Tropical macrophyte degradation dynamics in freshwater sediments: relationship to greenhouse gas production

Abstract

We investigate the potential of increasing temperature on CH4 and CO2 (C gas) production in order to predict the C gas production in tropical freshwater ecosystems. We explored seasonal variation of C gas production by simulations of the anaerobic decomposition of Utricularia breviscapa within the sediment of a tropical aquatic system, using a bioassay response, within the context of carbon cycling. We incubated U. breviscapa detritus in lake water from Oleo Lagoon, Brazil, and measured rates of CO2 and CH4 production over 138 days at four distinct temperatures simulating seasonal conditions of the lagoon from which the plant was collected. Periodically, subsamples of gases contained in the headspace of the decomposition chambers were collected, and the concentrations of CO2 and CH4 were quantified by gas chromatography. Methane production was limited at temperatures below 20 °C. Higher temperatures favored formation of CO2, while CH4 was a secondary product. The CO2 from the anaerobic metabolism of the sediment was the dominant final product. Based on the responses from the bioassays simulating the anaerobic decomposition of the prevalent U. breviscapa, we conclude that in the cooler ( 20 °C temperatures) indicate that CO2 and CH4 productions are both favored during warmer seasons, but CH4 appears with a delayed response. Within the range of temperatures of this tropical environment, biogenic C gas emissions from anaerobic degradation processes in sediments contribute strongly to the accumulation of C gas through carbon dynamics.