ON ASSESSMENT OF PREY INGESTION BY COPEPODS

The consumption of photosynthetic and heterotrophic cells by an abunda nt calanoid copepod species feeding on natural plankton communities wa s quantified with a state-of-the-art image-analysis system. Late copep odid stages of Eucalanus pileatus did not ingest bacteria, small photo synthetic and heterotrophic nanoplankton, or the abundant Ceratium spp . in quantifiable amounts. Although diatoms were by far the most abund ant cells (in terms of POC l(-1)), the copepods ingested a higher perc entage of ciliates in relation to their abundance than of diatoms and small heterotrophic dinoflagellates in the first experiment, and inges ted a higher percentage of dinoflagellates and ciliates compared to di atoms in the second experiment. Heterotrophic cells sufficiently large to be captured were repeatedly preferred by E. pileatus over autotrop hs of similar or larger size. Moreover, among the cells which could he individually perceived by this calanoid, larger ones were not preferr ed over smaller cells, implying that some aspect of food quality can b e as significant as prey size. These results support the notion (e.g. Kleppel, Mar. Ecol. Frog. Ser.,99, 183-195, 1993) that feeding by cope pods will be underestimated if ingestion of heterotrophic food organis ms is not quantified. While the proposed microscope-based method is co mparatively slow (similar to 1 h per sample), it is the only technique which provides detailed information on both the size and trophic comp osition of ingested prey.