Abstract 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 µn size fraction, the smallest size fraction measured. These 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 rates on the < 5 µm 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 µm phytoplankton were lower in magnitude, and were a lower percentage of phytoplankton growth rate than those on the < 5 µm size fraction. Grazing on the > 22 µm 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 µm phytoplankton in the harbour showed that the observed grazing rates were sufficient to prevent this size fraction from blooming.
Keywords phytoplankton; microzooplankton; grazing; dilution experiments; Manukau Harbour
New Zealand Journal of Marine and Freshwater Research, 1996, Vol 30: 423—434
0228-8330/96/3003-0423 $2.50/0 (c) The Royal Society of New Zealand 1996
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