Salinity is one of the major factors regulating zooplankton community composition, occurrence, and abundance in the brackish Baltic Sea. The dominant zooplankton groups are copepods, cladocerans, and rotifers, which occur within their preferred salinity ranges. Climate change and decadal-scale variability have modified the hydrographic conditions and decreased the salinity in large parts of the Baltic Sea over the last 50 years (Figure 16). In the coastal zone, these changes have resulted in a shift in species composition, from a dominance of large copepods of marine origin to freshwater cladocerans and rotifers. In the open sea, such compositional changes are less pronounced and diverge between sub-basins. For example, the total abundance of copepods (overlaying some decadal variability) shows an increasing trend, particularly in the Gulf of Finland. Changes in the ratio of abundance of the marine calanoid copepods, Pseudocalanus acuspes and Acartia bifilosa (which prefer brackish conditions), have not been observed (Figure 16). Instances of high Pseudocalanus abundance are more pronounced in the southern Baltic Sea (exemplified by the Arkona basin) and coincide with major saline inflow events. For the ratio of cladocerans to copepods, different patterns have occurred between southern and northern parts of the Baltic Sea. In the Gulf of Finland, this ratio has clearly decreased in the past ten years, probably due to decreased abundances of most marine cladoceran species. In the southern Baltic Sea, there is no clear trend.
