| Summary: | The Baltic Sea is a substantial brackish water system, and due to its shallow average depth, physicochemical dynamics affecting, for instance, phytoplankton growth, are controlled by atmospheric forces. This unique ecosystem is threatened by environmental changes, with implications for the water quality. Eutrophication remains the major challenge for the Baltic Sea ecosystem. The increases in nitrogen (N) and phosphorus (P) are not proportional to the carbon (C) input, which might cause imbalances in seston C:N:P stoichiometry and affect biogeochemical cycles. In turn, plankton growth, food web dynamics, and nutrient- / C-cycling, which plays a pivotal role in climate change, will be affected. The ongoing eutrophication supports the formation of high phytoplankton biomass, enhancing the spreading of anoxic bottom waters, affecting benthic communities, which might be worsened by an extended annual growing season caused by increasing sea surface temperatures (SST). Higher SST and effects of eutrophication are known to support harmful algal blooms such as the occurrence of toxic dinoflagellates and cyanobacteria in the Baltic Sea during summer. The spring bloom is the most productive period of the year (~50 % annual C-fixation), driven by high inorganic nutrient concentrations and improving light conditions. The co-existence of cold-water diatoms and dinoflagellates during this period is characteristic of the Baltic Sea. Heterotrophic bacteria will become more abundant due to increased concentrations of allochthonous organic matter, which could reduce the phototrophic contribution already in spring. The health of arctic and sub-arctic ecosystems is highly dependent on environmental factors such as temperature affecting, for example, sea ice conditions. Climatically driven changes have already caused variations in the plankton community structure during the Baltic Sea spring bloom. These alterations have led to lower diatom-dinoflagellate-proportions (DDP), affecting food web dynamics and biogeochemical cycles. ...
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