THE ROLE OF PELAGIC WATER MITES IN THE CONTROL OF CLADOCERAN POPULATION IN A TEMPERATE LAKE OF THE SOUTHERN ANDES

In a small temperate lake of the southern Andes, Bosmina longirostris and Ceriodaphnia dubia coexist with the predaceous water mite Limnesia patagonica. Sampling of natural populations and laboratory experiments were carried out. The field population of Limnesia did not show a numerical response to the density or biomass of its prey. Laboratory experiments showed that the water mite rejected C.dubia adults and juveniles as prey, but consumed Bosmina. The maximum predation rate was 40 prey predator-1 day-1, and a linear relationship between predation rate and prey density was obtained (R2 = 66%). The contribution of mortality due to predation and the predation risk are too low to provoke a prey suppression. By means of computer modelling, the densities of predator necessary to explain half of the total prey mortality were calculated. These densities were one or two orders of magnitude higher than those in the field. It is concluded that Limnesia could really be a suppressor, but the potential depends greatly on its density.