GROWTH AND DECLINE OF A DIATOM SPRING BLOOM - PHYTOPLANKTON SPECIES COMPOSITION, FORMATION OF MARINE SNOW AND THE ROLE OF HETEROTROPHIC DINOFLAGELLATES

During the spring of 1994, we determined the factors responsible for t he decline of the seasonal diatom bloom in the Gullmar fjord, on the w est coast of Sweden. Four species constituted >75% of the biomass-Deto nula confervacea, Chaetoceros diadema, Skeletonema costatum and Thalas siosira nordenskioeldii-reaching concentrations of 4900, 350, 8200 and 270 cells ml(-1), respectively. Growth of phytoplankton was exponenti al (growth rate = 0.12 day(-1)) from 3 to 21 March, after which a gale with winds >15 m s(-1) caused massive aggregation. A maximum of 130 p .p.m. (v/v) of marine snow aggregates was observed by in situ video at the peak of the bloom. Critical concentrations (Jackson, Deep-Sea Res ., 37, 1197-1211, 1990) were similar to observed showing that coagulat ion theory could explain the sudden decline of the bloom. The heterotr ophic dinoflagellate Gyrodinium cf. spirale increased exponentially af ter the peak of the bloom with maximum (temperature-adjusted) growth r ates. After the rapid aggregation and sedimentation of the bloom, they were able to control any further growth of diatoms. Nitrate and silic ate were never depleted, but phosphate may have been limiting by the e nd of the study period. We conclude that mass aggregation during a gal e marked the end of the bloom, and that intense grazing by heterotroph ic dinoflagellates prevented any subsequent increase of diatoms.