Impact of different zooplankton structures on the microbial food web of an Andean oligotrophic Lake

MODENUTTI BEATRIZ, QUEIMALIÑOS CLAUDIA, BALSEIRO ESTEBAN & REISSIG MARIANA

Porto - Portugal

Congreso; Plankton Symposium II; 2001

In oligotrophic Andean lakes omnivorous calanoid copepods are the dominant zooplankters and, at the same time, nanoflagellates and mixotrophic ciliates are the prevailing primary producers. In Lake Rivadavia (Patagoni, Argentina) a centropagid assemblage constituted by Boeckella michaelseni and Parabroteas sarsi was observed. In addition, the large Daphnia middendorffiana was the dominant cladocerans, whereas small-bodied species were scarce. This particular zooplankton constitution let us to hypothesize that its effect on the microbial food web would differ substantially depending on the presence of cladocerans or copepods. To determine the effect of predation in the pelagic microbial food web in this lake, we conducted a series of field experiments consisting in the manipulation of different zooplankton structures in two liters enclosures. Results showed that the presence of B. michaelseni depressed ciliates and nanoflagellates while no important changes were observed in autotrophic picoplankton and total bacteria abundances. On the contrary, the presence of Daphnia was decisive in decreasing autotrophic picoplankton abundances. Parabroteas sarsi was observed to prey on B. michaelseni copepodites and adults and a weak cascading effect on the microbial fraction could be detected.