The spring bloom in the Baltic Sea, as in most temperate aquatic systems, is by far the most important phase of the annual succession in terms of primary production as typically 40 to 60% of the annual carbon fixation takes place during a few weeks. Two phytoplankton groups are predominant during spring in the Gulf of Finland: diatoms and dinoflagellates. The relative abundances of these algal groups may have a large effect on remineralization of the organic matter. Diatoms settle quickly to the sea floor whereas dinoflagellates either lyse before reaching the sediment, releasing the organic material to the water. Alternatively, they produce resting cysts that do not easily decompose in the sediment. The dominance by either phytoplankton group may thus have direct effect on both the summertime nutrient pools of the water column, and the input of organic matter to the bottom sediments, to contrasting directions.

In parts of the Baltic Sea, dinoflagellates are getting more abundant at the expense of diatoms during spring bloom. What effect this change in phytoplankton community composition will have on biogeochemical fluxes is currently not known, and this is the key issue that I am working with at the moment. The goal is to better understand how the phytotplankton community composition may affect the pathways of organic material in the system.