Bloom and decline of the toxic flagellate Chattonella marina in a Swedish fjord

Abstract

During March 2001, after a spring diatom bloom, the toxic flagellate Chattonella marina bloomed in surface waters of the Gullmar fjord, Sweden, to cell concentrations of 1 to 2 x 10 3 ml -1 , with a strong subsurface maximum (∼2 to 5 m). While the diatoms' isotopic composition indicated a preference for nitrate as an N source, the flagellate seemed to utilize a higher fraction of recycled nutrients (ammonium) when nitrate was severely depleted by the diatoms, allowing it to remain in surface waters once the diatoms sank from the surface. C. marina had production rates of ∼20 mg C m -3 h -1 and turnover rates of 2 to 5 d -1 . The size distribution shifted from larger to smaller cells by the end of the bloom, and the total carbon that C. marina contributed to the system was about 1000 mg C m -2 . Concurrently, heterotrophic dinoflagellates (especially Peridinella danica) grew to a total biomass of 1500 mg C m -2 , and had a net growth rate of 0.06 d -1 . Our results suggest that heterotrophic dinoflagellates had the capacity to be primary grazers of C. marina, and could have easily accounted for the decline of the C. marina bloom in surface waters. Both C. marina and the heterotrophic dinoflagellates made a measurable but small (20 to 40 mg C m -2 ) contribution to sedimented carbon.