A COMUNIDADE FITOPLANCTÔNICA DE UM LAGO TROPICAL NO SUDESTE DO BRASIL (LAGO DOM HELVÉCIO, ESTADO DE MINAS GERAIS)

The aim of this study was to analyze the structure and seasonal and spatial variability of the Dom Helvécio Lake phytoplankton, a warm monomitic lake belonging to the lacustrine system of Rio Doce Valley, Minas Gerais State. In as much as four field trips were accomplished in eleven sampling stations for collecting phytoplanktonic material and for the following physical and chemical variables: pH, electric conductivity, temperature, dissolved oxygen, nitrite, nitrate, ammonium, total nitrogen, ortophosphate, total dissolved phosphorus, total phosphorus, ortossilicate, organic and inorganic suspended matter. Also TN/TP ratio, trophic state index, specific richness, equitability, dominance and the diversity of phytoplanktonic community were estimated. It was verified through quantitative analyses that phytoplankton was composed by 124 taxa, belonging, in majority, to the taxonomic groups Chlorophyceae, Conjugatophyceae and Cyanophyceae. Higher total densities occurred during stratification period, and were coincident with diversity indexes reduction. The Conjugatophyceae and Cyanophyceae classes showed higher relative abundance during stability period and the Chlorophyceae class during circulation period. The results indicated that the main forcing function in relation to phytoplankton was constituted by the climatic factors which determines periods of water column stratification and circulation. The Lake Dom Helvécio was classified as oligotrophic, considering species composition, population densities, and diversity of the phytoplanktonic community