David J. Rowan

Why Don’t Great Lakes Fish Reflect Environmental Concentrations of Organic Contaminants? – An Analysis of Between-Lake Variability in the Ecological Partitioning of PCBS and DDT☆

We have reviewed the literature on PCBs and DDT in the Great Lakes ecosystem, in an attempt to explain between-basin and between-species variation in fish contamination. Empirical models were developed, using log-linear multiple regressions, to link tissue contaminant concentrations to environmental levels (water and sediments) as well as basin-specific ecological attributes. Concentrations of PCBs and DDT in sediment and water can explain between-basin variability in fish contaminant levels only when basin-specific ecological attributes are taken into account. The important factors that appear to determine the ecological partitioning of persistent organic contaminants are fish lipid content, trophic level of the fish in question, and the trophic structure of the food chain. Multiple regressions of these variables explain 59% (DDT) to 72% (PCBs) of the variation in contaminant concentrations of 25 species of Great Lakes fish.

The elimination of radiocaesium from fish

1. A general model for fish to predict the elimination rate of radiocaesium from a regression of water temperature, weight and a categorical variable for exposure (steady state, non-steady state), was developed from published data. 2. The generality of the new model for the elimination of radiocaesium from fish provides aquatic ecologists with the means to simplify greatly and improve existing methods for estimating ration for fish through the use of 137 Cs based bioenergetics models. 3. Comparisons of the general model with other less complete data suggests that the model is applicable to many of the common taxa of north temperate fish. 4. Fish whose tissue pools are equilibrated to ambient concentrations (steady state) appear to eliminate radiocaesium at a rate much slower than those in non-steady state. Application of previous laboratory models to steady state field situations would overestimate the elimination rate of radiocaesium by 1.4-fold.

The fate of radiocesium in freshwater communities - why is biomagnification variable both within and between species?

In the Ottawa River, levels of 137Cs exhibit classic biomagnification with about a 4-fold increase with each trophic level. For a given trophic level, organisms with a benthivorous diet have about 2-fold higher 137Cs levels than planktivorous organisms of the same trophic level. Lower Bass Lake exhibits a similar pattern of biomagnification, as does Great Slave Lake. 137Cs concentrations in 18 species of fish from the three systems average about 3-fold those of their food, and always exceed the concentration of 137Cs in their food. However, there are no significant differences in biomagnification within functional groups or between functional groups and systems. These results conclusively demonstrate food-chain biomagnification of 137Cs. Within species variation in biomagnification follows three patterns: linear increase with age (4 species); higher biomagnification in mature fish than immature fish, increasing in a step-like pattern (3 species); no increase with age (1 species). These patterns can be explained by the ratio of assimilated consumption to growth and elimination: a linear increase in biomagnification occurs when the ratio of assimilated consumption to growth and elimination increases with age; a step-wise increase in biomagnification at maturity occurs when the ratio of assimilated consumption to growth and elimination increases at maturity; no increase in biomagnification with age occurs when the ratio of assimilated consumption to growth and elimination remains constant throughout the fish lifecycle. This explains why no consensus has been reached regarding 137Cs biomagnification and fish age and points to the predominance of consumption rates in determining the biomagnification of 137Cs by fish. Thus, to accurately predict 137Cs concentrations in fish under steady-state or dynamic conditions, consumption rates need to be known.

Wave Velocity Thresholds for Fine Sediment Accumulation in Lakes, and Their Effect on Zoobenthic Biomass and Composition

Deposition of fine sediments occurs at depths where water velocity at the bottom is less than the threshold velocity required to keep the particles in suspension. In this paper we demonstrate that the depositional boundary depth (DBD) can be predicted from models based on wave height and lake morphometry, and that such depositional boundaries greatly affect the biomass and composition of the zoobenthos. Lake Memphremagog was chosen for this study since it exhibits a great deal of variation in exposure and bottom slope, which make the DBD highly variable from place to place. In spite of this variation, our model predicted its location very accurately even along transects with highly complex topography. Sites that our models predicted would be depositional, always had fine sediments and had more than twice the zoobenthic biomass of non-depositional sites, which had sediments made up of silt or silty-sand. Epilimnetic sites had more than twice the average biomass of hypolimnetic sites of the same depositional type. PCA on the benthic community data ordinated our study sites along 2 distinct components: Component I reflected mainly the thermal environment and had positive loadings from epilimnetic taxa; Component II reflected mainly the depositional regime, and had positive loadings from most taxa. Most zoobenthos in lakes depend on sedimenting fine seston and its associated microflora for food. The close relationship between the depositional regime and the abundance and composition of the zoobenthos indicates that the sedimentation behavior of seston is very similar to that of fine clay-sized inorganic particles. This similarity allows us to use models developed for clay particle deposition to predict zones of high benthic productivity.

Bioaccumulation factors and the steady state assumption for cesium isotopes in aquatic foodwebs near nuclear facilities.

Steady state approaches, such as transfer coefficients or bioaccumulation factors, are commonly used to model the bioaccumulation of (137)Cs in aquatic foodwebs from routine operations and releases from nuclear generating stations and other nuclear facilities. Routine releases from nuclear generating stations and facilities, however, often consist of pulses as liquid waste is stored, analyzed to ensure regulatory compliance and then released. The effect of repeated pulse releases on the steady state assumption inherent in the bioaccumulation factor approach has not been evaluated. In this study, I examine the steady state assumption for aquatic biota by analyzing data for two cesium isotopes in the same biota, one isotope in steady state (stable (133)Cs) from geologic sources and the other released in pulses ((137)Cs) from reactor operations. I also compare (137)Cs bioaccumulation factors for similar upstream populations from the same system exposed solely to weapon test (137)Cs, and assumed to be in steady state. The steady state assumption appears to be valid for small organisms at lower trophic levels (zooplankton, rainbow smelt and 0+ yellow perch) but not for older and larger fish at higher trophic levels (walleye). Attempts to account for previous exposure and retention through a biokinetics approach had a similar effect on steady state, upstream and non-steady state, downstream populations of walleye, but were ineffective in explaining the more or less constant deviation between fish with steady state exposures and non-steady state exposures of about 2-fold for all age classes of walleye. These results suggest that for large, piscivorous fish, repeated exposure to short duration, pulse releases leads to much higher (137)Cs BAFs than expected from (133)Cs BAFs for the same fish or (137)Cs BAFs for similar populations in the same system not impacted by reactor releases. These results suggest that the steady state approach should be used with caution in any situation where reactor releases are episodic or pulse in nature, even if the magnitude of these releases is small.

Development and evaluation of a regression-based model to predict cesium-137 concentration ratios for saltwater fish

Data from published studies and World Wide Web sources were combined to develop a regression model to predict (137)Cs concentration ratios for saltwater fish. Predictions were developed from 1) numeric trophic levels computed primarily from random resampling of known food items and 2) K concentrations in the saltwater for 65 samplings from 41 different species from both the Atlantic and Pacific Oceans. A number of different models were initially developed and evaluated for accuracy which was assessed as the ratios of independently measured concentration ratios to those predicted by the model. In contrast to freshwater systems, were K concentrations are highly variable and are an important factor in affecting fish concentration ratios, the less variable K concentrations in saltwater were relatively unimportant in affecting concentration ratios. As a result, the simplest model, which used only trophic level as a predictor, had comparable accuracies to more complex models that also included K concentrations. A test of model accuracy involving comparisons of 56 published concentration ratios from 51 species of marine fish to those predicted by the model indicated that 52 of the predicted concentration ratios were within a factor of 2 of the observed concentration ratios.