Les transformations microbiennes de l’azote dans les grandes rivières
Rivers receive nitrogen (N) from their watershed and are the final sites of nutrient processing before delivery to coastal waters. Transformations of dissolved inorganic N (DIN) to gaseous N are highly variable and can impact both coastal eutrophication and greenhouse gas emissions. With anthropogenic N loading to aquatic ecosystems on the rise, nitrous oxide (N2O) emission from rivers should increase. Direct measurements of N2O from lake St. Pierre (LSP), an enlargement of the St. Lawrence River (SLR) indicate that although LSP is a net atmospheric source of N2O to the atmosphere fluxes are low compared to others rivers. Emissions are seasonally and inter-annually highly variable due to changes in hydrological conditions. N2O: N2 is also influenced by hydrology and lower ratios are observed in conditions of higher water discharge and elevated N charge into the ecosystem.   In a cross system analysis, hydraulic load mitigates the relation between annual N2O flux and nitrate concentrations in rivers.  In SLR, cyanobacterial mats colonizing low nitrate areas are a net source of N with high negative di-nitrogen (N2) fluxes. Given that fixation occurred during daylight and that oxygen concentrations in the water column were supersaturated, we hypothesize that N2 fixation is performed by the dominant cyanobacteria in anoxic micro-zone of the mat and/ or possibly by heterotrophic diazotrophs. Our estimates indicate that N fixation in the mats account for the replacement of up to 33% of the N loss via denitrification in the entire ecosystem during the study period.  In the tidal Hudson River N2 production is highly variable between vegetated shallows and was associated with species-driven differences in dissolved oxygen (DO) dynamics during the ebb tide. N2 production was extremely high in invasive floating-leaved plants (Trapa natans) but was insignificant in submersed native vegetation. An estimate of summertime N2 production in Trapa beds suggests that these beds are a major seasonal hotspot for N removal.  Large Trapa beds represent only 2.7% of the total area of the tidal Hudson but they remove between 70 and 100% of the total N retained in this section of the river during summer months and contribute to as much as 25% of the annual N removal.
Nitrification Denitrification Nitrification Invasive species Ecosystem services Nitrogen limitation Cyanobacteria Nitrogen fixation nitrous oxide yield