Influence of Potamogeton pectinatus and microphytobenthos on benthic metabolism in a freshwater littoral sediment in an agricultural landscape: N assimilation vs N removal

The influence of Potamogeton pectinatus colonization on benthic nitrogen dynamics was quantified in the littoral zone of a lowland pit lake with high nitrate availability (~200 µM). Our hypothesis was that in aquatic environments where nitrogen availability is not limiting, colonization by rooted macrophytes changes the dynamics of the benthic nitrogen cycle, stimulating N assimilation and denitrification, and increasing the system capacity to mitigate external nitrogen loads. To test this hypothesis we quantified and compared light and dark seasonal variations of benthic metabolism, dissolved inorganic nitrogen fluxes, denitrification and N assimilation rates in an area colonized by P. pectinatus and a reference site colonized by microphytobenthos. In both areas the benthic system was net autotrophic and a sink for dissolved inorganic nitrogen (2241-2644 mmol m-2 y-1). Plant colonization increased nitrogen losses via denitrification by 30% compared to the unvegetated area. In contrast to what is generally observed in coastal marine systems, where the presence of rooted macrophytes limits denitrification rates, under the very high nitrate concentrations in the studied lake, both denitrification (1237-1570 mmol m-2 y-1) and N-assimilation (1039-1095 mmol m-2 y-1) played important and comparable roles in the removal of dissolved inorganic nitrogen from the water column.