Bloom and toxin dynamics of Alexandrium ostenfeldii in a shallow embayment at the SW coast of Finland, northern Baltic Sea

A B S T R A C T In the past years, some coastal areas in the central and northern Baltic Sea have experienced recurrent blooms of the potentially toxic dinoflagellate Alexandrium ostenfeldii. The population dynamics and spatial distribution of the species were studied in the Fogloarchipelago, a bloom area in Aland, northern Baltic Sea. During a two-year survey, cell and toxin concentrations of A. ostenfeldii were recorded in summer phytoplankton communities and related to environmental parameters. The studies revealed that A. ostenfeldii blooms were restricted to a small area in a narrow sound, where cells of this species were detected from May to September in 2009 and 2010. A. ostenfeldii proliferated to bloom concentrations (1.7-2.1 � 10 5 cells L � 1 ) only at water temperatures around 20 8C during warm periods in July and August. Abundance of A. ostenfeldii did not significantly correlate with dissolved inorganic nutrient concentrations (p = 0.4-0.6) but was related to high concentrations of resting cysts in the sediment (p < 0.001). The blooms were not monospecific but A. ostenfeldii was a part of a diverse productive phytoplankton community forming 30-60% of the total biomass during the abundance peaks. This study provides the first evidence of paralytic shellfish poisoning (PSP) toxins produced by dinoflagellate blooms in the Baltic Sea: PSP toxin dynamics correlated with the progression of the A. ostenfeldii bloom with toxin peaks mirroring the species abundance peaks. No other potentially toxic species were identified from the phytoplankton community. PSP toxin concentrations of up to 2.3 mg L � 1 measured in the A. ostenfeldii cell fraction suggest that blooms may have toxic effects on co-occurring biota.