[Indirect parameters: Ecosystem] Microbial community and processes

This overview focuses on bacterioplankton, comprising single-celled prokaryotes, i.e.,small organisms that lack a nucleus (Bacteriaand Archaea), in the water column, consuming organic carbon as an energy and carbonsource. Benthic prokaryotes and protozoa (i.e., unicellular zooplankton and zoobenthos) are also important but not included here. Bacteria are the major transformers of carbon, nitrogen, sulphur, and trace metalcycles in aquatic environments. The supply of organic carbon mainly controls bacterial biomass production. The bacterial community composition changes along the salinity andoxygen gradients of the Baltic Sea. Shifts in, e.g., food sources, temperature, and oxygen concentration result in rapid bacterioplankton community changes, with potential impact also on overall ecosystem functions, such as respiration, carbon consumption, and biomass production.