Piilevät mikropaleontologisena proksina ja niiden vuodenaikainen sukkessio jääpeitteisellä Itämerellä

Ongoing climate change alters Northern marine ecosystems, where annual sea-ice cover has a significant role. Changes caused by climate change, such as sea surface temperature and sea-ice season, affect the composition of the community of primary producers. Primary producers have an important role in the ecosystems and biological and geological cycle, and a slight change in their community can have a significant impact on the marine system. Past environments provide important information on the effects of future changes in the environment and climate as well as tools to control them. Diatoms are commonly used in micropaleontology and paleoecology as an indicator for past environmental conditions and are therefore important proxies for paleoenvironments and climates. To better understand the past and future changes in the environment and climate, it is important to study not only microfossils in the sediments but also the relationship of modern diatoms to environmental factors. In the Baltic Sea, seasons strongly regulate the environmental conditions, which are reflected in the diatom community. Different seasons are represented by diatoms adapted to different conditions, which could lead to misrepresentation of environmental conditions if seasonal patterns are not recognized. In this master’s thesis, modern diatom seasonal succession is studied, as well as the role of environmental factors on diatom species over one year period. A sediment trap was used to monitor seasonal diatom succession and sediment vertical flux in Tvärminne Storfjärden, Gulf of Finland between 2012–2013. New information was discovered on the ecology and succession of common diatom species in the Baltic Sea. Data shows a clear succession according to the season as diatom community evolved to represent winter and early spring community, late spring community and autumn community. In winter the diatom community consists mainly of sea-ice species such as Pauliella taeniata and cold-water species Thalassiosira levanderi. The role of Pauliella taeniata was smaller than expected, possibly due to long-term decreasing trend associated with changing environment. Other central species were sea-ice related Stauroneis radissonii and a species belonging to Chaetoceros group. Sea-ice species formed a bloom around sea-ice melt and again during the spring bloom. In contrast to sea-ice species Thalassiosira levanderi formed a bloom only in the early spring, although it was present throughout the year. The bloom was probably initiated by optimal environmental conditions and lack of competition. Dominant species during spring bloom were common spring species in the Baltic Sea Skeletonema marinoi and Diatoma moniliformis. The latter occurs in benthic and planktic environments that were discovered blooming in planktic on spring blooms in May of 2013. In summer diatoms were relatively scarce, but a group of small centric species (including Cyclotella Choctawhatcheeana, Cyclotella atomus, Minidiscus proschkinae) formed massive autumn blooms as turbidity and nutrient concentrations increased in September and August.