Studies of perturbed laboratory microcosm communities composed of algae and competing herbivorous microcrustaceans were conducted to assess the role(s) of various species compositions and patterns of resource consumption on the efficiency of resource utilization. Equilibrium microcosm communities, consisting of four competing species, three cladocerans, and an amphipod, were perturbed by different intensities of highly size—specific fish predation that resulted in altered species compositions (two or three additional species) and new equilibrium densities. Analyses of food particle consumption patterns, spatial distributions, and life history characteristics under the different predation intensities indicated the importance of microhabitat and food specializations in permitting coexistence of some species but exclusion of others. Age— and size—specific competition for limiting small (3—6 μm) particles was especially important among young animals at low predation levels, whereas under high predation, competition for larger particles among adults was more important. Analyses of particle uptake efficiencies in the different communities revealed very uneven use of available resources when the presence of efficient competitors for some foods produced developmental bottlenecks among the young of competing specis. Resource utilization efficiencies were much less variable at equilibrium when predation reduced competition on the young. The potential general importance of age— and/or size—specific competition in reducing the efficiency of resource utilization in communities of organisms other than terrestrial homeotherms is suggested and contrasted to the current community models of competition.

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To examine the relative demographic effects of predation and competition for food in rotifers during spring and summer in an oligotrophic lake, predator and competitor densities and food supplies were experimentally altered inside large enclosures. Abundances of rotifer species were positively correlated with experimental densities of fourth instar Chaoborus trivittatus larvae, a major crustacean predator in 1976. Experimental alteration of the densities of Daphnia rosea and Diaptomus leptopus and D. kenai in 1978 produced highly significant increases in rotifer biomass only under Daphnia removal, but not under copepod removal. Inorganic fertilizer additions to enclosures in 1978 and 1979 revealed minimal rotifer increases unless pulsed additions were large or Daphnia were also excluded. Large demographic reponses of rotifers to low fertilizer loadings in the absence of Daphnia confirmed the pre-eminence of competitive food limitation in producing rotifer scarcity in summer.

Grazing rates of a freshwater copepod (Diaptomus kenai) and a freshwater cladoceran (Daphnia rosea) on the green alga Selenastrum minutum were determined to be dependent on the nutritional status of individual cells. Cells that were less nitrogen limited were ingested at a greater rate than cells reared under more nitrogen limited conditions. Diaptomus displayed active discrimination, possibly via chemodetection, while Daphnia expressed passive selection, probably via differential retention on filters. These results suggest that the impact of grazing zooplankton varies with the physiological state of components of the phytoplankton community.