On assessment of prey ingestion by copepods

The consumption of photosynthetic and heterotrophic cells by an abundant calanoid copepod species feeding on natural plankton communities was quantified with a state-of-the-art image-analysis system. Late copepodid stages of Eucalanus pileatus did not ingest bactena, small photosynthetic and heterotrophic nanoplankton, or the abundant Ceratium spp. in quantifiable amounts Although diatoms were by far the most abundant cells (in terms of POC 1−1), the copepods ingested a higher percentage of ciliates in relation to their abundance than of diatoms and small heterotrophic dinoflagellates in the first experiment, and ingested a higher percentage of dinoflagel lates and ciliates compared to diatoms in the second experiment Heterotrophic cells sufficiently large to be captured were repeatedly preferred by E.pileatus over autotrophs of similar or larger size. More over, among the cells which could be individually perceived by this calanoid, larger ones were not pre ferred over smaller cells, implying that some aspect of food quality can be as significant as prey size. These results support the notion (e.g. Kleppel, Mar. Ecol Prog. Ser., 99, s183–195, 1993) that feeding by copepods will be underestimated if ingestion of heterotrophic food organisms is not quantified. While the proposed microscope-based method is comparatively slow (∽1 h per sample), it is the only tech nique which provides detailed information on both the size and trophic composition of ingested prey.