Application of fluorometry (Phyto-PAM) for assessing food selection by cladocerans

Abstract

Cladocerans feed on a variety of phytoplankton food sources, which are variable across space and time. Different phytoplankton groups represent different nutritional quality to Daphnia magna (Straus) due to differences in their Redfield stoichiometry or digestibility. We used chlorophyll fluorescence to directly measure phytoplankton group quantity and composition (green algae and cyanobacteria) in the guts of live D. magna and thereby directly assessed differences in gut transit time (GTT). We observed a decrease of GTT with lower food quality relative to the most suitable diet treatment composed only of the green alga: Scenedesmus obliquus (Turpin) Kutzing (GTT = 27 min 20 s). Mixed, lower quality diets composed of 75% cyanobacteria, Microcystis aeruginosa (Kutzing) and 25% S. obliquus resulted in faster GTT, as did diets exclusively composed of microcystin-lacking cyanobacteria (GTT = 19 min 42 s). The GTT could not be measured when diets were composed exclusively of microcystin-producing M. aeruginosa due to insignificant ingestion of the cyanobacteria. Using Ivlev’s Electivity index, we demonstrate that provided with mixed algal food, D. magna was able to avoid ingesting lower quality food (cyanobacteria) in favor of more valuable material (green alga). However, this ability decreases or is lost when exposed to food mixtures dominated by cyanobacteria.