Microzooplankton grazing of phytoplankton in Manukau Harbour, New Zealand

Grazing by microzooplankton on phytoplankton in Manukau Harbour was measured by size-fractionated dilution experiments at monthly intervals from October 1994 to October 1995. Grazing rates were always highest on the < 5 urn size fraction, the smallest size fraction measured. These rates ranged from 0.3 to 1.3 d" and were highest in November and March; values did not appear to vary with grazer abundance. Grazing rates on the < 5 |am 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 rate. Grazing rates on the 5—22 um phytoplankton were lower in magnitude, and were a lower percentage of phytoplankton growth rate than those on the < 5 um size fraction. Grazing on the > 22 urn phytoplankton was measurable only during the February bloom of the large diatom Odontella sinensis. The grazing rate was low, being a small percentage of phytoplankton growth rate in that size fraction. Mathematical simulation of the growth of the < 5 um phytoplankton in the harbour showed that the observed grazing rates were sufficient to prevent this size fraction from blooming. M96027 Received 8 May 1996; accepted 5 August 1996