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

anglais. This study was supported by the Academy of Finland (decisionsno 259164 and 263376) and Estonian Research Council (no1575P, KO). Further funding came from Walter and Andrée deNottbeck Foundation (KS and ToT) and the Swedish CulturalHeritage (ToT). LT acknowledges support also from COCOA—Nutrient cocktail in coastal zones of the Baltic Sea—EU BONUSproject.

anglais. The Baltic Sea is affected by a range of human induced environmental pressures suchas eutrophication. Here we synthesize the ongoing shift from diatom dominance towardmore dinoflagellates in parts of the Baltic Sea during the spring bloom and its potentialeffects on biogeochemical cycling of key elements (e.g., C, N, and P). The spring bloomis the period with the highest annual primary production and sinking of organic matter tothe sediment. The fate of this organic matter is a key driver for material fluxes, affectingecosystem functioning and eutrophication feedback loops. The dominant diatoms anddinoflagellates appear to be functionally surrogates as both groups are able to effectivelyexhaust the wintertime accumulation of inorganic nutrients and produce bloom levelbiomass that contribute to vertical export of organic matter. However, the groups havevery different sedimentation patterns, and the seafloor has variable potential to mineralizethe settled biomass in the different sub-basins. While diatoms sink quickly out of theeuphotic zone, dinoflagellates sink as inert resting cysts, or lyse in the water columncontributing to slowly settling phyto-detritus. The dominance by either phytoplanktongroup thus directly affects both the summertime nutrient pools of the water columnand the input of organic matter to the sediment but to contrasting directions. Theproliferation of dinoflagellates with high encystment efficiency could increase sedimentretention and burial of organic matter, alleviating the eutrophication problem and improvethe environmental status of the Baltic Sea.

anglais. This study was supported by the Academy of Finland (decisions no 259164 and 263376) and Estonian Research Council (no 1575P, KO). Further funding came from Walter and Andrée de Nottbeck Foundation (KS and ToT) and the Swedish Cultural Heritage (ToT). LT acknowledges support also from COCOA— Nutrient cocktail in coastal zones of the Baltic Sea—EU BONUS project.