Quantitative comparison of food niches in some freshwater zooplankton : A multi-tracer-cell approach

The abilities of some zooplankton (rotifers, cladocerans, copepods) to ingest different sizes and kinds of food cells were quantified by determining the relative efficiencies with which they ingested nine tracer-cell types, ranging from a coccoid bacterium (0.45 μm³) to the alga Cryptomonas erosa (800-920 μm³). These efficiencies were obtained by dividing the clearance rate of each zooplankton group (species population, developmental stage or size class of a species population) on each ³²P-labeled cell type by that of a simultaneously-offered, ³³P-labeled, standard cell type - Chlamydomonas reinhardtii. Similarities of efficiency patterns on these cell types (food niches) between all possible pairs of the 17 zooplankton groups from 4 ecosystems were determined by calculating correlation coefficients. Although the utilization of the tested cell types may vary greatly within a species, three feeding guilds could be distinguished - based primarily on the efficiencies with which the smallest cell types were ingested. Guild I (Poyarthra vulgaris, Keratella crassa, Diaptomus minutus nauplii) ate the smallest cells (<4 μm diameter) (bacterium, Synechococcus, Nannochloris) and Ankistrodesmus very ineffifently but the three Cryptomonas species very efficiently. Guild II (Bosmina longirostris, D. minutus copepodites and Adults) had higher efficiencies on Synechococcus, Nannochloris, Ankistrodesmus, Stichococcus, and Stephanodiscus than guild I but similarly low ones on the bacterium and high ones on the Cryptomonas species. Guild III (Conochilus inicornis, Keratella cochlearis, Ceriodaphnia quadrangula, Diaphanosoma leuchtembergianum) differed from guilds I and II in having uniformly high efficiencies on all the small cells as well ad the larger ones. Principal component analysis of the matrix of correlation coefficients provided objective confirmation of the three guilds and provided a visual representation of the food niches of the 17 zooplankton groups in 3-dimensional space.