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

SOSNOVSKY, A.; QUIRÓS, R.

LIMNOLOGICA

ELSEVIER GMBH

Año: 2009 vol. 39 p. 219 - 229

0075-9511

Trophic cascade hypotheses predict that fish will affect the structure and biomass of pelagic plankton communities. Inorder to investigate trophic cascade effects from fish down to phytoplankton, whole-lake studies were performed in fivehypertrophic (mean total phosphorus (TP) concentrations higher than 1000mgm3) shallow lakes located in the PampaPlain. The main climatic characteristic of this region is the alternation between periods of drought and flood, withcorresponding 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 zooplanktoncommunity. Fish were manipulated in four lakes (Capurro, Longinotti, Vedia 1, Vedia 2), while the fifth (Lake Vedia 3) wasleft undisturbed as a reference system. High abundance of planktivorous minnows (Jenynsia multidentata and Cheirodoninterruptus) dominated the fish community in the reference lake. In the manipulated lakes, fish stocks were largely reduced inlate autumn (May). During winter, Capurro, Longinotti and Vedia 1 were stocked with a visual planktivore, the pampeansilverside (Odontesthes bonariensis, Atherinidae). Fish stocking was 24, 33 and 19 kg ha1, respectively. In contrast, no fishwere stocked in Lake Vedia 2. Following fish removal, large Daphnia appeared in these lakes and grazed intensively on thephytoplankton. In contrast, no Daphnia were found in the reference lake (Vedia 3). The stocking of O. bonariensis in lakesCapurro, Longinotti and Vedia 1 led to a decrease in the percentage of large cladocerans and a rise in the phytoplanktonbiomass:TP ratio. Moreover, the lakes mentioned were stocked with different quantities of silversides over different periodsof 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 werestocked. 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 smallpampean lakes. Fish removal was short lived, however. This could be because in small pampean lakes fish recolonization isfavored, and minnows are highly prolific. Moreover, if manipulation of the trophic structure of lakes is undertaken in thepampean region, high nutrient loading from the watershed, climate and hydrology should also be taken into account.