Taxon-specific analysis of growth and mortality rates of harmful dinoflagellates during bloom conditions

Growth and mortality rates of natural single Alexandrium spp. cells were measured by the Landry-Hassett dilution technique during different phases of blooms. Taxon-specific experiments were conducted between May and October 2002 during 3 intense blooms of A. taylori and A. catenella at different locations of the Mediterranean Sea. In addition, dilution experiments using chlorophyll a as a proxy for total phytoplankton biomass were used to estimate daily rates of net growth and mortality of the total phytoplankton community. A. taylori growth rates ranged from 0.04 to 0.67 d–1 and mortality rates from –0.20 to –0.65 d–1. Growth rates of Gymnodinium sp., an accompanying dinoflagellate species during the A. taylori bloom studied, were similar to those measured for A. taylori, whereas their mortality rates (–0.58 to –0.82 d–1) were slightly higher. A. catenella growth and mortality rates were balanced (0.24 and 0.44 d–1 compared with –0.25 and –0.44 d–1, respectively). The highest mortality rates (–0.65 d–1) were measured during the decline phase of 2 A taylori blooms. At the decline of the blooms, A. taylori and A. catenella showed considerable mortality, but microzooplankton grazing was not confirmed to be the main cause of the bloom termination. In general, growth was not limited by nutrients in the experiments. There were a few cases of a potential nutrient limitation in these areas and, in general, blooms were not conditioned by nutrients. When changes in biomass (chlorophyll a) were measured, non-linearity of data due to saturation was observed. The interpretation of these results required a split-function model. Saturated grazing (Gs) was 28.9 μg chl a l–1 d–1, during which the saturating phytoplankton population represented a chl a concentration of 16 μg l–1 (Ps).