Oxygen depletion and carbon dioxide and methane production in waters of the Pantanal wetland of Brazil

Abstract

This study examines dissolved O2, CO2 and CH4 in waters of the Pantanal, a vast savanna floodplain in Brazil. Measurements are presented for 540 samples from throughout the region, ranging from areas of sheet flooding to sluggish marsh streams to the major rivers of the region. Dissolved O2 is often strongly depleted, particularly in waters filled with emergent vascular plants, which are the most extensive aquatic environment of the region. Median O2 concentrations were 35 μM for vegetated waters, 116 μM for the Paraguay River, 95 μM for tributary rivers, and 165 μM for open lakes (atmospheric equilibrium, 230–290 μM). Airwater diffusive fluxes were calculated from dissolved gas concentrations for representative vegetated floodplain waters, based on data collected over the course of an annual cycle. These fluxes reveal about twice as much CO2 evasion as can be accounted for by invasion of O2 (overall means in nmol cm-2 s-1: O2 0.18, CO2 0.34, and CH4 0.017). Methanogenesis is estimated to account for ca. 20% of the total heterotrophic metabolism in the water column and sediments, with the remainder likely due mostly to aerobic respiration. Anaerobic respiration is limited by the low concentrations of alternate electron acceptors. We hypothesize that O2 transported through the stems of emergent plants is consumed in aerobic respiration by plant tissues or microorganisms, producing CO2 that preferentially dissolves into the water, and thus explaining most of the excess CO2 evasion. This hypothesis is supported by measurements of gases in submersed stems of emergent plants.