SIGNIFICANCE OF PLANKTONIC CILIATED PROTOZOA IN THE LOWER ST-LAWRENCEESTUARY - 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 degr ees 10' W), the importance of ciliate carbon biomass (CilC) was evalua ted in relation to bacterial (BactC), phytoplankton (PhytoC), and part iculate organic (POC) carbon, from May to September 1992. These variab les generally peaked in the upper 10 m of the water column. Their seas onal peaks occurred in June and July when water temperature and strati fication index were at their highest levels. CilC averaged 22, 8, and 4% of BactC, PhytoC, and POC, respectively . Compared to bacteria, cil iates appeared able to respond more rapidly to an increase in phytopla nkton biomass, and were under higher predation pressure. Ciliates were dominated by individuals with sizes (19 to 187 mu m) up to 15 times l arger 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% (me an +/- SD = 54 +/- 21%) of the phytoplankton growth rates. Microzoopla nkton grazing rates in prescreened (<160 mu m) water samples were up t o 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 phyt oplankton assemblage was in a blooming state. These results thus sugge st that herbivory by microzooplankton can enhance trophic transfer fro m the microbial food web to higher trophic levels in the LSLE. This ma y explain the apparent paradox of low annual algal production and larg e standing stock of metazoan zooplankton in this environment.