Patterns in riverine carbon, nutrient and suspended solids export to the Eastern James Bay: links to climate, hydrology and landscape

The Eastern James Bay has been undergoing major shifts in its physical, chemical, and ecological functioning over the past decades, driven by a combination of climate and landscape changes, and human intervention that includes the damming of major regional rivers. Rivers play a role in the functioning of the Bay, delivering not only freshwater and key materials but also transmitting environmental and climatic signals. Here we present a study of the spatial variability of riverine export fluxes and yields of freshwater, dissolved organic carbon (DOC), total suspended solids, total nitrogen, and total phosphorus of 18 major rivers flowing into Eastern James Bay, in the boreal region of Québec (CA). We characterized discharge patterns and identified and modeled the main landscape drivers of riverine material concentrations in these watersheds. We then combined these to model the contribution of unsampled watersheds to generate a regional budget of riverine freshwater and material export to the Bay. The peak of water export occurs in spring for free-flowing rivers, whereas for the dammed river La Grande, which accounts for half of the freshwater inputs, the peak discharge was shifted to winter due to high energy demands. The large gradient of watershed areas, discharge and environmental conditions resulted in the wide range of material concentrations across these boreal rivers, and we show that overall, the James Bay is a hot spot of DOC loading to the entire Hudson Bay System. We further reconstructed past (pre-damming) riverine export and compared this with current, and potential future scenarios, and we demonstrate that damming and climate change are impacting the patterns of water and material export of these boreal rivers to the Eastern James Bay.