The brackish Baltic Sea hosts species of various origins and environmental tolerances. These immigrated to the sea 10,000 to 15,000 years ago or have been introduced to the area over the relatively recent history of the system. The Baltic Sea has only one known endemic species. While information on some abiotic parameters extends back as long as five centuries and first quantitative snapshot data on biota (on exploited fish populations) originate generally from the same time, international coordination of research began in the early twentieth century. Continuous, annual Baltic Sea-wide long-term datasets on several organism groups (plankton, benthos, fish) are generally available since the mid-1950s. Based on a variety of available data sources (published papers, reports, grey literature, unpublished data), the Baltic Sea, incl. Kattegat, hosts altogether at least 6,065 species, including at least 1,700 phytoplankton, 442 phytobenthos, at least 1,199 zooplankton, at least 569 meiozoobenthos, 1,476 macrozoobenthos, at least 380 vertebrate parasites, about 200 fish, 3 seal, and 83 bird species. In general, but not in all organism groups, high sub-regional total species richness is associated with elevated salinity. Although in comparison with fully marine areas the Baltic Sea supports fewer species, several facets of the system's diversity remain underexplored to this day, such as micro-organisms, foraminiferans, meiobenthos and parasites. In the future, climate change and its interactions with multiple anthropogenic forcings are likely to have major impacts on the Baltic biodiversity.
Assessments of the environmental status of the Baltic Sea as called for by the Marine Strategy Framework Directive (MSFD) must be based on a set of indicators. A pre-core indicator is the diatom/dinoflagellate index (Dia/Dino index), which reflects the dominance of diatoms or dinoflagellates during the phytoplankton spring bloom. Here we explain the principles of the Dia/Dino index and the conditions for its calculation using examples from two very different water bodies, the Eastern Gotland Basin and Kiel Bay. The index is based on seasonal mean diatom and dinoflagellate biomass values. A precondition for its applicability is the coverage of the bloom. As a criterion, the maximum value of diatom or dinoflagellate biomass has to exceed a predefined threshold, e.g., 1000 µg/L in the investigated areas. If this condition is not fulfilled, an alternative Dia/Dino index can be calculated based on silicate consumption data. Changes in the dominance of these two phytoplankton classes impact the food web because both their quality as a food source for grazers and their periods of occurrence differ. If diatoms are dominant, their rapid sinking reduces the food stock for zooplankton but delivers plenty of food to the zoobenthos. Consequently, the Dia/Dino index can be used to follow the food pathway (Descriptor 4 of MSFD: "food web"). Moreover, a low Dia/Dino index may indicate silicate limitation caused by eutrophication (Descriptor 5 of MSFD: "eutrophication"). The Dia/Dino index was able to identify the regime shift that occurred at the end of the 1980s in the Baltic Proper. Diatom dominance, and thus a high Dia/Dino index, are typical in historical data and are therefore assumed to reflect good environmental status (GES). In assessments of the environmental status of the Eastern Gotland Basin and Kiel Bay, Dia/Dino index GES thresholds of 0.5 and 0.75, respectively, are suggested. The GES thresholds as calculated by the alternative Dia/Dino index are 0.84 and 0.94, respectively.
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