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

Nitrogen fixation by periphyton and plankton was measured on the Amazo n floodplain using the acetylene reduction method calibrated with N-15 -N2. The average ratio (+/- SD) of moles C2H4 reduced per mole N2-N fi xed was 3.4 +/- 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrog en fixation averaging 26% of the average rates in the light. The avera ge daily (24 h) rates for periphyton nitrogen fixation in 1989 and 199 0 were 1.79 and 0.51 mmol N2-N.m-1.d-1 respectively, which are compara ble to summer rates in many temperate cyanobacterial assemblages. Nitr ogen fixation was depressed at NO3- Concentrations as low as 0.5 muM, and was below detection limits at concentrations of 4 muM, which occur red during periods of river flooding. Planktonic nitrogen fixation rat es were high (0.5-0.8 mmol N2-N.m-2.d-1) during the high-water and dra inage 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-). Nit rogen fixation by periphyton and plankton in 1989-1990 accounted for a pproximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.