Isotopic study of contaminant biomagnification and fish growth in the St. Lawrence River food web

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Several water masses with very distinctive characteristics make up the fresh-water section of the St. Lawrence River (SLR). In particular, carbonate-rich 'green' waters from the Great Lakes and the 'brown' waters from the Ottawa River draining the Canadian Shield stand out. These waters do not mix subtantially until they pass Lake Saint-Pierre, near Trois-Rivières. Green and brown waters differ in productivity, transparence, and contaminant loads. Using the research vessel Lampsilis, a unique platform to sample shallow areas of the SLR, we will collect invertebrates and six species of fish. State-of-the-art techniques such as stable isotope analysis and molecular genetics will allow us to identify fish species and age classes of fish that live in either one of the water masses or continually travel between them. We will test the hypothesis that affiliation to a water masse inferred from isotopic analyses can be used to predict concentration of mercury in fish of the SLR, a ubiquitous contaminant in this ecosystem. If confirmed, this result would then enable us to model contaminant fluxes from various water masses in the SLR from hydrological data, thus predicting a complex ecological variable (contaminant concentration) from a simple one (hydrology). Likewise, we will investigate if fish living in different water masses show different patterns in growth rate.