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

In order to assess feeding selectivity in freshwater zooplankton, we conducted feeding trials using Diaptomus minutus isolated from two Wisconsin lakes. 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 of photopigments were tracked using high-performance liquid chromatography. The removals of carotenoids and a-type phorbins (chlorophyll a and phaeopigments) from feeding suspensions were compared with their presence in the guts of animals, in fecal pellets, and in the final suspension. Diaptomus minutus generally removed either the diatoms primarily or all three algal cells equally. These removals were not reflected in gut extracts of the animals, however, where alloxanthin (marker of cryptomonads) was always present, but where fucoxanthin and diadinoxanthin (markers of diatoms) were never observed. Pigment disappearance was variable for total a-type phorbins, but frequently >90% for carotenoids, particularly for fucoxanthin and diadinoxanthin. 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 made with substantial caution and that differences in pigment processing are likely 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 approach to understanding the feeding selectivity of copepods.