NITROGEN-FIXATION BY PERIPHYTON AND PLANKTON ON THE AMAZON FLOODPLAIN AT LAKE CALADO

Nitrogen fixation by periphyton and plankton was measured on the Amazon floodplain using the acetylene reduction method calibrated with N-15-N2. The average ratio (+/- SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 +/- 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N.m-1.d-1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at NO3- Concentrations as low as 0.5 muM, and was below detection limits at concentrations of 4 muM, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5-0.8 mmol N2-N.m-2.d-1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3-), but rates were undetectable (< 0.05 mmol N2-N.m-2.d-1) when there was river flooding (high NO3-). Nitrogen fixation by periphyton and plankton in 1989-1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.