Shifting Diatom—Dinoflagellate Dominance During Spring Bloom in the Baltic Sea and its Potential Effects on Biogeochemical Cycling

The BalticSea is affected by a range of human induced environmental pressures such as eutrophication. Here we synthesize the ongoing shift from diatomdominance towards more dinoflagellatesin parts of the BalticSea during the spring bloomand its potential effects on biogeochemical cyclingof key elements (e.g. C, N and P). The spring bloomis the period with the highest annual primary production and sinking of organic matterto the sediment. The fate of this organic matteris a key driver for material fluxes, affecting ecosystem functioning and eutrophicationfeedback loops. The dominant diatomsand dinoflagellatesappear to be functionally surrogates as both groups are able to effectively exhaust the wintertime accumulation of inorganic nutrients and produce bloom level biomass that contribute to vertical export of organic matter. However, the groups have very different sedimentation patterns, and the sea floor has variable potential to mineralize the settled biomass in the different sub-basins. While diatomssink quickly out of the euphotic zone, dinoflagellatessink as inert resting cysts, or lyse in the water columncontributing to slowly settling phyto- detritus. The dominance by either phytoplankton group thus directly affects both the summertime nutrient pools of the water columnand the input of organic matterto the sediment but to contrasting directions. The proliferation of dinoflagellateswith high encystment efficiency could increase sediment retention and burial of organic matter, alleviating the eutrophicationproblem and improve the environmental status of the BalticSea.