Patterns of biodiversity

More than 4,400 known species live in the brackish Baltic Sea. Of these, 4 % are cyanobacteria, 51 % unicellularUnicellular eukaryotes (protists), 8 % macrophytes, 32 % invertebrates and 5 % vertebrates.In the Baltic Sea Area (Baltic Sea and the transition zoneTransition zone), the species richness of these five groups is >6,600, 50 % higher than in the Baltic Sea alone, while the water volumeWater volume increases by only 4 %.The higher richness in the transition zoneTransition zone is caused by North SeaNorth Sea species that still occur in the Kattegat and Belt Sea but cannot survive in the low salinity of the Baltic Sea. Unicellular organisms may be especially diverse in the transition zoneTransition zone as they move with the water masses of different salinities from the SkagerrakSkagerrak and the Baltic Sea that mix here.The true number of species is much higher than the diversity reported from both the Baltic Sea and the transition zone since most archaean and bacterial species, as well as many protistsProtist, fungi[lak]Fungi and small invertebrates, are still unknown.The dominant species in the Baltic Sea proper are mainly hardy, estuarine species, accompanied by a number of glacial relicts, freshwater species and ~130 (non-indigenous) brackish-water speciesBrackish-water species. In the three large gulfs of the Baltic Sea (the Gulfs of Bothnia, Finland and Riga), and near large freshwater discharges along the entire coasts, freshwater species dominate below a salinity of ~4.The species richness of cyanobacteria, heterotrophic bacteria and benthic diatomBenthic diatoms is not impeded in the Baltic Sea. These groups are highly diverse in both marine and freshwater and enter the Baltic Sea from both habitats.Macroscopic organisms show a species minimum at salinity 5–7. There are very few “true” brackish-water species in the Baltic Sea, and the loss of marine species, e.g. macroalgae, polychaetes, crustaceans and molluscs, along the large-scale Baltic Sea gradient is poorly compensated for by species entering the Baltic Sea from freshwater such as charophytes, vascular plants, oligochaetesOligochaete and insect larvae.Despite a pool of >1,500 macroscopic species, the evenness of the communities in the Baltic Sea proper is low, as they are typically dominated by mass occurrences of a few macroscopic species that build simple food webs in a highly productive system.With few species in each functional groupFunctional group (e.g. habitat-forming macrophy