In situ feeding rates of plantonic copepods: A comparison of four methods

During a spring phytoplankton bloom in the Kattegat and Skagerrak area (Denmark) comparisons were made of the natural feeding rates obtained by four different methods of seven species of planktonic copepods. Measurements of gut contents (gut fluorescence) of freshly caught copepods were converted into estimates of in situ algal grazing rates by means of independently estimated gut turnover times, and were compared with chlorophyll and particle-volume grazing rates of animals sampled simultaneously and incubated in water from the collection depth. In addition, egg-production rates of adult females were measured during incubation, and converted to crude estimates of ingestion rate by assuming 33% gross production efficiency. The four methods yielded similar grazing rates with the gut fluorescence method generally yielding the highest and the egg-production method the lowest estimates. The methodological problems of the different methods are discussed, and it is concluded that they all approach representative (although minimum) estimates of in situ feeding rates.