Assessment of grazing by the freshwater copepod Diaptomus minutus using carotenoid pigments: a caution

In order to assess feeding selectivity in freshwater zooplankton, we conduc ted feeding trials using Diaptomus minutus isolated from two Wisconsin lake s. Copepods were fed an algal assemblage comprised of an equal biomass of a centric diatom, a cryptomonad and a coccal green alga. The total amounts o f photopigments were tracked using high-performance liquid chromatography. The removals of carotenoids and a-type phorbins (chlorophyll a and phaeopig ments) from feeding suspensions were compared with their presence in the gu ts of animals, in fecal pellets, and in the final suspension. Diaptomus min utus generally removed either the diatoms primarily or all three algal cell s equally. These removals were not reflected in gut extracts of the animals , however, where alloxanthin (marker of cryptomonads) was always present, b ut where fucoxanthin and diadinoxanthin (markers of diatoms) were never obs erved. Pigment disappearance was variable for total a-type phorbins, but fr equently >90% for carotenoids, particularly for fucoxanthin and diadinoxant hin. Phaeophytin a was the major a-type phorbin detected after gut passage. Our results indicate that evaluations of zooplankton grazing which assume that algal carotenoids remain detectable throughout gut passage must be mad e with substantial caution and that differences in pigment processing are l ikely to occur among zooplankton species. Furthermore, grazing experiments designed to evaluate decreases of specific pigments in feeding suspension, rather than their appearance in animals' guts, may prove a more valuable ap proach to understanding the feeding selectivity of copepods.