Nitrogen Fixation by Periphyton and Plankton on the Amazon Floodplain at Lake Calado

• Published in: Biogeochemistry 1994-02, Vol.26 (1), p.41-66
• Main Authors: Doyle, Robert D., Fisher, Thomas R.
• Format: Article
• Language: English
• Subjects: Alkynes; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Floodplains; Fresh water ecosystems; Freshwater; Fundamental and applied biological sciences. Psychology; Lakes; Macrophytes; Meadows; Nitrogen; Nitrogen fixation; Periphyton; Plankton; Synecology; Watersheds
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/BF02180403

Nitrogen fixation by periphyton and plankton was measured on the Amazon floodplain using the acetylene reduction method calibrated with 15N-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 NO3- 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· m2· 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 $(