Predation effects on prey coexistence: habitats free of fish versus habitats with fish in Patagonia Argentina

TROCHINE, C., BALSEIRO, E. & MODENUTTI, B.

Natal

Congreso; XIII Congresso Brasileiro de Limnologia; 2011

Predation is one of the factors believed to have a major impact on competitive interactions by sustaining the stable coexistence of closely related species. However, its role have frequently been questioned by numerous examples of diversity reduced by predation. Many lakes in the Patagonia Argentina experienced salmonids introduction since the early 20th century, and its effects in trophic dynamics remained unknown. We carried out a plankton sampling in several Patagonian lakes (from 39º to 49ºS.) to compare habitats free of fish and habitats with introduced fishes. We analyzed zooplankton and phytoplankton composition, phytoplankton relative abundance, and zooplankton body size and mouthpart morphology. Our results showed differences between lakes without and with fishes, Daphnia and large endemic centropagid copepods dominated the zooplankton assemblage in the former while small centropagids and rotifers were the most abundant zooplankters in the latter. Moreover, three or four centropagid species that differed markedly in body size co-occurred in systems lacking fish because of differences in resource utilization. Interestingly, fish reduced the crustacean zooplankton community to a single small sized herbivore. Fish introduction also increased phytoplankton similarity in lakes from different basins and cyanobacteria dominated only in lakes with introduced fishes. We speculate that the elimination of Daphnia favored cyanobacteria proliferation because of nutrient rebalance. Our conclusion is that predation in this case hindered the probability of prey coexistence. Different factors could account for these results including e.g. sized selective predation by fish, increased mortalities of zooplankters due to reduction of protective pigments, unpalatability and low food quality of cyanobacteria, among the most relevant.