Prorocentrum minimum (Dinophyceae) in the Baltic Sea: morphology, occurrence—a review

The potentially toxic dinoflagellate Prorocentrum minimum (Pavillard) Schiller has successfully established in the Baltic Sea in the last two decades. A review of the invasion history is presented as well as new data on the spatial and inter-annual variability of this species and its relation to salinity, temperature, and nutrient concentrations. A short literature review of the morphological characters of the Baltic P. minimum is also included. From 1993 to 2002, P. minimum was a regular component of the summer and autumn plankton flora of the Baltic Sea proper and the Gulf of Finland. Its abundance varied considerably inter-annually and did not show any clear trends during the period. Abundance of P. minimum was significantly higher in the nutrient-enriched Bay of Mecklenburg (German coast) and the southern Baltic proper than in the central and northern Baltic proper and the Gulf of Finland, where its abundance was mostly sparse. In coastal waters P. minimum occasionally reached densities of several million cells per litre and dominated phytoplankton biomass (>90%). Abundance of the Baltic P. minimum was generally not related to salinity or temperature. It could be a dominant species at both high and low salinity (over 15 and 4.8 PSU), and its temperature range was broad (from 2.7 to 26.4 °C). However, dense populations usually occurred from July to October at temperatures above 10 °C. Further, there appears to be a positive correlation between the success of P. minimum in the Baltic Sea and high concentrations of total phosphorus and nitrogen. This tolerant and morphologically variable dinoflagellate seems to be a morphospecies without subtaxa, which can expand its range in the Baltic Sea, especially in nutrient-rich coastal waters.