Significance of planktonic ciliated protozoa in the Lower St Lawrence Estuary: Comparison with bacterial, phytoplankton, and particulate organic carbon

To investigate the potential role of microzooplankton as trophic links in the Lower St. Lawrence Estuary (LSLE, ca 48 degrees 50' N, 68 degrees 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 mu m) 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 (<160 mu m) water samples were up to 60% higher than in unscreened samples. We conclude that this was due to the removal of a substantial fraction of herbivorous micrograzers by metazoan zooplankton present in unscreened samples, though the phytoplankton assemblage was in a blooming state. These results thus suggest that herbivory by microzooplankton can enhance trophic transfer from the microbial food web to higher trophic levels in the LSLE. This may explain the apparent paradox of low annual algal production and large standing stock of metazoan zooplankton in this environment.