the expected total transfer (mean and ranges) for some different common fish species in Nordic lakes, and also the transfer after 3 years (F3) and 6 years (F6), respectively, in relation to the total expected transfer F. Annual and seasonal fluctuations and an increase of TE with time due to a future increased impact of factors controlling the secondary load (such as resuspension (Broberg and Andersson 1991; Hkkanson and Andersson, 1992) are possible. However, in small perch (which in this data set show a decreasing concentration of radiocaesium for the longest time, > 6 years), there is a tendency for an increase of TE during the last 3 years compared to the values (0.6
Turnover of radiocaesium in fish Several important controlling factors for radiocaesium turnover in fish were identified. Generally, both intake and excretion of radiocaesium vary with season and with the life stage of the fish. The turnover is rapid in summer, with both high intake and rapid excretion, and slower during winter with lower intake and slower excretion (Forseth et al., 1991; Meili, 1991). The dependence of uptake and elimination on temperature in a given type of organism may, however, differ significantly. Furthermore, turnover is faster in smaller than in larger fish, as small fish have a higher rate of metabolism. Food selection may also vary with season and fish size, thus influencing the intake of radiocaesium. Uptake of caesium from contaminated food is the major source of radiocaesium in fish, and

85

intake from the water is of negligible significance to the body burden in natural freshwater systems. The radiocaesium intake from food is thus determined by food selection, radiocaesium levels in the prey, and amount of food consumed (see above). On the other hand, excretion is probably determined by the metabolic rate (Carlsson, 1978; Evans, 1988) and may be species-specific. Several studies have shown that the elimination rate, similar to the metabolic rate, depends on both fish size and temperature (Ugedal et al., 1991 and references therein; Evans, 1989). In Nordic lakes, temperature varies with depth and time, with maximum temperatures in epilimnetic waters during summer. Excretion may therefore be linked to the habitat of the fish which, in some species or size groups, varies with respect to temperature. The same applies to the feeding rate of fish, but not necessarily in the same manner (Forseth et al., 1991). The biological half-life of radiocaesium was estimated experimentally in brown trout (Ugedal et al., 1991) and in one size class of roach (Evans, 1989). The former study confirmed the strong dependence on temperature, to a lesser degree there was also an effect of fish size. The latter study showed no effects of potassium addition to the water (not to the food) on the elimination rate of radiocaesium in roach. There exists a very clear inter-lake relationship between the total transfer to perch and to pike (Figure 2.5.2), i.e., lakes with a high total transfer to small perch also show a high total transfer to pike. The amount of appropriate data concerning trout and charr is not sufficient to make any similar comparisons of the total transfer between different lakes, but the high correlation during autumn 1987 (r2= 0.68, n = 12 lakes, Hiikanson et al., 1992) suggests that there exists a similar, but somewhat weaker, relationship as between pike and perch.

A

0.1

I 1

Fpike (mz kg-1 yr)

Figure 2.5.2. Relationship between the total expected transfer of Chernobyl fallout (F) to pike and small perch in 44 lakes, Fpike(6) = the time-integrated transfer to pike after 6 years. An attempt was made to relate the inter-lake variability in the activity ratio Cs-pike:Cs-perch to

the growth rate of pike expressed as mean age of 1-kg pike (Figure 2.5.3 A). However, the

86 inter-lake differences could not be explained by the difference in pike growth rate between the lakes. The total transfer to pike after 6 years (Fpike(6))did not show any clear relation to the mean age of pike, even if one might see a tendency to a faster transfer in lakes where the pike has a faster growth rate (Figure 2.5.3 B). Thus, other factors must be considered when explaining the inter-lake differences in transfer from fallout to fish.

A.

4

4,5

5 5,5 6 Mean age of pike (years)

6,5

7

B.

4

4,5

5

535

6

7

Mean age of pike (years)

Figure 2.5.3. The growth rate of pike expressed as mean age of I-kgpike compared to (A) the ratio of the concentration of 137Cs in pike and perch (Cs-pi:Cs-pe) during 1987, 1988 and 1992, and (B) compared to the time-integrated transfer to pike after 6 years, i.e. until 1992 (Fpike (6). m2 kg-1 yr-1).

The transfer to fish in relation to abiotic factors Earlier studies in Swedish lakes (Anderson et al., 1990; H&anson et al., 1992) have shown that the factors of highest statistical significance for the initial transfer of radiocaesium to fish were (sign

87 of correlation within brackets) water retention time (+) and any of the more or less intercorrelated parameters hardness (-), potassium concentration (-), humic content (+) and ionic strength (-). How are these parameters related to the total transfer factor (F), and what causal relationships could these correlations reflect? Andersson (1993) showed that the lake mean distribution coefficient (Kd = mean activity in settling particles / mean activity in lake water) for 137Cs varied between 1 * 104 and 7 * 104 cm3 g-1 in the Swedish lakes (n=15) most studied, a range which covers the values reported for Lake Ziirich (Santschi et al., 1990) and Lake Paijbne (Kansanen et al., 1991) and also most of the modelled, strongly time-dependent distribution coefficients in the epilimnion of Lake Constance during 1986 (Robbins et al., 1992). The inter-lake variation of Kd,Cs was quite strongly correlated to the natural (i.e. before liming operations) concentration of major base cations expressed as hardness (r2=0.58, n=15) and intercorrelated parameters like pH and alkalinity. h , c swas also well correlated negatively to the carbon content in settling material. The sedimentation rate of radiocaesium as expressed by the sedimentation coefficient (Ksed,Cs [d-I] = &,Cs

*

particle settling flux / lake mean depth) was well correlated to the natural

concentration of major base cations (r24.81, n=15) and intercorrelated parameters such as pH, alkalinity and conductivity (Andersson 1993). The higher scavenging capacity in lakes with higher concentration of major base cations was due to both higher particle sedimentation rates and higher & values in these lakes. However, water chemistry was probably not causal in this respect, despite the high correlation. Liming operations and potash addition caused a highly significant increase in mean values of hardness (and intercorrelated parameters) in lakes with initially low concentrations of major base cations (Hfkanson & Andersson, 1992), but this markedly increased concentration of major base cations in most lakes during 1988 and 1989 did not notably affect the mean distribution and sedimentation coefficients of 137Cs in the lakes (Andersson, 1993). The lack of effect of the remedial measms on the distribution and sedimentation coefficients of radiocaesium, suggests that a likely causal factor would rather be the amount and nature of scavenging agents, which in these lakes were well correlated to the natural concentration of base cations in the water. This is supported by the fairly good relationship between Ksed.Cs and the fraction of particulate inorganic matter in setding particles (PIM), which was very similar before and after remedies, respectively. The high correlation of Ksd,cs to natural water hardness might reflect that the concentration of major base cations in this calcite-poor region is positively correlated to the content of clay minerals in soils and sediments. Lakes with higher sedimentation coefficients generally also had a higher bioproduction as expressed by the concentration of total phosphorous, but the correlation of K,j,cs to total-P in water, and the ratio of C:N in settling matter, was much weaker compared to the correlation with hardness. In a model for scavenging of 137Cs in the epilimnion of Lake Constance (Robbins et al., 1992), the best fit to observed activities was obtained using a substitute (particulate aluminium) for clay minerals as the "reactive phase" of total suspended matter, while the affinity to calcite was found to be negligble. Cremers et al. (1988) and earlier works have shown that the division of

88 radiocaesium between solid and liquid phase in soils is regulated by a small number of highly selective ion-exchange sites, located at the frayed edges of micaceous clay minerals (illites). These observations are all consistent with the observed depletion of nuclear weapon radiocaesium in sediment inventories compared to the cumulative atmospheric deposition in North American softwater lakes (Heit and Miller, 1987), where low levels of binding clay minerals were suggested as a prime cause. Because of the mentioned reasons, however, this interpretation of the correlation between haxiness or ionic strength to scavenging capacity seems to be restricted to lakes within the zone of boreal forests and the relationship is different in geological areas dominated by calcite or dolomite weathering. The sedimentation coefficient Kse&csis inversely related to the residence time (Tsed, Eadie and Robbins, 1987) of 137Cs in the water body with respect to the removal by particle settling (Tsed =l/Ksed). The theoretical residence t h e of 137Cs in lake water (TcS)could then be obtained from:

where Tw= mean hydraulic residence time, Figure 2.5.4 shows the relationship between the mean hydraulic residence time and the theoretical residence time for 137Cs (TcS) within the lake water columns for different values of the sedimentation coefficient

(Ksed),

along with empirical values and modelled values based on natural

mean water hardness. It should be noted that there is a considerable variation between lakes in their theoretical retention of 137Cs. This variation is directly connected to the hydraulic residence time of the lakes, but also to the factors described earlier that influence the specific sedimentation rates of 137cs.

The theoretical residence time of 137Cs was determined in 15 lakes using sediment traps. As water hardness was found to be a good indicator of the scavenging capacity of 137Cs from lake water, the mean concentration of Ca+Mg in lake waters was used to calculate the removal rate due to sedimentation. In this context, it is important to note that the empirical values are based on data of gross sedimentation from sediment traps without accounting for any potential resuspension effects. The apparent residence time is almost always significantly longer than Tcs, as a result of resuspension of or possibly diffusion from Cs-contaminated sediments, and/or of input from the catchments. However, during the summer of 1986, after the large initial direct deposition and catchment-derived input that occurred within the f i s t month after Chernobyl, the lake water pools were comparatively large compared to the potential loading from resuspension and to the catchment input which declined very rapidly (Bergman et al., 1991; Santschi et al., 1988). Thus, it is likely that Tcs provides a fairly realistic description of the shape and magnitude of the pulse of radiocaesium activity through the lake water columns during the first important phase.

89

0

1

P

0.001 0.13

1.4

0.005

0.01

0.17 0.21

2.2 3.0

0.05

0.55

6.5

I

10

1000

100

Tw (days)

Figure 2.5.4. The relationship between the mean hydraulic residence time (Tw) and the theoretical mean residence time of 137Cs (TcS) for different values on the sedimentation coefficient (Ksed), along with empirical values based on sedimentation traps (n=15 lakes, unfilled diamonds) and modelled values (short lines) based on the natural mean hardness of lake water. This suggestion is supported by Figure 2.5.5, which shows the relationship between the theoretical residence time of 137Cs in lake water (Tcs) and the total transfer to pike. Three lakes with high transfer to fish despite low Tcs can be identified. Two of these are very shallow lakes (mean depths around 1 m), and a high degree of windwave-induced resuspension is likely. The third lake is situated closely downstream (< 500 m) of a much larger lake, and should in this context be regarded as a part of the larger lake with its much higher Tcs value. The relationship given in Figure 2.5.5 is therefore valid for lakes without major secondary inputs of 137Cs to lake waters from either major resuspension activity (or possibly diffusion) in sediments, or from major temporary traps within the catchment (lakes or bogs). From the rather high frequency of shallow lakes in the studied data set one would expect more outliers from the general pattern which does not account for resuspension or sediment-mediated uptake. This lack of outliers indicates that the biological availability of sediment-bound radiocaesium is generally rather low, even though the effect of resuspension or sediment-mediated uptake must be considered in certain lakes. Table 2.5.2. Normal ranges for the total expected transfer (F)of Chemobyl fallout to pike, small perch and brown trout in Nordic lakes at different theoretical residence times of 137Cs in lake water (Tcs).

Tcs (days)

Fpike (m2 kg-1 yr)

Fperch (m2kg-1 yr)

Feout (m2 kg-1 yr)

<30 30-50 50- 100 100-365 365-

<0.50 0.30-0.70 0.60- 1.2 0.80- 1.5 1.2-3.3

<0.25 0.20-0.40 0.35-0.60 0.50-1 .O 0.80-1.5

ca 0.20 ca 0.30 ca 0.50 ca 0.70 ca 1.0

90

Table 2.5.2 provides information about the ranges for the total transfer to some common fish species in Nordic waters at different ranges of the theoretical residence time of 137Cs (TcS). By combining the information in Figure 2.5.4 and Table 2.5.2 it is possible to obtain a fairly accurate estimation of the total transfer of Chernobyl caesium to, e.g., pike in a certain lake, based only on a knowledge of the hydraulic residence time and water hardness. -

10

c *

7 rn

*

N

€

-Bn v

1

L

0,1

1

10

'CC!

:2 c o

TCs (days)

Figure 2.5.5. The relationship between the theoretical mean residence time of 137Cs in lake water v c s ) and the expected total transfer to pike (Fpike) in 45 Nordic lakes, transfer values marked with unfilled squares are probably due to major secondary inputs (resuspension or upstream input, see text).

CONCLUSIONS The maximum transfer of Chernobyl caesium to fish was reached within the f i s t three years for all species and normally in the order; small perch - trout and charr - larger perch - pike, a sequence that seems to reflect the trophic level each species occupies. Thus, pike is the species with the highest total time-integrated transfer and the largest remaining fraction of the total transfer. The transfer from fallout to fish differed over an order of magnitude between lakes, and lakes with a high total transfer to small perch also show a high total transfer to pike. Inter-lake differences in the transfer from fallout to fish could not be explained by differences in pike growth rate. A better predicton was obtained from the mean hydraulic residence time and the scavenging capacity of the lakes, where the amount and nature of scavenging agents (possibly clay minerals) in this calcite-poor region were well indicated by the natural concentration of base cations in lake water. It seems likely that these, easily monitored, abiotic factors which are causally linked to the residence time of 137Cs in lake waters will be good predictors also of the transfer to fish after possible, future fallout events. ACKNOWLEDGEMENTS We are grateful to all people involved in sampling and data collection and Lars Sonesten for age determination of fish. The overall study "Liming-Mercury-Cesium" was financially supported by the

91 National Swedish Environmental Protection Agency ( S N V ) and the National Swedish Institute of Radiation Protection (SSI).

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