1. We analysed the functional composition of coastal phytoplankton communities (n=7941) along the gradient from marine to brackish waters of the Baltic Sea, using species-specific morphological and ecological functional traits (ability to fix atmospheric nitrogen, mixotrophy, use of silica in cell walls, formation of chains or colonies, motility, accessory pigment composition, and size), to describe and measure the functional differences between species.
2. Mean pairwise functional distance of phytoplankton communities increased from spring to mid- and late summer in all regions, due to higher pigment diversity, increased share of mixotrophic and nitrogen fixing species, more diverse size distribution, and reduced dominance of silica users.
3. A null model that simulated the expected community composition from empirical spatial distribution and environmental preferences of individual taxa was used to partition the effects of habitat filtering and biotic interactions on the community assembly.
4. About every fourth community departed significantly from random expectations, signaling the notable effect of biotic interactions in the assembly of natural phytoplankton communities.
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