Food type and concentration affect chlorophyll and carotenoid destruction during copepod feeding

Abstract

To evaluate the use of pigments as tracers for determining copepod grazing rates and selectivity, we examined the stability of several biomarker pigments during copepod feeding incubations. During these incubations, we measured changes in phytoplankton‐derived chlorophylls and carotenoids in the particulate and dissolved pools. Budgets were calculated to determine changes in pigment concentrations in the food, copepod fecal pellets, copepod guts, and dissolved/colloidal fraction. For each of six algal diets, adult female Acartia tonsa copepods were fed limiting and saturating food concentrations, approximately 100 and 500 µg C L−1 , respectively. Thalassiosira weissflogii, Rhodomonas lens, Chroomonas salina, Dunaliella tertiolecta, and two Tetraselmis strains were used in the feeding experiments to investigate the fate of fucoxanthin, alloxanthin, lutein, chlorophyll (Chl) b, and Chl a. In all experiments using saturating food concentrations, the dissolved/colloidal pool contained no more than 3% (usually less than 1%) of any pigment, whereas in experiments using limiting food conditions, pigments were undetectable in the dissolved/colloidal pool. Pheopigments were present in fecal pellets and copepod guts in most of the experiments. Chl a destruction (conversion to colorless products) was variable among the different experiments, depending on algal species and food concentration and, in most cases, Chl a was destroyed to a greater extent than the carotenoids. In all cases, pigment destruction was higher when copepods were fed limiting rather than saturating food concentrations. These data attribute the variability in pigment destruction to algal species and concentration, and suggest caution when pigments are used as tracers of herbivory. In such studies, assumptions about conservative behavior, even for the carotenoids, would need to be verified for each set of experimental conditions and grazers.