Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Abstract. Grazing by microzooplankton on autotrophic and heterotrophic picoplankton as well as>0.7 µm phytoplankton (as measured by chlorophyll a) was quantified during July, August, October, January and April in the surface layer of Logy Bay, Newfoundland (47°389140N, 52°399360W). Rates of growth and grazing mortality of bacteria, Synechococcus and>0.7 µm phytoplankton were measured using the sea water dilution technique. Microzooplankton ingested 83–184, 96–366 and 64–118 % of bacterial, Synechococcus and>0.7 µm phytoplankton daily potential production, respec-tively and 34–111, 25–30 and 16–131 % of bacterial, Synechococcus and>0.7 µm phytoplankton standing stocks, respectively. The trends in prey net growth rates followed the seasonal cycles of prey biomass, suggesting that microzooplankton are important grazers in Newfoundland coastal waters. Ingestion was lowest during January and October (~2 µg C l–1 day–1) and highest in August (~20 µg C l–1 day–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%) of carbon ingested, bacterioplankton and <1 µm phytoplankton represented most of the carbon ingested (~40–100%). Although microzooplankton have here-to-fore been unrecognized as an important grazer population in Newfoundland coastal waters, these results suggest that they play an important role in carbon flow within the pelagic food web, even at low temperatures in Logy Bay