Selecting potential summer phytoplankton eutrophication indicator species for the northern Baltic Sea

Abstract

The composition, abundance, and biomass of aquatic flora, including phytoplankton, belong to the key biological indicators according to the EU Water Framework Directive. The changes in physico-chemical conditions lead directly or indirectly to impacts on biological qualify elements. In order to identify which phytoplankton taxa may be indicative of ambient nutrient concentrations, we analysed the seasonal biomass pattern of selected dominant species and simultaneously measured hydrographical and chemical parameters. High frequency data were collected on the transect of the central Gulf of Finland in summer periods (from June to August) of 1997-2004. The relationships between abiotic and biotic variables were studied by linear correlation, regression, and multivariate BIO-ENV analyses. Among 14 selected taxa we found only 3 species (the diatoms Skeletonema costatum, Cyclotella choctawhatcheeana, and Cylindrotheca closterium) that responded regularly to enhanced nutrient concentrations having higher biomasses in coastal areas and following a similar temporal pattern. We suggest that some species (e.g. Heterocapsa triquetra, Eutreptiella gymnastica) are opportunistic and short-term changes in their biomass are not related to eutrophication levels.