The overall aim of the thesis was the quantification of nitrification and the investigation of its role in the coastal nitrogen turnover. Two estuaries in the Baltic Sea, the eutrophied Vistula estuary and the pristine Öre estuary were studied with a focus on nitrification rates in river plume and BBL. Also the influence on nitrification by distinct riverine nitrogen loads and sedimentary ammonium release was inverstigated. Nitrogen recycling via nitrification is a crucial part of the coastal filter function in coastal zones of the Baltic Sea.