Nodularin analyses and toxicity of a Nodularia spumigena (Nostocales, Cyanobacteria) water-bloom in the western Gulf of Finland, Baltic Sea, in August 1999

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Abstract
During 2–12 August 1999, the development of a surface bloom dominated by the cyanobacterium Nodularia spumigena was followed in the western Gulf of Finland, Baltic Sea. Intense surface scums of aggregated N. spumigena existed on calm days. The low-nutrient N. spumigena-dominated water mass was separated by a front from a nutrient-rich water mass dominated by Aphanizomenon cf. flos-aquae (Nostocales, Cyanobacteria). Owing to inadequate mixing of the two water masses, it is likely that N. spumigena growth was based mainly on the use of internal phosphorus stores. Signs of cell decay were observed on 5 August, and the proportion of empty filaments in the community increased towards the end of the survey period. A high number of Nitzschia spp. (Bacillariophyceae) was recorded within the N. spumigena aggregates on 8 August, suggesting nutrient leakage from the N. spumigena filaments. Senescence of N. spumigena and a decrease in the proportion of intact N. spumigena cells in dry material resulted in decreasing concentrations of cell-bound nodularin (NODLN), from 2.1 to 0.5 g kg−1 dry weight. No clear temporal trends in NODLN concentrations (< 0.5–2.6 μg l−1) were observed in Gulf waters. We hypothesize that the bloom was linked to a fish kill that occurred during the same time period. Threespine sticklebacks (Gasterosteus aculeatus, Gasterosteiformes, Gasterosteidae) were found floating on the surface during the water-bloom. The sticklebacks contained approximately 35–170 μg NODLN kg−1 dry weight (microcystin-LR equivalents). Empty N. spumigena cells and trace amounts of NODLN were also found in surficial sediment, indicating that toxin-producing N. spumigena had reached the seafloor.

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