Phytoplankton responses to nutrient status: application of a screening method to the northern Baltic Sea

摘要

Phytoplankton monitoring data from the coastal northern Baltic Sea were analyzed in order to investigate whether changes in species composition could be related to nutrient levels. In this study 76 phytoplankton taxonomic units, identified to either species or genus level, that were present in at least 5% of all samples (N = 2217), were examined for the Gulf of Riga, the Gulf of Finland and the Archipelago Sea. The presence of a particular species/genera was modeled by means of response surfaces describing the probability of that species/genus being present in a sample as a function of salinity and total nitrogen (TN), total phosphorus (TP) or N:P ratio. The probability of presence was significantly related to nutrient levels for only half of the phytoplankton units; 4 species/genera had probabilities ranging over 30 % for changing nutrient levels; only 1 species (Plankto-thrix agardhii) allowed the detection of a change in the probability of presence from realistic sample sizes (N = 50) when TN was decreased by 20%, and only for a limited range of salinity and TN levels. Significantly, responses to changing nutrient levels were not specific to any of the different taxonomical divisions. Our results confirm that phytoplankton composition changes with different nutrient levels but that the composition does not shift abruptedly and changes are small for moderate increases in nutrient levels. The method developed enables screening of phytoplankton taxa that occur in response to elevated nutrient status, providing a tool for the selection of potential indicator species. This could prove useful in the further development of classification systems for coastal waters.