DIURNAL-VARIATION OF RATES OF DENITRIFICATION AND NITROGEN-FIXATION OF PERIPHYTON ASSOCIATED WITH ORYZA-GLUMAEPATULA STEUD IN AN AMAZONIAN LAKE

The rates of denitrification and biological nitrogen fixation of the p eriphyton communities associated with the culms and adventitious root of Oryza glumaepatula in Lake Batata, State of Para, Brazilian Amazoni a, were investigated over the course of 24 h. N2O was not detected dur ing the period, demonstrating that denitrification was not occurring. The low concentration of nitrate in the water column was indicated as the main factor limiting the denitrification process. The rates of bio logical nitrogen fixation (BNF) in culms were lowest at night and earl y morning, while the highest values were measured at 1200 h. BNF rates in periphyton associated with adventitious roots were lowest during t he morning, while highest values were measured at 1600 h. The differen ce in the daily patterns of BNF rates in the periphyton of culms and r oots of O. glumaepatula in the euphotic zone was attributed to deffere nces in the availability of organic carbon which supports this process . In the periphyton of the culms, the BNF process was carried out by h eterocystous cyanobacteria, which was photoautotrophs. In the periphyt on of the adventitious roots, BNF was carried out by periphytic bacter ia, and is heterotrophically mediated.