Effects of fish manipulation on the plankton community in small hypertrophic lakes from the Pampa Plain (Argentina)

Trophic cascade hypotheses predict that fish will affect the structure and biomass of pelagic plankton communities. In order to investigate trophic cascade effects from fish down to phytoplankton, whole-lake studies were performed in five hypertrophic (mean total phosphorus (TP) concentrations higher than 1000 mg m(-3)) shallow lakes located in the Pampa Plain. The main climatic characteristic of this region is the alternation between periods of drought and flood, with corresponding changes of lake depth and conductivity of lake water. All lakes were studied from April to December 2000. Samples were taken of their physical and chemical characteristics and biotic communities, focusing on the zooplankton community. Fish were manipulated in four lakes (Capurro, Longinotti, Vedia 1, Vedia 2), while the fifth (Lake Vedia 3) was left undisturbed as a reference system. High abundance of planktivorous minnows (Jenynsia multidentata and Cheirodon interruptus) dominated the fish community in the reference lake. In the manipulated lakes, fish stocks were largely reduced in late autumn (May). During winter, Capurro, Longinotti and Vedia I were stocked with a visual planktivore, the pampean silverside (Odontesthes bonariensis, Atherinidae). Fish stocking was 24, 33 and 19 kg ha(-1), respectively. In contrast, no fish were stocked in Lake Vedia 2. Following fish removal, large Daphnia appeared in these lakes and grazed intensively on the phytoplankton. In contrast, no Daphnia were found in the reference lake (Vedia 3). The stocking of O. bonariensis in lakes Capurro, Longinotti and Vedia I led to a decrease in the percentage of large cladocerans and a rise in the phytoplankton biomass:TP ratio. Moreover, the lakes mentioned were stocked with different quantities of silversides over different periods of time. These differences were reflected temporarily in the plankton dynamics, affecting mainly larger sized zooplankton. Nevertheless, the presence of Daphnia was short lived in the lake where fish had been removed and no O. bonariensis were stocked. Competition for resources and recruitment of remaining fish probably caused a collapse in the zooplankton biomass. Our results support the idea that fish predation on zooplankton and its effect on phytoplankton is very intense in small pampean lakes. Fish removal was short lived, however. This could be because in small pampean lakes fish recolonization is favored, and minnows are highly prolific. Moreover, if manipulation of the trophic structure of lakes is undertaken in the pampean region, high nutrient loading from the watershed, climate and hydrology should also be taken into account. (C) 2008 Elsevier GmbH. All rights reserved.