Influence of dissolved organic matter from terrestrial origin on the changes of dinoflagellate species composition in the Gulf of Riga, Baltic Sea

A mesocosm experiment was used to investigate the effect of terrestrial-origin dissolved organic matter (DOM) on the development of dinoflagellates in natural summer phytoplankton from the Gulf of Riga. Seawater was collected in the central part of the Gulf of Riga and at the entrance of the Gulf in June 1999. DOM was extracted from Pärnu River water by use of tangential ultrafiltration. Experimental series were enriched with DOM, DOM in combination with nitrate and phosphate, and only with inorganic nutrients. Enrichments were added in ranges of their natural concentrations. Dinophysis acuminata, Protoperidinium brevipes and Gymnodinium spp. were dominant species in the initial dinoflagellate community. During the experiment, the best growth of dinoflagellates was observed in treatments with DOM and DOM in combination with phosphate, mainly due to active growth of D. acuminata (maximum μ = 0.8 day^-1). Significant uptake of dissolved organic nitrogenous compounds was seen, indicating the importance of heterotrophic nutrition among the phytoplankton species.