Legacy contaminants in waterbird eggs: divergent temporal and spatial trends in the St. Lawrence River, Canada

Waterbirds are frequently used as indicators of environmental pollution in aquatic ecosystems. However, species accumulate contaminants differently and can provide insights to diverse aspects of ecosystem health. In this study, we compared temporal and spatial trends of legacy contaminant levels (total mercury [Hg], hexachlorobenzene [HCB], p,p'-dichlorodiphenyldichloroethylene [DDE], dieldrin, Mirex, and polychlorinated biphenyls [PCBs]) in eggs of two species collected between 1972 and 2022: American Herring Gulls (Larus smithsonianus) and Great Blue Herons (Ardea herodias). We evaluated their use as environmental indicators in fluvial and marine sectors of a large drainage system with a long history of anthropogenic disturbance, the St. Lawrence River. Generalized Additive Models were applied to investigate the temporal and spatial trends due to the flexibility in modeling non-linear patterns. There were significant decreases in the concentrations of most contaminants over five decades, except Hg, in both species, and DDE and dieldrin in herons in the marine sector of the St. Lawrence River. Gulls had higher Hg and PCBs concentrations in the fluvial sector, and HCB and Mirex in the marine sector of the river, while herons only showed higher Hg concentrations in the marine sector. When comparing between sectors, PCBs were higher for both species in the fluvial sector than the marine sector. We also found higher concentrations of HCB in herons and Hg in gulls in the fluvial compared to the marine sector of the river. Declines in concentrations of legacy contaminants over the past five decades demonstrate gradual ecological recovery of the ecosystem in the St. Lawrence River, while differences in accumulation patterns between species over time reflect variations in availability and pathways of contaminants within different sectors of the St. Lawrence River.