Significance of planktonic ciliated protozoa in the Lower St. Lawrence Estuary : comparison with bacterial, phytoplankton, and particulate organic carbon
Résumé
To investigate the potential role of microzooplankton as trophic links in the Lower St. Lawrence Estuary (LSLE, ca 48'50' N, 68" 10' W), the importance of ciliate carbon biomass (CilC) was evaluated in relation to bacterial (BactC), phytoplankton (PhytoC), and particulate organic (POC) carbon, from May to September 1992. These variables generally peaked in the upper 10 m of the water column. Their seasonal peaks occurred in June and July when water temperature and stratification index were at their highest levels. CilC averaged 22, 8, and 4 % of BactC, PhytoC, and POC, respectively. Compared to bacteria, ciliates appeared able to respond more rapidly to an increase in phytoplankton biomass, and were under higher predation pressure. Ciliates were dominated by individuals with sizes (19 to 187 pm) up to 15 times larger than that of the blooming diatoms. Direct observations revealed protozoan individuals with up to 10 ingested bloom-forming algal cells within their body. Microzooplankton grazing rates (estimated In July 1992 using a dilution technique) in unscreened water samples from the depth of maximum chlorophyll a concentration represented up to 70% (mean & SD = 54 * 21 %) of the phytoplankton growth rates. Microzooplankton grazing rates in prescreened (