Impact of water level management on organic carbon availability and nitrogen transformations in wetland sediments

The impact of water level management via water retention on benthic carbon and nitrogen fluxes was studied in a wetland of the Seine estuary. Carbon and inorganic nitrogen fluxes at the sediment-water interface were determined during periods of intermittent and permanent immersion along a lateral gradient. In addition to fluxes, nitrate reduction rates, quantity and quality of both sedimentary and dissolved organic carbon, and organic matter lability via external enzymatic activities were analyzed. During both periods, the sediments subject to water level management facilitated nitrogen removal, with potential NO₃^- fluxes averaging -109 ± 31 nmol NO₃^- cm^-2 h^-1 under permanent immersion and -34 ± 13 nmol NO₃^- cm^-2 h^-1 under intermittent immersion. During permanent immersion, more water retention favors a higher input of dissolved organic matter including fresh and labile compounds, which most likely explained the significantly higher NO₃^- influxes. Intermittent immersion resulted in a lower quantity of retained dissolved organic matter, which likely explains the low N fluxes. The results of this study indicate the implementation of water retention strategies can markedly enhance NO₃^- removal by increasing the availability of organic matter. This underscores the importance of considering water-level management of wetlands to sustain the ecological functions of these valuable ecosystems, which are often the first barriers against environmental disturbance.