Seasonal shifts in phytoplankton ingestion by Daphnia galeata, assessed by analysis of marker pigments

To determine food selectivity in Daphnia galeata, a common freshwater cladoceran, the carotenoid pigments in the guts of animals collected in a meso-eutrophic reservoir were analysed. Using the ratios of carotenoid to chlorophyll a for the main marker pigments enabled an investigation of the algal diet of this herbivorous zooplankton at the class level in natural conditions. To allow a comparison with data from pigments, food-removal experiments were carried out using Daphnia spp. isolated from the lake, fed with the algal assemblages present in the lake at different times. For both methods, based on gut-pigment analysis and food removal, the total and class-specific grazing rates were calculated, and food selection over time was assessed from the mean of selectivity indexes. Except for three dates, both methods agreed on the qualitative composition of the diet of D. galeata. This herbivorous species fed preferentially on the most available algae in the edible size range, and showed positive selection for cyanobacteria and cryptophytes during most of the growing season. The gut-pigment method seems to be an interesting tool for following the algal selectivity of freshwater cladocerans in natural conditions.