Cyanobacterial blooms in the Baltic — A source of halocarbons

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During summer, cyanobacteria become highly dominant in the Baltic Sea, forming extensive blooms. It is well established that algae form volatile halogenated organic compounds, halocarbons, but it has only been suggested that cyanobacteria are capable of a similar production. During a cruise in the Baltic proper in 29–31 July 2004, the halocarbon formation from a cyanobacterial bloom was studied. Incubation experiments were performed with samples taken from the same bloom during a 36 h period. Halocarbons, as well as photosynthetic pigments, were measured in all samples. Significant amounts of CHBr 3, CH 3I, CH 2Br 2 and CHBr 2Cl were produced in the bloom, with production rates up to 0.3 pmol [μg chl a] − 1 h − 1 (CHBr 3, midday). In order to distinguish between the production of halocarbons by large size organisms caught in a plankton net, and smaller organisms present in the bloom, alternatively abiotic processes, a mathematical model was constructed. In this model, measured surface water concentrations were compared to concentrations calculated from measured production rates, air–sea exchange and turbulent eddy diffusion in the mixed layer. It was shown that the production of CHBr 3, CH 3I, CH 2Br 2 and CHBr 2Cl, was indeed associated with the large size, cyanobacteria, fraction. Since this fraction did not include the major species of micro-algae found in the bloom, a cyanobacterial production is suggested. The calculated total production, of each compound, in the Baltic Sea would be in the Mg y − 1 range for a cyanobacterial bloom period of 1 month and a maximum bloom coverage of 130 000 km 2 (HELCOM, 2005), with a bloom density equivalent to 5 μg L − 1 of chl a.