Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado

Abstract

Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with¹⁵N-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⁻²·d⁻¹ respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N0₃ ⁻ concentrations as low as 0.5 μM, and was below detection limits at concentrations of 4 μM, 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.