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 grazi ng and diet in two freshwater zooplankters, experiments were carried out to check chlorophyll a and xanthophyll conservation during feeding. For both animals, two sets of experiments were conducted by incubating animals in th e laboratory, either isolated from a reservoir (the calanoid copepod, Eudia ptomus gracilis) or cultured under high-food conditions (the cladoceran, Da phnia galeata). For both animals, gut pigments and clearance rates on diffe rent types of algae were determined from the same incubations. Chlorophyll a and derivatives, as well as major algal carotenoids, were analysed by Hig h Performance Liquid Chromatography (HPLC). In copepods, the pigment profil es from the gut extracts reflected the diet of the animals poorly. The anim al extracts contained almost exclusively alloxanthin (or an alloxantfiin-li ke pigment) in large amounts, whereas the other pigments were lost in high proportions (>70% for lutein and fucoxanthin; 57 and 78% for a-phorbins). T he 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 o f lutein and fucoxanthin may have occurred (18.7 and 30%). The loss rate fo r alloxanthin seemed more variable (0 and 68%), possibly depending on food concentration. As for the transformation of a-phorbins, E.gracilis and D.ga leata behaved quite differently. The HPLC profiles of copepod extracts alwa ys showed a very small chlorophyll a peak, along with phaeophytin a and pyr ophaeophytin 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 experimental conditions but converted chlorophyll a mainly into phaeophorbide. From a comparison of our results w ith data from other studies, it seems that in these two zooplankters, use o f gut pigment data for quantitative grazing assessment should be considered with caution.