Floating macrophyte influence on the phytoplankton from a wetland of the lower Paraná river (Otamendi Natural Reserve, Buenos Aires, Argentina)

O'FARRELL, INES; DE TEZANOS PINTO, PAULA; ALLENDE, LUZ

Utrech, Holanda

Congreso; 7th INTECOL Wetland Conference; 2004

INTECOL

The effect of floating macrophyte cover on the phytoplankton is analyzed using in situ mesocosmos. The experiment consists of three treatments: macrophyte cover removal, dense macrophyte cover and macrophyte removal with addition of an obscure mesh simulating the light attenuation produced by floating vegetation. The latter treatment represents 98% decrease of the incident light. The temporal evolution of physical and chemical parameters, species composition, algal assemblages, morphological variability of the dominant populations, and nutrition and mobility strategies of the phytoplankton community is studied over a period of six days. Picoplankton is analyzed at the beginning and at the end of the experiment using microscopy. Initial anoxic condition at all the enclosures was reversed in the treatment free of macrophyte achieving oxygen super saturation and pH increase after 96 hours. Likewise, density and species composition revealed strong changes. In the treatments with light attenuation, densities remained near 100.000 ind/mL whereas in free water surface mesocosmos density achieved almost 25.000.000 ind/mL. Cyanophyceae were represented by taxa of different functional groups, dominant under light deprived conditions. In the treatment with free waters, a marked increase of small chloroccocales and volvocales was observed after 96 hours. Both heterotrophic picoplankton and picocyanobacteria densities remained similar at the three treatments, whereas the picoeucariotic fraction increased strongly in the treatment without cover.