ABSTRACT: The complex role of the microbial food web was investigated through a series of size fractionated grazing experiments conducted in the north-east of Manukau Harbour, a large shallow coastal embayment in the Auckland region, New Zealand. The harbour site chosen for this study is considered eutrophic, with total inorganic nitrogen levels generally exceeding 35 µM and soluble reactive phosphorus levels varying between 3 and 6 µM. A late summer bloom of large diatoms (>22 µm) is a regular feature and results in chlorophyll a (chl a) concentrations between 20 and 60 mg m^-3. For the rest of the year, mean chl a concentrations are around 5 mg m^-3 and phytoplankton cells are generally <22 µm. Microzooplankton grazing on phytoplankton and heterotrophic prey was investigated in 2 experiments which were run simultaneously at monthly intervals on water collected from this site. These experiments were able to identify the separate grazing impacts of <22 µm and 22 to 200 µm microzooplankton. The <22 µm microzooplankton grazer population was dominated by heterotrophic nanoflagellates (HNF) and was responsible for the majority of grazing on organisms <5 µm in diameter. Results from these experiments indicated that, on average, HNF alone consumed all the prokarotic picophytoplankton production, 87% of the bacteria production and 75% of the eukaryotic <5 µm phytoplankton production. Heterotrophic prey in the form of bacteria was the dominant prey in the <5 µm size fraction throughout the year, with HNF consuming an average of 1.9 times as much bacterial biomass as phytoplankton biomass. The <200 µm microzooplankton population was dominated by ciliates and grazed 37% of the 5 to 22 µm chl a weight specific production. These grazers were also partially sustained by feeding on heterotrophic food sources, with 63% of the HNF production being grazed.

KEY WORDS: Microzooplankton · Nanoflagellates · Ciliates · Grazing · Dilution experiments

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Published in AME Vol. 29, No. 1 (2002) on September 3
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2002