Nitrogen cycling, retention and export in a eutrophic temperate macrotidal estuary

The ecological functioning of estuarine environments is greatly influenced by human activity. One of the most important perturbations lies in the increased inputs of nutrients, mainly nitrogen (N), to these systems. This N is submitted to various biogeochemical processes leading to recycling, retention and export to the sea. In this article we present results from 15N-incubation experiments allowing the simultaneous determination of NH4+ and NO3– uptake, nitrification and ammonification rates in the waters of a eutrophic temperate macrotidal estuary (the Scheldt in Belgium and The Netherlands) for different seasons. These are compared to whole estuarine nitrogen budgets. Results showed that nitrogen cycling in this system is dominated in all seasons by the mineralization of organic matter leading to a supply of dissolved inorganic nitrogen (DIN) exceeding river inputs. During winter, summer and fall seasons, only 6 to 10% of this DIN is recycled by microorganisms and most is eliminated (50 to 60%), probably by denitrification in the sediments, and exported (30 to 40%) to the sea. In spring, a larger fraction of DIN (around 70%) is recycled by microorganisms, while only 2% is eliminated and 26% is exported. Overall, we conclude that turbid eutrophic temperate estuaries as the Scheldt may have low DIN recycling efficiencies because of low in situ primary production except in spring.