Summary
Reverse-phase thin layer chromatography was used to quantify algal carotenoid degradation resulting from grazing by Daphnia magna in suspensions of lyophilized Oscillatoria utermöhlii, O. limnetica, Anabaena flos-aquae and Synechococcus sp. Three samples were removed every 24 h from 0 h to 144 h and the carotenoid, carbon and nitrogen content of the remaining algal-fecal mixture was determined. Grazing by Daphnia reduced carotenoid concentrations by 30% -80% after 144 h, with myxoxanthophyll being the most labile pigment in experiments with filamentous cyanophytes. However, as a group, the carotenoids were more resistant to the combined effects of grazing by Daphnia and bacterial action than were particulate algal carbon (78% to 82% lost by 144 h) or nitrogen (86%-90%) during the period in which easily digestible material remained. The maximal extent of carotenoid enrichment relative to particulate carbon ranged from 150% to 300% of initial pigment concentrations but declined with repeated coprophagy. On the basis of these results and published pigment budgets, we conclude both that fecal transportation of pigments may be an important determinant of carotenoid accumulation rates and that carotenoid stratigraphies should record predator-mediated changes in zooplankton community structure, especially in oligotrophic conditions.
Methodology
Daphnia magna Straus was obtained from Dr. P. D. N. Hebert, University of Windsor, Windsor, Ontario, Canada. Animals were cultured under continuous ('cool-white') fluorescent light in glass microfiber-filtered (Whatman GF/C), aerated well water. Cultures were maintained at 19° ± 2° and Daphnia were fed a diet of fresh, washed Oscillatoria utermöhlii. O. utermöhlii, O. limnetica, and Synechococcus sp. which were isolated by Dr. S. R. Craig. Oscillatoria cultures were isolated from Lake Opinicon, Ontario (44°33' N, 76°26' W) and Synechococcus from Little Round Lake, Ontario (44°47' N, 76°26' W). Anabaena flos-aquae (UTEX 1444) was obtained from Dr. R. Starr, University of Texas, Austin, Texas. All algal species, except A. flos-aquae, were cultured in a half-strength dilution of the mineral medium defined by Daley & Brown (1973b). One-litre cultures were grown at 19° ± 2° in 2.8-1 Fernbach flasks and received continuous fluorescent light (165 μEm-2s-1) provided by Sylvania Supersaver and Growlux lamps. Algal species were subcultured using aseptic techniques every 14 d although no attempt was made to achieve culture axeneity. A. flos-aquae was cultured in full strength medium buffered to pH 8.4 with CHES (2[N-cyclohexylamino] ethane sulfonic acid) at a final concentration of 20 mM.