Assessing Trends in Organochlorine Concentrations in Lake Winnipeg Fish Following the 1997 Red River Flood

As we move toward the virtual elimination of persistent organic pollutants (POPs) in the environment our understanding of how short-term variability affects long-term trends of POPs in natural populations will become increasingly more important. In this study we report short-term trends in organochlorine (OC) levels in fish from Lake Winnipeg in the months and years following the 1997 100-year flood of the Red River ecosystem. Our goal was to understand the effects of an episodic event on OC levels in benthic and pelagic invertebrates and in fish. Despite elevated loading of OCs into the south basin of Lake Winnipeg during the flood there were no differences in OC levels of surface sediments or emergent mayflies. After adjusting for differences in lipid content and length among sample times, we did find significant increases in total DDT (ΣDDT) and total polychlorinated biphenyl (ΣPCB) post-flood (March 1999) in top predators including walleye and burbot. Significant increases were also observed in OC concentrations of zooplankton and yellow perch (> 2 fold in ΣPCB, ΣDDT, total chlordane (ΣCHL), total chlorobenzenes (ΣCBZ)) and walleye (1.4 fold ΣPCB) over a 2-month period in the summer following the flood. Analysis of specific congener patterns over time suggest that the major changes in fish OC levels pre- and post-flood did not appear to be linked to transport of new compounds into the Lake during the flood, but to species shifts within the plankton community. Our results indicate that short-term variation (∼2 months) in OC distributions within biota may be equal to or greater than those resulting from episodic events such as spring floods.