Seasonal dynamics of PCB and toxaphene bioaccumulation within a Lake Michigan food web

Seasonal variations in PCB and toxaphene concentrations were measured in bulk zooplankton, mysid shrimp, benthic amphipods, alewife, and bloater chub collected from Grand Traverse Bay, Lake Michigan between April and September of 1997 and 1998. Concentrations of PCBs in the dissolved phase of the water column were consistent over time; however, seasonal changes in contaminant concentrations within the biota were significant. Seasonal changes were most pronounced in zooplankton, which displayed highest PCB burdens in April and decreased by as much as 75% through September, coincident with changes in phytoplankton biomass, species composition, and changes in the particulate pools of PCBs within the water column. Mysis sp. display a similar PCB trend as zooplankton, while Diporeia sp. displayed maximal PCB concentrations in late summer during 1997. PCB trends in the primary forage fish alewife (Alosa pseudoharengus) and bloater (Coregonus hoyi) were correlated more to shifts in lipid content and seasonal diet preferences. PCB concentrations were higher (p < 0.01) in bloater (310 +/- 98 ng/g wet weight) than alewife (233 +/- 70 ng/g wet weight), however alewife possessed higher toxaphene burdens (198 +/- 72 ng/g wet weight) than bloater (88 +/- 36 ng/g wet weight). Alewife contaminant burdens were high in spring and fall of both years, decreasing by as much as 60% in midsummer, and were reflective of changes measured in their lipid content associated with gamete production and spawning. These results suggest that despite invariant concentrations of the dissolved Pool of PCBs within the lake, accumulation of PCBs by the biota on seasonal scales is controlled appreciably by growth and lipid dynamics, and foraging behavior.