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

Abstract

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. . 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 ) 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 (G s ) 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 (P s ). KEY WORDS: Dilution technique · Growth rate · Mortality rate · Dinoflagellates · HAB · Alexandrium · GymnodiniumResale or republication not permitted without written consent of the publisher

Comparison of the functional and numerical responses of resistant versus non-resistant populations of the copepod Acartia hudsonica fed the toxic dinoflagellate Alexandrium tamarense

“…We are aware that Alexandrium spp. most commonly occurs as part of a mixed assemblage of phytoplankton (Garcés et al, 2005;Turner and Borkman, 2005), and other grazers can select against -but not entirely avoid -Alexandrium (Teegardern and Cembella, 1996;Teegarden, 1999). Hence, we cannot generalize the results observed here to other grazers.…”

“…Grazing is hypothesized to be an important factor in the population dynamics of harmful algae (Turner and Tester, 1997;Calbet et al, 2003;Garcés et al, 2005;Dam and Colin, 2005). The grazing impact is determined by the summed impact of both metazoan (e.g., copepods) and protist predators.…”

“…Often, microplankton are found to be the dominant grazer on toxic Alexandrium spp. and their grazing rates (day À1 ) are often at levels just below or above Alexandrium growth rates (Calbet et al, 2003;Garcés et al, 2005). Therefore, at times, their grazing impact alone may be sufficient to suppress the development of HABs.…”

Impact of micro- and nanograzers on phytoplankton assessed by standard and size-fractionated dilution grazing experiments

“…Previous reports of microzooplankton grazing upon the phytoplankton community in Mediterranean waters were either restricted to particular semi-enclosed environments, such as harbors during harmful algal bloom episodes (Calbet et al 2003, Garcés et al 2005, and lagoons (Collos et al 2005), or to open waters for a limited period of time (June; Latasa et al 2005). The phytoplankton mortality and growth rates obtained in the present study are in the wide range of values reported for coastal ecosystems (Landry & Hassett 1982, Burkill et al 1987, Neuer & Cowles 1994, Archer et al 2000, Fileman et al 2002, and the average percentage of PP daily consumed by microzooplankton (56%) is close to that expected for coastal waters (60%, Calbet & Landry 2004).…”

Modulation of nearshore harmful algal blooms by in situ growth rate and water renewal

“…Proliferations of species of either or both genera are becoming common along the coasts of the Mediterranean (Garcés et al 2005), particularly in areas of restricted water exchange such as pocket beaches and ports where their swimming ability allows for the maintenance of critical viable patches, even at early bloom stages.…”