Spatial variation in contaminants in fish from large river ecosystems : The Dark Side of riverine connectivity?

English Abstract:

Hydrological connectivity in large river ecosystems plays a key role in the life cycles of many species of fish. However, certain river areas or tributaries may also constitute important sources of contaminants that can be biomagnified by fish and transported throughout the river network. We used a biogeochemical approach, based on carbon stable isotopes, to investigate how Hg biomagnification in five species of fish is influenced by patterns of migration between a contaminated tributary and the main stem of the St-Lawrence River, Canada. To estimate trophic linkage to the Saint-Maurice river, a tributary where Hg in biota has been observed to be 2-3X higher than in the SLR, we first developed a two end-members mixing model based on large differences observed in δ¹³C of sessiles organismes collected in the SLR (δ¹³C≈ -20‰) and the Saint-Maurice (SMR δ¹³C≈ -30‰). Multi-species and single species models including trophic linkage to the SMR tributary, fish trophic position (based on nitrogen isotopes), and fish size as independant variables predicted 45-81 % of the observed variation in Hg. These results clearly show how stable isotopes can track the impact of extended trophic exposure to the highly contaminant SM river, resulting in high Hg levels in more sedentary species or individuals compared to migrants.