Veden laadun, ilmaston ja sijainnin vaikutukset piileväyhteisöihin pohjoisella Itämerellä

The structure of a local community depends on interacting local and regional filters. During the last few decades, the effects of local and regional factors have been studies in communities formed not only by macro-organisms but also by micro-organisms like diatoms. In the northern parts of the Baltic Sea, unfortunately, this kind of research has been quite scarce and often spatially limited. Studying of brackish water seas like the Baltic Sea is important, however, because their conditions and species composition differ from those of oceans, lakes and rivers, and their biodiversity is usually quite low. Also, they are surrounded by continents and are therefore exposed to heavy pollution. The goal of this thesis is to examine what are the most important factors explaining the species richness and species composition of the epilithic diatom communities in the littoral zone of the Gulf of Finland and the Gulf of Bothnia, Baltic Sea. The explaining factors used in the study are water chemistry, salinity, climate, exposition and geographical location. The diatom and water samples were collected in July 2013 from 37 sites covering the whole Finnish coastline of the Gulf of Finland and the Gulf of Bothnia. Also data received from the Finnish Meteorological Institute and the Finnish Environment Institute were used after interpolation. Generalized linear models (GLM) and generalized additive models (GAM) were used for the species richness analyses, and the spatial autocorrelation was examined with Moran's I -index. The species composition of the communities was analyzed with variation partitioning, redundancy analysis and Mantel's test. According to the GLMs, the species richness was best explained by total nitrogen, pH and oxygen. According to the GAMs, the best explaining factors were pH, oxygen and salinity. Species richness was not spatially autocorrelated. The variation partitioning showed that the species composition was mostly affected by climate and location, whereas water quality had little impact. However, the strongest affecting factor was the cooperative action of water quality, climate and location. According to the Mantel's test, there was a statistically significant correlation between species composition and geographical location, whereas between species composition and water quality or species composition and climate the correlations were non-significant. The redundancy analysis indicated that the most important factors regulating the species composition were air temperature, silicon oxide, total phosphorus, water temperature, salinity and pH. The results suggest that the species richness of the diatom communities in the northern Baltic Sea is mostly regulated by the local factors, while geographical location and climate have little impact. When it comes to the species composition, geographical location has stronger effect than water quality or climate. However, the most important regulator of species composition is the cooperative action of water quality, climate and location, because location affects climate and both location and climate affect water quality.