Towards a deeper Understanding of Cyanobacteria Blooms in the Baltic Sea

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In the Baltic Sea, cyanobacteria species regularly form massive surface scums or blooms in late summer. Because they produce toxins (that are suspected to enter the food web) they receive more and more attention from both - researchers and the public. Less prominent, but equally important for the Baltic Sea ecosystem, is their ability to fix atmospheric nitrogen which substantially impacts on the biogeochemical cycle of the Baltic, because cyanobacteria supply the already over-fertilized system with additional bioavailable nitrogen. This input can be substantial (up to 40% compared to the overall nutrient loads from rivers and atmospheric decomposition). Thereby cyanobacteria have the potential to counteract management decisions to limit eutrophication.Despite their crucial importance, cyanobacteria blooms are surprisingly little understood which is reflected in major difficulties to model and forecast blooms acurately. Towards a more comprehensive understanding we propose to systematically explore the predictive skill of a suite of potentially important factors. In contrast to existing studies we aim to elucidate controls during the early stages of bloom formation rather than focusing on the bloom peaks. Another novel element will be a combination of high resolution ocean circulation model output and observational data to back-trace the origin and conditioning of cyanobacteria blooms. One major focus will be set on abiotic factors, as e.g. photosynthetically active radiation, salinity and temperature, during bloom formation. In addition, the accustomed theory on controlling effects of the ratio of nitrogen and phosphorus supply will be reexamined. New hypotheses will be tested and compared to the forecast skill of an existing state-of-the-art biogeochemical model for the Baltic Sea. The Baltic Sea is particularly suitable for these investigations, since the amount of available observations is extraordinary high. Anticipated results may well have major implications for modeling nitrogen fixation (diazotrophy) in the global oceans.