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Bottle incubation experiments are widely used in mesozooplankton grazing studies.However, we have shown here that traditional particle removal experiments with Calanus finmarchicusand C. helgolandicus as grazers on natural plankton may yield low or even statistically significant(p < 0.05) negative grazing estimates, even though negative grazing rates are impossible.Low grazing rates are often reported, especially on smaller prey types, despite abundant food andsignificant egg production. Microzooplankton, such as ciliates, show higher biomass-specific grazingrates on algae than do copepods and other mesozooplankton. Instead, copepods often selectivelyfeed on the microzooplankton. Thus, apparent negative rates would be expected when the release ofmicrozooplankton grazing pressure outweighs the copepod grazing rates on the same food items inthe incubation bottle. We show that this potentially large bias increases with microzooplanktoncommunity grazing pressure in the control. A simplified general method to correct for this bias ispresented and compared with the original method (Nejstgaard et al. 1997, Mar Ecol Prog Ser147:197–217). Although complexity and the need for taxonomic accuracy are reduced in the generalmethod, the results are not significantly different between the 2 methods. Both methods also show agood fit with ingestion rates estimated from faecal pellet production. We suggest that the generalmethod be combined with automated sample treatment in future studies. In addition, we argue thatcarefully estimated faecal volume production provides a simple and quick overall feeding estimatewith important advantages over the common gut pigment technique, and it may be used as an independentmethod in bottle incubation experiments

Correcting for underestimation of microzooplankton grazing in bottle incubation experiments with mesozooplankton