Spatial heterogeneity and biological productivity in a large river ecosystem

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The long-term objective of my research program is to model interactions between physical and chemical factors and lower food web processes in the St. Lawrence River (SLR). Identifying the primary sources of carbon supporting food webs is a fundamental step towards understanding the structure and function of aquatic ecosystems. Some studies have argued for a dominant role of terrestrial carbon and heterotrophy in river food webs while others have stressed the greater importance of in situ photosynthetic sources. In either case, the transported matter in the river is transformed by light or bacterial activity which affects its bioavailability and flow to downstream consumers. This study aims at understanding how the transfer of matter (e.g. nutrients, phytoplankton) and organisms between habitats (connectivity) is controlling food availability and biodiversity, important for determining carbon sources supporting food webs and productivity in large river ecosystems. Lac St. Pierre (LSP) is the largest fluvial lake (~400 km2) of the SLR and is well recognized for its high productivity. However, there is a large gap in knowledge about the fate of matter leaving LSP into the fluvial estuary, which connects to the estuarine transition zone (ETZ). The ETZ is light-limiting for photosynthesis and marks where freshwater meets saltwater. It is recognized for its very high productivity, and as a large nursery area for several fish species. I hypothesize that much of the productivity in the ETZ is dependent on transported material from the fluvial estuary, which in turn receives high dissolved organic matter and nutrients from LSP. I recently established that LSP can contribute significantly to downstream productivity through large populations of ice-algae drifting during the ice-break period in spring. This finding opens a totally new perspective for understanding aquatic productivity in large rivers. This NSERC program on the SLR ecosystem integrates:  1) ETZ as a key habitat of large rivers, acting as a gateway to the sea through the exchanges of carbon and sediments; and 2) winter biological activities in north-temperate rivers, in which river ice biota may substantially contribute to annual primary productivity and carbon transfer along the food web.