Hydrological and Landscape Features Driving Riverine Carbon, Nutrients and Suspended Solids Export to the Eastern James Bay

Abstract:

Regional runoff and river discharge have been identified as major drivers of carbon and nutrients export from land to the oceans, but export is further modulated by patterns of material leaching from land, which depend on geology, land cover and human activities across watersheds. Hydropower projects and climate change impacts can lead to significant shifts in both terrestrial leaching and discharge patterns, and therefore on fluvial exports to coastal ecosystems, with further consequences for their functioning and productivity. Here we examined the spatial and seasonal concentrations and export of dissolved organic carbon (DOC), total suspended solids (TSS), and nutrients (total nitrogen and phosphorus – TN and TP) from natural and regulated rivers flowing into the James Bay in the boreal region of Québec, and identified the main watershed-scale drivers of their variability. We performed seasonal sampling of eighteen rivers (ranging from Strahler order 4 to 10) over a 600 km latitudinal transect, encompassing a large gradient of watershed areas, streamflow and environmental conditions, which ultimately resulted in a wide range of material concentrations across rivers. DOC increased as a function of wetland coverage in the watershed, and both DOC and TN concentrations declined with increasing proportion of water coverage within the watersheds. TSS and TP were very variable across rivers and seasons, but strongly positively correlated to each other (but not to TN and DOC). The peak riverine export occurred in spring for natural rivers, while for La Grande dammed river, the largest one, the streamflow shifts towards high energy demands in winter. Although large rivers export the greatest absolute amounts of materials to the Bay, driven by the magnitude of their discharges, smaller rivers typically have much higher material concentrations and generate local hotspots of DOC, TSS, or nutrients within coastal habitats, with major consequences to the functioning of these ecosystems.