Preliminary Investigation of the Ecological Role of Microzooplankton in the Kariega Estuary, South Africa

A preliminary investigation of the ecological role of microzooplankton (20–200 μm) was conducted in the Kariega Estuary along the south coast of South Africa during summer (November) 1994. Microzooplankton grazing impact on phytoplankton was estimated at 15 and 20 °C in the laboratory using the dilution technique. Size-fractionated chlorophyll studies showed that the nano- and picophytoplankton dominated phytoplankton standing stock, comprising between 53 and 78% of total chlorophyll. During the entire investigation, the microzooplankton were numerically dominated by protozooplankton with densities ranging between 900 and 2850 cells l−1. Based on biovolume: carbon estimates, microzooplankton contributed between 13 and 43% of the total biological seston in the size range 2–200 μm. Amongst the protozoans, ciliates and dinoflagellates numerically dominated. Microzooplankton densities co-varied with chlorophyll concentrations (r2=0•45;P<0•05). Analysis of variance indicated that the grazing impact of microzooplankton was not significantly different between the two temperatures (F=12•68;P>0•05). The instantaneous grazing rates of microzooplankton on nanophytoplankton varied from 0•010 to 0•105 day−1, and those on microphytoplankton from 0•007 to 0•091 day−1. This level of grazing corresponds to a daily loss of about 5 and 2% of the initial standing stock and about 50 and 65% of the potential production of the two size fractions, respectively. During this study, the grazing rates of microzooplankton correlated with the growth rates of nanophytoplankton (r2=0•51;P<0•05) reflecting tight coupling between these two fractions. These data suggest that microzooplankton are important consumers of phytoplankton production in the Kariega Estuary.