Temporal trends of mercury and organohalogen contaminants in great blue heron eggs from the St. Lawrence River, Québec, Canada, 1991-2011, and relationships with tracers of feeding ecology.

Since 1991, great blue heron (Ardea herodias) eggs have been collected and analyzed for mercury (Hg), persistent organic contaminants (OCs), brominated and non-brominated flame retardants (FRs) as well as stable isotopes δC and δN. In the present study, temporal trends of contaminants were analyzed in eggs sampled in four regions along the St. Lawrence River (Quebec, Canada) and inland sites using new and previously published data. Most contaminants declined significantly over time in most regions. Globally, the highest annual change, -17.5%, was found for pp'-DDD, while the smallest annual decline, -0.54%, was observed for Hg. Concentrations of ΣDDT and ΣFR (sum of 8 congeners) decreased by -11.6% and -7.3%, respectively. Declines in ΣPCBs differed among regions, from -5.6% in the fluvial section to -14.7% in the inland region. The highest concentration of ΣFR was measured in eggs from Grande Ile in the fluvial section of the river in 1996 (2.39μg/g). Stable isotope ratios also showed temporal trends in some regions: δC decreased in the fluvial section and increased in Gulf region, while δN decreased in the fluvial section and increased in the upper estuary. Significant positive relationships were found between ΣDDT, ΣPCBs and ΣFRs and δN and δC in freshwater colonies, but not in estuarine or marine colonies. These results suggest that changes in trophic level and foraging areas over time were influential factors with respect to contaminant burden in great blue heron eggs in the fluvial section, but not in the other regions.