Seasonality of coastal phytoplankton in the Baltic Sea: Influence of salinity and eutrophication

In this study long-term (1984-2001) phytoplankton and physico-chemical monitoring data representing different salinity regimes of the Baltic Sea were compiled from HELCOM, national and regional databases. The aim was to define seasonal succession patterns of phytoplankton in seven different areas of the Baltic sea, characterised by different salinity, climate, and trophic conditions and to delineate a set of phytoplankton community indicators that are independent of season and salinity, but indicative of trophic status of different coastal areas. The cluster analysis of the combined data set resulted in eight phytoplankton community types, common for all locations, and characterised by different taxonomic composition representing different stages of seasonal succession. A hierarchy of explanatory variables that best predicted the communities, dominated by either diatoms, cyanophytes, cryptophytes or dinoflagellates, was revealed through a redundancy analysis (RDA). Nutrients were not found to be significant factors shaping the common phytoplankton community types for all locations. RDA analysis at the location level, covering all seasonal succession stages, confirmed phytoplankton community composition to be sensitive to nutrient concentrations. Even with the limitations of utilizing databases from different sources we identified community types that were indicative of climatic conditions (particularly temperature), salinity and eutrophication. The dominance of cyanobacteria as such, would not be an appropriate indicator of trophic conditions in the Baltic Sea, in the areas where cyanobacteria blooms occur naturally. The structure of both diatom- and cyanophyte-dominated communities is governed by salinity, and thus the abundances of these groups cannot be directly used as an indicator across the whole Baltic Sea.