PCB Concentrations of Lake Michigan Invertebrates: Reconstruction Based on PCB Concentrations of Alewives (Alosa pseudoharengus) and their Bioenergetics

J. Great Lakes Res. 21(1):112-120 Internal. Assoc. Great Lakes Res., 1995

PCB Concentrations of Lake Michigan Invertebrates: Reconstruction Based on PCB Concentrations of Alewives (Alosa pseudoharengus) and their Bioenergetics Leland J. Jackson and Stephen R. Carpenter

Center for Limnology, University of Wisconsin 680 North Park Street Madison, Wisconsin 53706 ABSTRACT. Invertebrate PCB concentrations are a poorly quantified but crucial step in the trophic transfer of organochlorine contaminants to fishes. In fact, current attempts to quantify PCB fluxes in the Lake Michigan pelagic food web are hampered by poor knowledge of invertebrate PCB concentrations. Models exist that estimate PCB concentrations in fish based upon PCB concentrations in their food. We have used a complementary approach and estimated invertebrate PCB concentrations based upon historical records of alewife PCB concentrations in Lake Michigan. We first developed a model of total PCB accumulation in alewife (Alosa pseudoharengus) using a bioenergetics-based approach to growth. The PCB assimilation efficiency between invertebrate prey and alewife predators was estimated to be about 0.40. We then used the model to hindcast PCB concentrations in the invertebrates that comprised alewife prey. We estimated that median PCB concentrations of Lake Michigan invertebrates have dropped roughly 10-fold from 1976 to 1990; for example, the estimated Diporeia hoyi PCB concentration has droppedfrom ca. 0.48 mg-kg- l wet weight to 0.04 mg-kg- l wet weight over this time period. The biomagnification ratio (alewife PCB/zooplankton PCB) is about 16-fold (Diporeia hoyi) to 40-fold (copepods/ cladocerans). The PCB concentrations in Lake Michigan invertebrates and alewife that we have estimated for 1993 and 1994 should be viewed as predictions, testable as data become available. Because historic data on invertebrate PCB concentrations in Lake Michigan are exceedingly scarce, our estimates should be useful for studies that attempt to quantify the Lake Michigan PCB fluxes or future modeling efforts that attempt to incorporate multiple levels of the Lake Michiganfood web. INDEX WORDS:

Lake Michigan, invertebrates, bioenergetics, alewives, PCB.

Invertebrates that consume phytoplankton and zooplankton, and are themselves consumed by planktivorous fishes, are undoubtedly important in the trophic transfer of PCBs in the Lake Michigan food web. Indeed, Diporeia (previously Pontoporeia) and Mysis are thought to be important links in the transfer of contaminants from sediments to pelagic food webs (Jensen et al. 1982, Breck and Bartell 1988) but are not well quantified. Attempts to quantify the PCB cycle in Lake Michigan, whether by mass-balance budgets or simulation models, are hampered by few data on PBC concentrations in the invertebrates. Monitoring PCBs in invertebrates is a logical way of assessing PCB mobilization to the food web today. Assessments of change, whether with regard to flows or concentrations of PCBs in the food web, are invariably the

INTRODUCTION

Laboratory studies and analyses of survey data have demonstrated that trophic transfer is the major pathway for organochlorine contaminant accumulation in fishes (Gobas et al. 1993, Rowan and Rasmussen 1992, Rasmussen et al. 1990, Thomann and Connolly 1984), and ultimately in humans and wildlife that consume fishes. The PCB concentrations of sport fishes, and to a lesser degree, prey fishes, have been monitored because of their importance to human health (Dar et al. 1992, Fiore et al. 1985). Organochlorine binding to phytoplankton and other particles can be calculated using sorption kinetics that have been studied by a number of workers (Thomann and Connolly 1984, Swackhamer and Armstrong 1987, Eisenreich et al. 1989, Dean et al. 1993).

112

Reconstructed PCB Concentrations of Lake Michigan Invertebrates measure of success or failure of contaminant management policy. A comprehensive assessment of change necessarily requires estimates of invertebrate PCB concentrations, and thus must face the problem of virtually no historical data for comparison. Models exist that calculate PCB accumulation by fishes, given PCB concentrations in their food. In fact, levels of PCBs in zooplanktivorous fish and piscivores have been shown to be related to PCBs in their prey (Hesselberg et aI. 1990, Madenjian et aI. 1994). It is also possible to use these models to hindcast PCB concentrations in prey given the concentration in the predator. Thus, historical records of alewife PCB concentrations can be used to estimate historical alewife prey PCB concentrations. This paper applies this method to estimate temporal trends in PCB concentrations of Lake Michigan invertebrates using records of total PCB concentration in alewife.

Model Development Our procedure for hindcasting Lake Michigan zooplankton PCB concentrations used a computer simulation model composed of two submodels: (1) an alewife bioenergetics submodel, and (2) a submodel for estimating invertebrate PCB concentrations from alewife PCB concentrations (Fig. 1). The bioenergetics model is therefore a tool that provides a link between the PCB concentrations in alewives and their diet. The bioenergetics submodel was identical to that of Stewart and Binkowski (1986) for the alewife in Lake Michigan except for two modifications. First, we used diets that reflect seasonal and ontogenetic changes (Hewett and Stewart 1989) and second, we added a subroutine (detailed below) to account for PCBs in the alewife's diet. The bioenergetics model is a mass-balance where growth (G) is the net balance between consumption (C) and the sum of respiration (R), egestion (EG), excretion (EX) and specific dynamic action (SDA): G = C - (R + EG + EX + SDA)

(1)

All terms in the mass-balance are typically expressed as calories and are functions of body size and temperature. 1 July was day 1 of the model year. Our simulations did not include larvae and began with 0.161 g young-of-year fish on 20 August (simulation day 50) of their 0+ year. Years were divided into four time periods consisting of 51 (summer), 69 (fall), 231 (winter), and 14 (spring)

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1. Calculate best fit model (eq.6) to Wisconsin DNR alewife PCB time series (1976-1991)

~

2. For each model year calculate the median (Ax) and coefficient of variation (CV) of the alewife [PCB]

!

3. Empirically calculate the invertebrate [PCB) (Zx) necessary to lead to the fit alewife median [PCB) (Ax)

1

4. Produce an invertebrate population using empirical median (Zx) and alewife CV

!

5. Drive alewife bioenergetics model with yearly invertebrate population

1

Initialize alewife bioenergetics model

! Invertebrate consumption and growth (3 years)

! Accumulation of PCBs

1 6. Final output for comparison to original time series and analysis of invertebrate [PCB]

FIG. I. Diagram of the procedure used to hindcast historic PCB concentrations in Lake Michigan invertebrates based on the PCB record for alewives and their bioenergetics. See text for details.

days to account for variable seasonal growth dynamics, primarily the result of a seasonal cycle of body energy density (Flath and Diana 1985). We adjusted the proportion of maximum diet consumed (P-value; Table 1) so that simulated alewife growth (Fig. 2a) with the diets of Hewett and Stewart (1989) matched the growth trajectory presented in Stewart and Binkowski (1986). Alewives are assumed to seek the warmest water available without exceeding their preferred temperature; therefore, the model incorporates different temperature regimes for young-of-year, yearling, and adult fish. Norden (1967) estimated gonad mass of Lake

Jackson and Carpenter

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TABLE 1. Modeled growth (expressed as weight gain) of the average alewife in Lake Michigan and the proportion of maximum consumption (P-value) by growth season (simulation days) required to obtain such growth. The simi/ation began with 0.161 g post-larval fish on 20 August (simulation day 50) of model year l. Model year (age class)

Weight gain (g)

Summer

Fall

Winter

Spring

(1-50)

(51-119)

(120-351)

(352-365)

1 (a-I) 2 (I-II) 3 (II-III) 4 (III-IV) 5 (IV-V) 6 (V-VI) 7 (VI-VII) 8 (VII-VIII)

7.47 16.29 10.80 7.34 6.25 6.85 5.59 5.74

0.55 0.44 0.20 0.21 0.21 0.21 0.21 0.21

0.55 0.51 a 0.27 0.28 0.28 0.28 0.29 0.29

0.55 0.33 b 0.22 0.21 0.20 0.21 0.20 0.20

0.55 0.40c 0.18 0.18 0.18 0.18 0.18 0.18

Simulation days 51-76 Days 77-323 c Days 324-365

a b

801

:960 f-

I4Q

<.'J

~

20

A O'r-'--.---...,......--...,.--.---.---...,.----,-------,'

o

2

3

4

5

6

8

7

3r-----------------~

B

o

!,-,---r-~-____"T-____"T-___,--,__-_r____T

o

2345678 AGE (years)

FIG. 2. (A) Simulated growth oj an average Lake Michigan alewife based on the bioenergetics model of Stewart and Binkowski (1986) with diets modified to contain ontogenetic and seasonal variation as detailed in Hewett and Stewart (1989). The simulations began on 20 August (simulation day 50) with 0.161 g post larval fish. (B) PCB concentration of the average Lake Michigan alewife, calibrated to ca. 1980. Michigan alewives to be about 10.3% for females and 5.1 % for males. Stewart and Binkowski (1986) reported a 90% reduction in gonad mass between age III and age IV ripe and spent individuals. The average mass of gametes lost by all adults would

therefore be about 6.9%, and we have assumed that all adults lose 6.9% of their body mass at spawning. Growth was simulated to the lifespan of 8 years. See Stewart and Binkowski (1986) for more details on the specific components and parameters of the alewife bioenergetics model. The PCB accumulation subroutine considered consumption and egestion as the principal PCB input and output terms for the alewife. PCB uptake and excretion across the gills and loss via metabolism were considered negligible. Ignoring any exchange of PCBs between alewives and the water via the gills or excretion is consistent with experimental evidence that suggests that strongly hydrophobic chemicals like PCBs are primarily absorbed by fish from their food (Thomann and Connolly 1984, Gobas et aZ. 1993). Therefore, the daily increment in an alewife's body burden (MB, ~g PCB) was described as the net balance between uptake from the food (lPCB' ~g PCB), and loss by egestion (E pCB ' ~g PCB): (2)

Daily uptake from food was modeled as: (3)

where TI = total amount of ingested PCB (~g PCB) and EI = PCB assimilation efficiency (dimensionless; determination outlined below). TI was calculated as: n

T[ =

L (consumptiond * F; * [PCB]i) i=!

( 4)

Reconstructed PCB Concentrations of Lake Michigan Invertebrates where consumption d = daily consumption (g wet wt.), i is an index for diet items; F j = proportion of daily consumption that was diet item i (dimensionless), and [PCB]j = PCB concentration of diet item i (f..lg'g wet wt.- 1). Loss by egestion (EpCB ' f..lg PCB) was modeled simply as a fixed loss proportional to the amount of PCBs consumed:

5 ---- --- - ----------------------- - --

i; ~.

--:::~_• .:.~ • _~ ===~=="!

co 2 U1 a.

o ~

••

n

Estimating Historic PCB Content of Lake Michigan Zooplankton The PBC concentrations of alewives from the Wisconsin DNR time series (Fig. 3, circles; summarized raw data in Appendix A) represent composite samples of fish from the southern basin of Lake Michigan. The number of fish per composite sample and the sample sizes between years were not consistent, and there were some years for which no data were available (1977, 1980-82, 1989). Because we were interested in estimating PCB concentrations of the invertebrates comprising alewife prey, we used a model fit to the alewife PCB concentration time series to estimate a median (AX> PCB concentration (Fig. 1, steps 1 and 2; raw data in Appendix A) for each year of the time series (Stow et al., in press). This procedure also allowed an es-

~

m

~

~

~

00

~

YEAR

(5)

where L = 0.16. A 16% loss of consumed food by egestion is consistent with the bioenergetics model (Stewart and Binkowski 1986) and an 84% food assimilation efficiency is also in the range of published values of many fish (Brett and Groves 1979, Penczak 1992). PCB concentrations in the diet items were based upon estimates from ca. 1980 (Diporeia hoyi and Mysis relicta measured directly by Evans et al. 1982; copepods and cladocerans from modeled estimates by Connally and Thomann 1982). The PCB assimilation efficiency, EI' was determined empirically with these invertebrate estimates and alewife PCB concentrations collected by the Wisconsin DNR (Stow et al. in press). We have no information regarding PCB loss at spawning by alewives. As a first approximation, we assumed the PCB concentrations of eggs and sperm were similar to the body burdens and modeled PCB loss at spawning simply as a loss (6.9%) proportional to average weight spawned by both sexes (Stewart and Binkowski 1986). This is higher than the 1.9 % and 2.7 % PCB body burden lost at spawning by lake trout (Madenjian et al. 1993) and rainbow trout (Niimi 1983), respectively.

115

FIG. 3. Time series of PCB contents of Lake Michigan alewives (circles) from the approximate date of PCB ban (ca. 1975) until 1991. The line of best fit is the center line (equation 6 in the text); the outer two lines represent +/- 2 SE. See text for details of model parameters.

timation of alewife PCB concentrations for those years with missing data and to project the time series to 1994. A first order exponential decay with non-zero asymptote (Fig. 3) better described the Wisconsin DNR alewife time trend than either an exponential decay with zero asymptote or a double exponential model (Stow et al., in press). The model of best fit, with 2 SE in parentheses, is: PCB!

(mg'kg- 1) = 6.05 (6.08) e -0.32 (0.20)! + 0.53 (0.19) (6)

r 2 = 0.57, n = 40

where t = 0 at 1974. Because the Wisconsin DNR alewife time series lacks data for some years and the coefficient of variation is highly variable between years (ca. 17% to 105%), we chose to use the smoothed line of best fit and associated variability (Fig. 3) to estimate invertebrate PCB concentrations. For each year of the time series, we calculated the invertebrate PCB concentration that would be necessary to lead to the median alewife PCB concentration estimated by the model of best fit (eq. 6; Fig. 1 step 3). Mysis relicta, copepods, and cladocerans were scaled to Diporeia hoyi (with ratios equivalent to 1980 values), and the D. hoyi PCB concentration was adjusted until the invertebrate PCB concentrations, when run through the alewife bioenergetics model, resulted in an alewife PCB concentration that was within 1% of the median alewife PCB concentration (of the smoothed fit). We considered this calculated PCB concentration to represent the median diet (invertebrate) PCB concentration for each year. We assumed that inverte-

Jackson and Carpenter

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brate PCB concentrations, like PCB concentrations for individual fish that have been measured, were lognormally distributed with weight and that invertebrate and alewife PCB concentrations were equally variable (specifically, that they had equal coefficients of variation, CVs). The CV from the smoothed alewife distribution was therefore used as a first approximation of the invertebrate PCB CV. The estimated invertebrate median PCB concentration that lead to the median alewife PCB concentration, together with the estimated invertebrate CV were used to simulate 200 invertebrate PCB concentrations for each year of the time series (Fig. 1 step 4). These concentrations were used to drive the alewife bioenergetics model (Fig. 1 step 5). Because Lake Michigan salmonids selectively attack large alewife (Stewart and Ibarra 1991), we assumed the average Lake Michigan alewife was 3 years old, and hence alewife growth, for the purpose of determining PCB accumulation, was only simulated to 3 years. The output obtained from running the alewife bioenergetics model with the simulated invertebrate PCB concentrations was then compared to the Wisconsin DNR best fit model for alewife (Fig. 1 step 6).

RESULTS AND DISCUSSION

Alewife Growth and PCB Accumulation in Lake Michigan The calibration of our PCB subroutine to ca. 1980, given the established alewife bioenergetics parameters, resulted in a PCB assimilation efficiency of E1 = 0.40. This value is lower than assimilation efficiencies previously reported for lake trout in Lake Michigan (0.8; Madenjian et al. 1993) and for goldfish and guppies in laboratory experiments (ca. 0.6; Gobas et al. 1993), yet is consistent with a pattern of lower gross conversion efficiencies of mass for alewives compared to other fishes (Stewart and Binkowski 1986, Ursin 1979). The estimated PCB assimilation efficiency of 0.40 is lower than the food assimilation efficiency of 0.84 from the bioenergetics model. We have taken a parsimonious approach and allowed PCB assimilation to be a free parameter accounting for the net uptake of PCBs by alewives from their food based on PCB concentrations measured in invertebrates and alewives. Gross uptake might have an efficiency greater than 40 % with a concomitant loss via exchange across the gills (e.g., for low K ow congeners) or some other unmeasured depuration

mechanism. The nearly linear rise in PCB concentration following the initial increase (Fig. 2b) therefore reflects net PCB accumulation. The minor loss of PCBs (6.9%) at spawning has little effect on the overall trend. However, the amount of gametes shed by males and females ranges from about 5% (males) to 10% (females) of body mass, and thus some variation in PCB concentration might be explained by sex differences. The modeled accumulation of PCBs in Lake Michigan alewives (Fig. 2b) followed a less variable pattern than did growth (Fig. 2a). The rapid increase in PCB accumulation during the fall of model year 1 likely reflects alewife beginning their simulated life as post-larval fish with no PCB burden because our model excluded larval fish and began on 20 August (day 50 of the simulation) with fish weighing 0.161 g (Norden 1967) and containing no PCBs. Females lose some of their PCB mass during spawning and larval alewife feed on small zooplankton (Norden 1967) that would contain some PCBs; therefore, the latter assumption is not strictly true. However, we have no data to estimate the PCB content of larval alewives and it seems reasonable to assume that their PCB concentration is small relative to their PCB concentration as adults. Our model incorporates the ontogenetic and seasonal variation in alewife diets described by Hewett and Stewart (1989) and as a result, we found agespecific differences in the proportion of maximum consumption (P-value; Table 1) necessary to match the growth pattern of the original alewife bioenergetics model (Stewart and Binkowski 1986). Differences in P-values therefore reflect differences in diets between our simulation and that of Stewart and Binkowski (1986) as we have forced weight at age to be similar to theirs, but have used different diets that reflect ontogenetic and seasonal shifts in diet. Alewife growth is strongly influenced by the energy density of the alewife and its prey; hence, changes in diets would necessarily require a different consumption to achieve the same growth, assuming other factors, e.g., temperature, remained the same. However, alewife diets in Lake Michigan may, in fact, have changed since the mid-1980s. Bythotrephes cederstroemi invaded the Great Lakes by the mid-1980s, and was found in Lake Michigan alewife stomachs by the summer of 1988 (Keilty 1990). B. cederstroemi is a relatively large microcrustacean zooplankter that, at times, has been thought to be a large component of alewife diets in Lake Ontario (Mills et al. 1992). Thus, if there was a switch from Mysis and Diporeia to an alternative

Reconstructed PCB Concentrations of Lake Michigan Invertebrates prey such as B. cederstroemi, one might expect alewife growth rates to decline because microcrustacean zooplankton have a lower energy content than Mysis relicta and Diporeia hoyi (Hewett and Stewart 1989). There are no published data on PCB concentrations in B. cederstroemi, but because they feed on zooplankton they should have higher PCB concentrations than their zooplankton prey. Whether or not alewife PCB concentrations change with the inclusion of B. cederstroemi in their diet would depend on the PCB concentration of B. cederstroemi relative to Mysis and Diporiea. However, the potential interaction between decreased growth rates, which should tend to increase alewife PCB concentrations, and decreased diet PCB concentrations, which should tend to decrease alewife PCB concentrations, has not been explored. The mean alewife weight of 43 g used to calibrate our model (1980) represents an alewife of age 3+. Our model predicts that an alewife achieving its potential lifespan of 8 years in 1980 would have accumulated, on average, 2 mg PCB-kg- 1 (fresh weight). However, today's average alewife in Lake Michigan (and Lake Ontario) probably does not live longer than 4 years (Stewart and Ibarra 1991). The shorter lifespan, and reductions in prey PCBs since the early 1980s, would be expected to result in alewives having lower PCB concentrations. This is consistent with the observed trend of lower PCB concentration in alewives over the last 15 years (median total PCB in 1976 was 3.73 mg.kg- 1 compared to 0.55 mg-kg- 1 in 1991). We have run our model from 1974 onward, and in doing so assume that the decreasing pattern of PCB concentrations in zooplankton and alewives occurred at, or before, 1974. Other fish species in Lake Michigan have followed a decreasing pattern; for example, many salmonids show a decreasing pattern from at least 1974 onward (Stow et al., in press) and the bloater chub, a planktivore like the alewife, has been decreasing in PCB concentrations since at least 1972 (Hesselberg et al. 1990). Our extrapolation to 2 years prior to the time series therefore appears reasonable, although the uncertainty surrounding the extrapolated median PCB concentrations is large. Estimated Time Trend of Total PCB in Lake Michigan Invertebrates The estimated Diporeia hoyi median PCB concentrations (25th percentile; 75th percentile) range from 0.48 (0.37; 0.61) mg-kg- I wet wt. in 1974 to

117

0.04 (0.03; 0.05) mg-kg- 1 wet wt. in 1994 (Fig. 4). Scaling M. relicta, copepods, and cladocerans to D. hoyi assumes that the relative concentrations of these four groups have remained constant during our modeling scenario. We scaled PCBs of Mysis relicta, copepods, and cladocerans to Diporeia hoyi because there is no information on how invertebrate PCB concentrations between species/groups might have changed over time. Other systems, e.g., Lake Ontario, have a similar paucity of data on invertebrate PCB concentrations, and thus offer little insight on variability among invertebrate species although in the eastern and western basins of Lake Ontario the relative degree of contamination was found to be the same as in Lake Michigan with P. hoyi > M. relicta > copepods and cladocerans

4-

3]1 2 jl ;

I

I

Diporeia hoyi

II

I,

i

~ 11I

j

~~lW- ~ ~ ~ ~ ~ c;

-.lI::

01

1.5 cladocerans/copepods

1.0

.§, co 0.5

0

a..

t-

1

0.0

1

1

2.0 Mysis relicta

1.5 1.0 0.5 0.0

t---

1974

1

1978

1982

1986

1990

1

1

1994

MODEL (SfMUU\TION) YEAR

FIG. 4. Box and whisker plots illustrating the estimated 25th (bottom), median (center, joined), and 75th (top) percentiles of PCB concentrations of Diporeia hoyi, cladocerans, and copepods, and Mysis relicta in Lake Michigan reconstructed from historic PCB records for the alewife and its bioenergetics. The whiskers represent +/- 1.5 interquartile distances.

118

Jackson and Carpenter

(Whittle and Fitzsimons 1983, Borgmann and Whittle 1983). This ranking reflects different ecologies for the four groups: D. hoyi is a benthic amphipod, M. relicta occupies the hypolimnetic waters and feeds on algae, small zooplankton, and perhaps even benthic detritus and small Diporeia, and copepods and cladocerans occupy the water column and consume mostly phytoplankton. Our modeling scenario predicts that in 1994, the four groups of alewife prey we have considered contain levels of total PCBs at ca. 20-40 Ilg'kg-1 fresh weight in Lake Michigan (Fig. 4). Alewives consuming this diet should contain (-2 SE; + 2 SE) 0.62 (0.49; 0.79) mg total PCB'kg- 1 fresh wt. These concentrations suggest that PCBs are biomagnified (alewife PCB/invertebrate PCB) by factors ranging from 16fold for Diporeia to about 40-fold for cladocerans and copepods. These biomagnification ratios are higher than the ca. 1 to 3-fold ratios calculated by Stow et al. (in press) for the piscivore/planktivore step in Lake Michigan. Hence, biomagnification may be greater at the planktivore/invertebrate level than at the piscivore/planktivore level. We used the variability about the line of best fit for the Wisconsin DNR alewife time series as a first approximation to the variability of invertebrate PCB concentrations. This assumption likely overestimates the true invertebrate PCB variability. Stow et lli. (in press) indicated that weighting the alewife samples by the number of fish per composite artificially inflated the true alewife PCB concentration variance because analytical variance was effectively multiplied by the number of fish per composite. The population variability of the measured alewife PCB concentrations is problematic because the measured time series contains inconsistent sample sizes, inconsistent numbers of fish per composite sample, and lapses in the years when fish were sampled. Our assumption of equal variability of alewife and their prey PCB concentrations should therefore be viewed as a conservative hypothesis testable by measurements of PCBs in individual predators and prey. Interestingly, the alewife exponential decay coefficient, when applied to the invertebrate data of 1980, yields a time series for the invertebrate PCB concentrations that is similar to the input to the bioenergetics model (Fig. 4). Evidently the nonlinearities of the bioenergetics model do not cause large departures from the expectations of a simplistic exponential decay model. This finding suggests that a relatively simple model (e.g., first-order decay) may explain PCB dynamics in lower trophic levels.

Testable Predictions Our model, together with equation 6, can be used to produce three sets of predictions, testable as new data become available. A 3-year extrapolation of the model of best fit to the Wisconsin DNR alewife PCB time series predicts that alewife PCB concentrations will be between 0.46 and 0.62 mglkg fresh weight during 1993 and 1994. If measured alewife PCB concentrations do fall within this range, then the model of best fit will remain unchanged. Should alewife PCB concentrations not fall within the predicted bounds, then one of the alternative models explored by Stow et al. (in press) will likely be a better fit to the complete time series. The implications of the alternative models are discussed by Stow et al. (in press). The second set of predictions center around the invertebrate PCB concentrations we have estimated for 1993 and 1994. These predictions are made from the extrapolated alewife PCB concentrations over the same years and the model we have presented. If the predictions of invertebrate and alewife PCB concentrations are met, then our approach and our model parameters are validated. If the alewife predictions are met, but the invertebrate predictions are wrong, then our model, some component of our model, or one (or more) of our assumptions is wrong and will require re-examination. For example, if alewife diets or growth rates have changed over the last few years (e.g., due to Bythotrephes) or the assumption of constant ratios of the PCB concentration of the invertebrate prey is not valid, then the predicted PCB accumulation in the alewife may still be true (because it derives from the measured time series), but the invertebrate concentrations may differ from the predictions. It is possible that invertebrate and alewife PCB concentrations may differ from their predicted values and our model may still be valid if the bioconcentration ratios we calculated are correct (l6-fold for Diporeia to 40-fold for copepods and cladocerans) because the invertebrate PCB concentrations would have been derived from incorrect alewife PCB concentrations. Testing the accuracy of the predictions and model validity will occur by using new invertebrate PCB concentrations to drive the model, and accurately estimating the alewife PCB concentrations measured over the same time period. A third set of predictions could be made by using the estimated (hindcast) invertebrate PCB concentrations as input to a model for an alternative species of equivalent trophic level as the alewife

Reconstructed PCB Concentrations of Lake Michigan Invertebrates (e.g., the boater chub), to calculate a PCB concentration time series for that species. The calculated PCB concentration time series could then be compared to a measured time series. This would establish strong corroborating evidence that the historic invertebrate PCB concentrations we have estimated are reasonable estimates, because the bloater chub has different physiological rates (Le., its bioenergetic parameters are much different than the alewife's) and a different diet ratio of the invertebrates we examined, compared to that of the alewife. If the bloater time series can be accurately predicted, then there is independent support for the historic invertebrate PCB concentrations we have estimated. The estimated invertebrate PCB concentrations we have presented here are one possible scenario for the historic time trend of total PCBs in Lake Michigan invertebrates. The values we have projected for 1993 and 1994 should be thought of as testable predictions and compared to data as they become available. The values we have estimated will be important for future modeling efforts that

119

attempt to incorporate multiple levels of the Lake Michigan food web and that need to consider historic patterns of invertebrate PCB concentrations. ACKNOWLEDGMENTS We thank A. Trebitz and K. Cottingham for advice and suggestions with the programing. This study greatly benefitted from numerous conversations with C. Stow. An early draft of the manuscript benefited from thoughtful reviews of K. Cottingham, D. Schindler, C. Stow, and A. Trebitz. We would like to thank D.J. Stewart and P. Bertram for many constructive comments on the manuscript. This work was funded by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and from the State of Wisconsin. Federal grant NA90AA-D-SG469, project RlMW-41. This work was also supported by a Natural Sciences and Engineering Research Council of Canada post-doctoral fellowship to L.J.J.

APPENDIX A. Measured values, smoothed fit, and bioenergetics model output ofPCB contents (all concentrations are mg'kg- 1 fresh wt.) of Lake Michigan alewives. The number of composite samples (n) is indicated. Model Output Measured values Year 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

Median (n)

-2SD

+2SE

2.92 (2)

2.14

3.97

2.15 (13) 1.72

1.05 0.62

3.04 2.30

1.03 0.83 0.93 0.71 0.65 0.33

0.85 0.96

1.60 1.26

0.12 0.25 0.25

0.82 1.11 0.91

(6) (6) (1)

(4) (2) (2)

0.64 (1) 0.56 (1)

Smoothed fit -2SE Median 4.11 6.58 4.92 3.37 3.72 2.77 2.86 2.28 2.22 1.87 1.76 1.54 1.42 1.26 1.04 1.18 1.00 0.89 0.87 0.78 0.78 0.70 0.71 0.65 0.66 0.61 0.62 0.58 0.60 0.55 0.58 0.53 0.56 0.51 0.55 0.49 0.55 0.48 0.54 0.47 0.54 0.46

+2SE 10.53 7.21 5.01 3.57 2.62 2.00 1.60 1.32 1.12 0.97 0.86 0.80 0.71 0.67 0.65 0.63 0.63 0.63 0.62 0.62 0.62

Bioenergetics -2SE Median +2SE 7.65 5.98 9.78 4.85 3.57 6.26 3.01 3.96 5.10 2.81 3.56 4.50 2.79 2.15 3.64 1.61 1.25 2.08 1.32 1.04 1.68 1.30 1.03 1.64 0.93 0.73 1.92 1.15 0.89 1.47 0.92 0.72 1.18 0.82 0.66 1.58 0.80 0.63 1.49 0.76 0.60 0.76 0.68 0.53 0.88 0.51 0.40 0.65 0.58 0.46 0.73 0.81 0.62 1.06 0.73 0.57 0.94 0.62 0.48 0.81 0.62 0.49 0.79

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