The Baltic Sea Observatory A new holistic approach to understand the coastal ocean

The Baltic Sea, a European semi-enclosed marginal sea, is driven by the estuarine circulation of dense, saline North-Sea water entering the Baltic Sea and mixing with the freshwater input due to rivers and precipitation. The mixed brackish water leaves the Baltic Sea at the surface through the Danish Belts and the Sound. These processes lead to a strong vertical stratification of the Baltic Sea water masses, the halocline. The slow water exchange causes a mean residence time of the water of 30 years, which leads an accumulation of nutrients. A major consequence of the long residence time and the halocline are low oxygen concentrations below the halocline, with virtually permanent anoxic conditions in the deep basins of the Baltic Sea. To what respect climate change, with the warming of the Baltic Sea as one effect, is impacting the Baltic Sea ecosystem is an open and very relevant research question. One potential consequence could be a further spreading of low or anoxic zones towards the coastal areas, which is already being observed. A less well understood part of the Baltic Sea are the shallow coastal zones, but recent research points to the direction of a strong relevance for e.g. the nutrient turnover. To develop a fundamental understanding of the relevant processes and their coupled effect on the biota, it is therefore essential to measure, monitor and predict the shallow water processes along the margins of the Baltic Sea and how they alter the state of the sea at a basin-wide scale. To address these research questions, the IOW is establishing an interdisciplinary network of long-term and short-term observations in the coastal area of the southern Baltic Sea. This involves deploying moorings in shallow water that send their data to the institute in real time, where they are made immediately available to the general public. The essential ocean parameters (EOP) acquired will be used to control specialized sampling. A measurement program on biogeochemical nutrient turnover, sediment dynamics, geophysics, phytoplankton and zooplankton takes place regularly and is linked to physical data on current patterns, wave intensity and turbulence. The in-Situ measurements are combined with high-resolution numerical modeling to be able to extrapolate the field measurements and to develop numerical experiments. Different stakeholders groups will be involved to provide society and authorities with the latest findings of the measurement campaigns.