The number of species decreases with increased distance from the North Sea. In the Baltic Sea, several essential ecosystem functions are supported by only a few species, which are of either freshwater or marine origin and live at the border of their physiological salinity tolerance (Figure 25) (HELCOM, 2010). For example, there is a decrease in diversity of benthic sediment communities with decreasing salinity, from 25 functional groups of benthic species in Skagerrak (K. Norling, Rosenberg, Hulth, Gremare, & Bonsdorff, 2007), to only 5 groups in the Baltic Sea sediments (Bonsdorff & Pearson, 1999).