Nitrogen fixation estimates for the Baltic Sea indicate high rates for the previously overlooked Bothnian Sea.

Dense blooms of diazotrophic filamentous cyanobacteria are formed every summer in the Baltic Sea. We estimated their contribution to nitrogen fixation by combining two decades of cyanobacterial biovolume monitoring data with recently measured genera-specific nitrogen fixation rates. In the Bothnian Sea, estimated nitrogen fixation rates were 80 kt N year-1, which has doubled during recent decades and now exceeds external loading from rivers and atmospheric deposition of 69 kt year-1. The estimated contribution to the Baltic Proper was 399 kt N year-1, which agrees well with previous estimates using other approaches and is greater than the external input of 374 kt N year-1. Our approach can potentially be applied to continuously estimate nitrogen loads via nitrogen fixation. Those estimates are crucial for ecosystem adaptive management since internal nitrogen loading may counteract the positive effects of decreased external nutrient loading.