Depuration of PCBs in the Lake Michigan ecosystem

A suite of physical processes controls the rate at which polychlorinated bi phenyl (PCB) concentrations change in Lake Michigan fauna. To the extent th at these processes are homogenous in space, predator and forage fish specie s are exposed to the same water column and sediment concentrations of PCB i n Lake Michigan, whereas exposure due to diet varies with trophic position. We can think of observed PCB concentrations in a particular species as rea lizations of an unknown process consisting of many sources of variability. Because all species are utilizing the resources of the same ecosystem, a co mponent of variability of PCB concentration is shared by all organisms in t he system, whereas other components are not. Individual species were modele d with parameters that were estimated as the overall, or reference mean and a shared rate of change, plus an individual species offset from that refer ence 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 determin e the extent to which observed declines in PCB concentrations in various sp ecies in the Lake Michigan ecosystem are similar and synchronous. A high de gree of similarity and synchrony between species would imply a high degree of spatial homogeneity of the physical processes controlling PCB concentrat ions in Lake Michigan fauna. A lack of synchrony suggests that other factor s, such as food web interactions or spatial patterns, may be important.