Denitrification and bacterial community structure in the cascade of six reservoirs on a tropical river in Brazil

Denitrification activity was measured by the acetylene blockage technique in six reservoirs of the Tiete River, southeastern Brazil, in March 1999 and July 1999. The total number of bacteria was measured by DAPI while bacterial community structures were estimated with fluorescent in situ hybridization (FISH) using oligonucleotide probes specific to ribosomal RNA of the domain Bacteria; the alpha, beta and gamma subclasses of the Class Proteobacteria; the Cytophaga-Flavobacterium group; the Order Planktomycetales; and the Domain Archaea. Denitrification was detected only where anoxia or low concentrations of dissolved oxygen were observed. Maximum in situ denitrification integrated in the water column ranged from 1.36 to 3.79 mmol-N2O m(-2) day(-1) in the rainy season and in the dry season, respectively, both at Station 1, the upstream portion of Barra Bonita Reservoir influenced by the sewage carried out by the river from Sao Paulo City. Bacterioplankton community analysis by FISH showed that the Alpha subclass of the class Proteobacteria was the main subgroup in the middle and low Tiete River (Fig. 3), represented by 10.3-19.2% of total, followed by the Beta Subclass of Proteobacteria and the Cytophaga/Flavobacterium group, represented by 8.3-13.3% and 3.4-10.9%, respectively). This result differs from the proportion of bacterial subgroups in water bodies of temperate regions, suggesting that bacterial communities of water bodies of the tropical region may differ from those of temperate regions.