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GLERL 2001 Milestone Reports
GOAL: SUSTAIN HEALTHY COASTS
OBJECTIVE 2: Promote Clean Coastal Waters to Sustain Living Marine Resources and to Ensure Safe Recreation, Healthy Seafood, and Economic Vitality
PM: Number of coastal and Great Lake states provided with improved predictive capabilities and understanding of coastal processes
Milestone: Complete and report on studies of the impact of seasonal factors on the bioaccumulation of PCB congeners by the amphipod Diporeia spp. in the Great Lakes.
Scientist: P.F. Landrum GLERL
Purpose: Because sediment-associated contaminants are likely a major source of persistent organic contaminants to the food web in the Great Lakes, establishing the factors that influence their accumulation and improving prediction of their contribution to the to the food web is critical to predicting the accumulation and effects of persistent contaminants to the food chain upper trophic levels. As Part of the US EPA Lake Michigan Mass Balance Program, a food web model was to be constructed to predict the accumulation of polychlorinated biphenyls, one class of persistent organic contaminants, in the top predators of the food web. However, one of the critical connections the sediment-pelagic coupling via Diporeia remained a weak link in the model. The infaunal amphipod, Diporeia spp, is an extremely important food web component in the Great Lakes. It is a major prey item for most of the fish species at some stage in their lifecycle. Thus, Diporeia serve as a major source of sediment-associated contaminants to the food web. Other work at GLERL had established several of the fundamental factors that govern accumulation by the organism and pointed to a potentially strong seasonal variation in Diporeia accumulation of persistent organic contaminants. This work was designed to directly examine the seasonal signal in the accumulation process and to determine the importance of its incorporation into a food web model.
Efforts: Diporeia, surficial sediment, and settling detritus were collected monthly while water and suspended particles were collected quarterly from 1997 and the first quarter of 1998 at a 45 m station off Grand Haven, MI. Congener specific PCB concentrations were determined by gas chromatography with electron capture detection in each of the samples along with the organic carbon content of the sediment and suspended sediments and the lipid content of the Diporeia. The data were evaluated for quality and compared to two steady state models for PCB accumulation by Diporeia.
The seasonality of bioaccumulation expressed as the biota-sediment accumulation factor (BSAF) was clearly observed for the PCB accumulation by Diporeia (Figure 1). The BSAF is the concentration in the organism normalized its lipid content divided by the sediment concentration normalized to its organic carbon content.
Figure 1. The seasonal variation in BSAF values for PCB congeners in Diporeia are displayed for congeners of increasing hydrophobicity as described by the log of the octanol-water partition coefficient (log Kow). In addition to displaying the octanol water partition coefficient for each congener, the congener number is provided for additional reference.
Equilibrium partitioning (EqP) theory indicates that if the organism is at steady state with the sediment then the BSAF should be constant and approximately 2. The theory was propagated on the organism being at steady state while the sediment particles are at chemical equilibrium with the interstitial water. In Figure 1, it is clear that the observed BSAF ranged from less than 0.5 to greater than 3 for various PCB congeners and that there was a seasonal variation in BSAF. Both of these findings are contrary to the expectations of the EqP theory. Before any evaluation of EqP theory could be made, it was important to demonstrate that the organisms were at steady state. Two steady-state models developed from laboratory kinetics were used to predict PCB concentrations by congener in Diporeia and compared to the field data. The simple temperature dependent BSAF model was useful but a new steady-state model based on incorporating new data on feeding behavior improved the prediction.
Where Ca is the concentration in the organism, ku is the uptake rate coefficient from water, Cw is the concentration in water, F is the feeding rate, AE is the absorption efficiency from ingested food, Cf is the concentration in the food and ke is the elimination rate coefficient. The slope of the predicted concentration versus measured concentration ranged from 0.6 to 1.5 when the concentration in the surficial sediment was used as the food source. It is clear from these analyses that the Diporeia are at steady state with the exposure environment. Further, if settling detritus is used as the food source, the slopes ranged from 1 to greater than two. This likely results from a slower feeding rate on the freshly settling detritus, which would contain a more nutritious food quality. If the feeding rate is adjusted then there is good comparison between the two materials, surficial sediment and settling detritus as a food source for Diporeia (Figure 2). Thus, there was good prediction across season less than a factor of two for the accumulation of PCB by the amphipod, Diporeia.
Figure 2. After adjusting for differences in feeding rate, both settling detritus and surficial sediment are equally useful for predicting the bioaccumulation of PCB congeners by Diporeia spp.
Customers: This work is of particular interest to the Great Lakes scientific community to better establish the role of sediment-associated contaminants in food web transfer. It is specifically of interest to the US EPA to parameterize their food web model. It is also of interest NOAA and the USACOE to provide improved predictions of the bioaccumulation of sediment associated PCBs in infaunal benthos and the factors that influence those processes.
Significance: This is the first effort to examine the seasonal variation in the bioaccumulation of sediment -associated contaminants on a seasonal basis for any infaunal benthos and to examine the potential sources of bioaccumulation for organisms that may have several exposure sources, overlying water, settling detritus and surficial sediments. This also represents a useful data set for testing the utility of equilibrium partitioning theory, as advocated by the US EPA, for predicting the accumulation of non-polar persistent organic contaminants and comparing the utility of two laboratory models for bioaccumulation of PCB congeners by this important food web organism.
Success: This work clearly demonstrated the importance of seasonality in the bioaccumulation of sediment-associated contaminants. We were also successful in applying a steady state model to predict the concentration of sediment associated PCBs to Diporeia on a seasonal basis. Further the above results suggest some limits to the predictability of bioaccumulation (exposure) via the EqP theory, which is the basis for the US EPA prediction of exposure to sediment-associated non-polar contaminants.
Next Steps: Manuscripts on the study are being generated and the new findings will be incorporated into the models of bioaccumulation from sediments. This effort will be combined with studies investigating the differences in accumulation among sediments from different stations in Lake Michigan and the continued evaluation of the factors that drive the bioaccumulation of persistent organic contaminants from sediment to improve our predictability for a wide range of organisms and sediments.
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Last updated: July 9, 2002 mbl