Phytoplankton community structure in an Amazon floodplain lake (Lago Catalão, Amazonas, Brazil)

This study analyzed the attributes of the phytoplankton community (species richness, diversity, equitability, density, size structure and rate of community compositional change) associated with some physical and chemical characteristics of a mixed water lake (white and black water) for four consecutive weeks in the dry and rain season, in four sampling sites and at three depths (surface, middle and bottom). The presented differences in limnological characteristicas in the rainy and the dry season. The phytoplankton was composed, mainly by nanoplanktonic algae, especially Chlorophyceae and Cyanobacteria. In both periods the functional group X1 was the predominant association in terms of organism density. Considering the sampling seasons, the highest values of richness, equitability and diversity were recorded in the dry season. Chlorophyceae, Euglenophyceae, Bacillariophyceae and Cyanobacteria were the groups with the highest number of species in the dry season, while Chlorophyceae and Cyanobacteria were the groups with the highest species richness in the rainy season. The main physical and chemical variables that determine the distribution of the species were dissolved oxygen, temperature, electrical conductivity and ratio of mixing zone to euphotic depth. Key words: phytoplankton, floodplain, Amazon Lake.