Grazing experiments with two freshwater zooplankters:fate of chlorophyll and carotenoid pigments

In order to evaluate the validity of the gut pigment method to assess grazing and diet in two freshwater zooplankters, experiments were carried out to check chlorophyll a and xanthophyll conser- vation during feeding. For both animals, two sets of experiments were conducted by incubating animals in the laboratory, either isolated from a reservoir (the calanoid copepod, Eudiaptomus gracilis) or cultured under high-food conditions (the cladoceran, Daphnia galeata). For both animals, gut pigments and clearance rates on different types of algae were determined from the same incuba- tions. Chlorophyll a and derivatives, as well as major algal carotenoids, were analysed by High Performance Liquid Chromatography (HPLC). In copepods, the pigment profiles from the gut extracts reflected the diet of the animals poorly. The animal extracts contained almost exclusively alloxanthin (or an alloxanthin-like pigment) in large amounts, whereas the other pigments were lost in high proportions (>70% for lutein and fucoxanthin; 57 and 78% for a-phorbins). The cladocerans fed on the main types of algae abundant in the suspensions, with a preference, however, for small cells. Although the main xanthophylls from these algae were detected in the Daphnia extracts, some destruction of lutein and fucoxanthin may have occurred (18.7 and 30%). The loss rate for alloxan- thin seemed more variable (0 and 68%), possibly depending on food concentration. As for the trans- formation of a-phorbins, E.gracilis and D.galeata behaved quite differently. The HPLC profiles of copepod extracts always showed a very small chlorophyll a peak, along with phaeophytin a and pyrophaeophytin a. Those from the cladoceran exhibited a large phaeophorbide a peak, along with some chlorophyll a and phaeophytin a. In fact, D.galeata did not destroy a-phorbins under our experi- mental conditions but converted chlorophyll a mainly into phaeophorbide. From a comparison of our results with data from other studies, it seems that in these two zooplankters, use of gut pigment data for quantitative grazing assessment should be considered with caution.