EFFECT OF FISH AND MESOZOOPLANKTON MANIPULATION ON THE PHYTOPLANKTON COMMUNITY IN THE PATOS LAGOON ESTUARY, SOUTHERN BRAZIL

An enclosure experiment in the Patos Lagoon estuary, southern Brazil (32-degrees-S, 52-degrees-W) investigated the response of phytoplankton to the enhancement of mesozooplankton and fish abundance. Addition of nutrients (NO3- and PO4-) stimulated the growth of 3-20 mum diameter phytoplankton, especially the diatom Cylindrotheca closterium, which, in turn, was heavily grazed by the dominant mesozooplankter, the copepod Acartia tonsa. Acartia did not consume small (2-3 mum) autotrophic flagellates and the cyanobacteria Anabaena sp., despite their high cell number. Largest grazing of C. closterium by the copepod occurred only after a decrease of ciliate abundance. The addition of mesozooplankton did not change the levels of primary production, but it significantly increased the phytoplankton assimilation number. Highest chlorophyll a concentrations were measured in enclosures stocked with juveniles of the fish Xenomelaniris brasiliensis. The large phytoplankton biomass and low mesozooplankton abundance found in this treatment indicates an effective predator-mediated action on the phytoplankton community.