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 Based on a variety of available data sources (published papers, reports, grey literature, unpublished data), the Baltic Sea, incl.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.

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Ecological and evolutionary processes have shaped current biodiversity patterns. For brackish-water ecosystems, Remane's Artenminimum ('species minimum') concept argues that taxonomic diversity in organisms is lowest within the horohalinicum, which occurs at salinity 5 to 8. This concept developed from macrozoobenthos data; it originated from, and is still applied to, the geologically young Baltic Sea, the world's largest semi-enclosed, brackish water body with a unique permanent salinity gradient. We re-assessed pelagic biodiversity in the Baltic Sea, which had long remained underestimated. We show that phyto-and zooplankton in Baltic waters exhibit unexpectedly high diversity (> 4000 taxa), with dominance by protists. Protists in the Baltic Sea follow a binomial distribution mode, while metazooplankton diversity decreases exponentially with higher salinity; however, species richness of both groups peaks in the horohalinicum. Drifting within large water masses, planktonic organisms are affected by only moderate salinity fluctuations (compared to benthic species), and thus prosper in brackish environments. The present study challenges Remane's concept for large water bodies with relatively stable salinity gradients and substantiates a novel ecological perspective of the previously overlooked high protistan diversity in brackish waters. We infer that the pronounced adaptability and advanced osmoregulation strategies of protists are the result of large-scale ecological and evolutionary processes. The novel brackish-water biodiversity pattern underpins the proposed protistan species-maximum concept, which refines Remane's model by discriminating between the effects of the horohalinicum on the biodiversity of small motile versus large sessile organisms.

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