Nitrogen turnover during the spring outflows of the nitrate-rich Curonian and Szczecin lagoons using dual nitrate isotopes

•Dual isotopes in nitrate of two outflows from highly eutrophied rivers in the Baltic Sea were determined.•Assimilation and coupled nitrification/denitrification dominated the fate of N.•Cycling of N in sandy sediments led to highly enriched delta values in nitrate in the water column.

Coastal zones that receive high nitrogen loads from rivers often suffer from intense eutrophication. This is the case in the Baltic Sea, where nutrient concentrations decrease rapidly offshore, but uptake and turnover processes in the highly eutrophic near-shore areas are not well understood. The Oder and Nemunas Rivers, the second and third largest nitrogen contributors (70 and 45 kt N y−1), drain into the Szczecin and Curonian lagoons, respectively, and thus into the coastal waters of the Baltic. In this study, nitrate turnover processes in the coastal zones of the Baltic Sea were determined in March 2009 by measuring nutrient concentrations, nitrate uptake rates, and dual isotopes in nitrate (δ15N-NO3− and δ18O-NO3−) in the lagoon outflows. While mixing processes dominated in the outflow of the Curonian lagoon, a phytoplankton spring bloom largely accounted for the nitrogen processes in the outflow of the Szczecin lagoon. Here, nitrate assimilation in the surface waters was evidenced by a parallel enrichment of 15N and 18O. In the near-bottom waters of the Szczecin lagoon, a deviation from the 1:1 relationship between δ18O-NO3− and δ15N-NO3−, typical of denitrification, suggested the coupling of nitrification and denitrification, triggered by the presence of organic material from the spring bloom. The particulate material may be rapidly sequestered in the sandy sediments off the outflow and generate the isotope signal of nitrate. The highly enriched δ15N-NO3− and δ18O-NO3− values (up to 28‰) in the near-bottom layer implied that the impact of isotopic enrichment from sediment processes in the water column is higher in sandy than in muddy sediments.