Influence of floating macrophyte on the phytoplankton from a wetland of the Lower Paraná River: an experimental approach.

O'FARRELL I.; DE TEZANOS PINTO P.; ALLENDE L.

Utrecht, Holanda

Congreso; VII Intecol International Wetlands Conference; 2004

INTECOL, Utrecht University

The effect of floating macrophyte cover on the phytoplankton is analyzed using in situ mesocosms. 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 treatments represent 98% decrease of the incident light. The temporal evolution of physical and chemical parameters, species composition, algal assemblages, morphological variability of the dominant populations, nutrition and mobility strategies of the phytoplankton community is studied over a period of six days. Picoplankton is analized at the beginning and at the end of the experiment using epifluorescence microscopy.             Initial anoxic conditions at all the enclosures were reversed in the treatment free of macrophytes achieving oxygen supersaturation and pH increase after 96 hours. Likewise, density and species composition revealed strong changes. In the treatments with light attenuation densities remained near 100000 ind/ml whereas in free water surface mesocosms density achieved almost 25000000 ind/ml. Cyanophyceae, represented by taxa of different functional groups, dominated under light deprived conditions. In the treatment with free waters, a marked increase of small chorococcales 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.