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The impact of microzooplankton community grazing on different size classes of algae was investigated at 11 stations between Dogger Bank and the Shetlands in early spring (March-April) and in summer (July-August). This work is part of a larger study designed to test the hypothesis that size-differential grazing of phytoplankton populations plays a crucial role in regulating food web structure. Dilution experiments, in which loss rates of the algae due to microzooplankton grazing can be estimated from the relation between growth rate of the prey and dilution, failed in many cases due to high variance. The present paper analyses the problem and puts forward a solution which involved pooling data from comparable stations into 1 average grazing estimate per algal size class. In early spring, estimates of grazing from measurements of chlorophyll a (chl a) were obtained only for algae >5 μm, and average grazing rate was 0.23 d -1 at the deeper stations. At the shallower more southern stations where a phytoplankton bloom was in progress the average grazing rate on algae >5 μm was 0.5 d -1 . Flow cytometry was more successful in the <5 μm algal fraction, yielding grazing rates of 0.25 d -1 for the southern and 0.31 d -1 for the northern stations. In summer, microzooplankton grazing incubations yielded significant results only when flow cytometry was used, and only when results were pooled for different areas. Grazing rates ranged from 0.07 d -1 in the 1-2 μm algal size class to 0.74 d -1 for algae of 3-4 μm and were 0.25 d -1 for the <5 μm cluster as a whole.

Grazing impact of microzooplankton on different size classes of algae in the North Sea in early spring and mid-summer