Structure et fonctionnement des ecosystemes du haut-rhone francais. 14-etude des preferences alimentaires de la loche franche (noemacheilus barbatulus L.) par une methode des points modifiee

The net effect of zooplankton on phytoplankton productivity was investigated through experiments using natural concentrations of plankton in the epilimnion of Castle Lake, California. Zooplankton enhanced algal productivity during July and August, and nocturnal grazers caused greater proportionate increases than did daytime densities. Consumers had neutral or negative effects in September. Animal biomass was associated significantly with enhanced productivity for the experiment in late July, and with decreased growth rates in the last trial of September. The direct relationship between the activities of zooplankton and primary productivity observed in other lakes is qualified by this seasonal change in net effect. The removal of algae by grazing, increases in the productivity to biomass ratio through nutrient regeneration and temporary photosynthetic inhibition from ammonia excretion appear to have shifted in relative impact during the 3 month experimental period.