Impact of grazing by microzooplankton in the Northwest Arm of Halifax Harbour, Nova Scotia

Abstract

Impact of grazing by natural assemblages of microzooplankton was measured in 5 in situ experiments in Halifax Harbour, Nova Scotia (Canada) using a seawater dilution method. The microzooplankton assemblages, dominated numerically by oligotrich chates, exerted a seasonally variable grazing impact; 38 % of the initial standing stock of chlorophyll a d-' was consumed 47 % of potential chlorophyll production d-l) in June when flagellates < 12 pm dominated the phytoplankton. No significant grazing occurred in November during a bloom of large diatoms. In March, when grazing and phytoplankton growth were in balance, 100 % of the daily chlorophyll production was grazed. Assumptions of the dilution method that threshold feeding does not occur and that phytoplankton nutrients are not limiting were examined, and although probably violated in some cases, were found not to affect the results of the experiments.