Depuration of PCBs in the Lake Michigan Ecosystem

A suite of physical processes controls the rate at which polychlorinated biphenyl (PCB) concentrations change in Lake Michigan fauna. To the extent that these processes are homogenous in space, predator and forage fish species are exposed to the same water column and sediment concentrations of PCB in Lake Michigan, whereas exposure due to diet varies with trophic position. We can think of observed PCB concentrations in a particular species as realizations of an unknown process consisting of many sources of variability. Because all species are utilizing the resources of the same ecosystem, a component of variability of PCB concentration is shared by all organisms in the system, whereas other components are not. Individual species were modeled with parameters that were estimated as the overall, or reference mean and a shared rate of change, plus an individual species offset from that reference mean and shared rate of change. This approach allows species-specific differences to be manifest via the offset, whereas the shared components of variability are known with greater precision than would be possible if we considered only a single species. The objective of this work is to determine the extent to which observed declines in PCB concentrations in various species in the Lake Michigan ecosystem are similar and synchronous. A high degree of similarity and synchrony between species would imply a high degree of spatial homogeneity of the physical processes controlling PCB concentrations in Lake Michigan fauna. A lack of synchrony suggests that other factors, such as food web interactions or spatial patterns, may be important.