Predatory - prey relationships in subtropical zooplankton: water mite against cladocerans in an Argentine lake.

In a small lake in northern Argentina pelagic water mite Piona sp. had the maximum of population density in January following with a five-day delay after the peak of zooplankton dominant - Daphnia laevis. The mite density was highly predicted by the previous variations of Daphnia density during 4 months of observation (December-March). Daphnia density was a negative delayed function of the predator density but only in December-January when Piona was abundant. During that period Daphnia death rate, d was also correlated with the mite density (r 2=0.80, P<0.005). In laboratory experiments water mites killed 1-7 Daphnia · predator-1 · hour-1 in a broad range of prey density. Another zooplankton component, Diaphanosoma birgei, was consumed at the same rates. The mite hardly consumed any copepods. In the pelagium during 24 hours the mite was more associated with Daphnia, than with Diaphanosoma, probably, because of the coincidence in photoreactions with Daphnia. Piona contribution to the death rates of its prey estimated by using the data on functional and numerical responses as well as by means of Edmondson-Paloheimo model, could reach 53% for Daphnia and 40% for Diaphanosoma. A computer experiment on the reconstruction of prey dynamics after subtraction of predator influence showed that the mite could have caused a depression in Daphnia numbers observed in the lake, but the declines in Diaphanosoma population were caused by other factors. After the "removal" of mite pressure model Daphnia population increased its average density 10-fold. Experiments on Piona feeding revealed a strong effect of interference among predators. This was eliminated by putting one mite per experimental vessel, which led to a 20-fold increase in predation rate. The effect explains the low feeding rates of Piona obtained by the previous authors who ignored the possibility of interference.