In situ nitrification rates and activity of present nitrifiers in the bottom water layer of two Baltic coastal zones affected by different riverine nutrient loads

Baltic coastal zones are among the most eutrophied in the world receiving high loads of nitrogen from riverine inputs. However, not only the loads but also the internal dynamics in coastal zones might have positive feedback on eutrophication through efficient remineralisation of organic material in the bottom water. Therefore, we studied nitrification, which is a vital remineralisation process, near the seafloor along with the community of nitrifying microorganisms. We hypothesize that a high nutrient and organic matter load leads to elevated ammonium concentrations in coastal waters and thus stimulates nitrification rates and alters the nitrifying community. Here we present results from 3 cruises combining nitrification rate measurements by ¹⁵N-incubations with sequence-based analyses of present and active nitrifiers in the bottom water of two sites in the Baltic Sea receiving different nutrient loads. The first results from the Bonus projects COCOA and BLUEPRINT indicate an increase of nitrification rates with depth as well as distance from the river mouth. In situ rates in the bottom water of the nutrient rich Vistula plume range from 53 to 197 nmol L^-1 d^-1 and from 10 to 646 nmol L^-1 d^-1 during winter and summer, respectively. In the nutrient poor Öre estuary rates increased significantly by 11 nmol L^-1 d^-1 from the river mouth to the outermost station. The relationship between nitrification rates, nitrifiers and trophic state of the coastal zone shall be discussed.