Tropic interactions in pampean shallow lakes: evaluation of silverside predatory effects in mesocosm experiments

Planktivorous fish directly suppress some prey species and indirectly affect species that interact with these prey (HRBACEK 1962, BROOKS & DODSON 1965, HALL et al. 1970, DRENNER et al. 1986). Visually feeding zooplanktivorous fish are size-selective predators, directly suppressing large zooplankton, and indirectly temporarily enhancing small zooplankton and phytoplankton (HURLBERT & MULLA 1981, DRENNER et al. 1990). The intensity of the direct and indirect effects of planktivores is dependent on fish density (THRELKELD 1988). Planktivorous fish are sometimes considered to promote eutrophication process in lakes (QUIRÓS 1998). Its trophic effects sometimes cascade through zooplankton to the base of the food web, promoting temporary algal growth by means of the suppression of large herbivorous cladocerans (CARPENTER et al. 1985). The young pampean silversides (Odontesthes bonariensis), dominant species in pampean shallow lakes, are visual zooplanktivore feeders. Moreover, pampean silverside abundance was shown to be indirectly related to phytoplankton biomass, after nutrient effects have been taken into account (QUIRÓS 1995). To evaluate the direct and indirect effects of this planktivore, two outdoor mesocosm experiments were conducted, simulating average conditions of salinity, nutrient status and plankton in the pelagic zone of a typical pampean shallow lake.