Sediment implication in shift in nutrient limitation and impact on purification capacity: Application on a common Atlantic east coast pond

Salt marshes are widely recognized as regulation areas of nutrient flows to the ocean. However, poor knowledge of nutrient dynamics in these environments counteracts the understanding of nutrient regulation within the land-sea continuum, previous works mainly focusing on the water compartment. To overcome this gap, an experimental pond was monthly (open system) and weekly (closed system) monitored between February and October 2021. Bio(geo)chemical parameters (in water and sediment) and nutrient fluxes at the sediment-water interface (SWI) were relevantly explored to highlight the coupling between the benthic and pelagic compartments. A significant shift from P limitation (February-June period) to N limitation (July-October period) was observed and attributed to the phosphate mobilization (named soluble reactive phosphorus, SRP) processes in the sediment and its effluxes toward the water column. This SRP mobilization was strongly linked to the respiration of iron oxides for mineralization of organic matter (OM) and anammox processes. Surprising high carbon production in September (>1200 mgC m^-3 d^-1) in a low nutrient concentration range indicated significant regenerated N production and potentially new P production. Monitoring in enclosed system indicated a 3-week water retention time for a maximum nutrient removal and minimal impact for water export to the coastal waters. In non-bloom period, sediment SRP efflux from the sediment could contribute up to 40 % of SRP increase in the water column in September and to 60 % of the P-production demand in October. This work highlighted the importance of sediment for water nutrient re-supply and the temporality of water retention time in pond system to optimize purification effect and limit risks of eutrophication in coastal waters. While the eutrophication potential associated with nitrogen seemed to be reduced in such pond, that of phosphorus needs to be more considered.