MICROZOOPLANKTON GRAZING OF PHYTOPLANKTON IN MANUKAU HARBOR, NEW-ZEALAND

Grazing by microzooplankton on phytoplankton in Manukau Harbour was me asured by size-fractionated dilution experiments at monthly intervals from October 1994 to October 1995. Grazing rates were always highest o n the < 5 mu m size fraction, the smallest size fraction measured. The se rates ranged from 0.3 to 1.3 d(-1) and were highest in November and March; values did not appear to vary with grazer abundance. Grazing r ates on the < 5 mu m phytoplankton exceeded growth rate by more than a factor of two in June when growth rate was seasonally depressed; but for most of the year the grazing rate averaged about 90% of growth rat e. Grazing rates on the 5-22 mu m phytoplankton were lower in magnitud e, and were a lower percentage of phytoplankton growth rate than those on the < 5 mu m size fraction. Grazing on the > 22 mu m phytoplankton was measurable only during the February bloom of the large diatom Odo ntella sinensis. The grazing rate was low, being a small percentage of phytoplankton growth rate in that size fraction. Mathematical simulat ion of the growth of the < 5 mu m phytoplankton in the harbour showed that the observed grazing rates were sufficient to prevent this size f raction from blooming.