Contribution of Photosynthesis-Driven Oxic Methane Production to the Methane Cycling of a Tropical River Network
Latika Patel, Rashmi Singh, Shoji D. Thottathil
Abstract
Oxygenated surface layers of aquatic systems are ubiquitously oversaturated with methane (CH 4 ). A growing number of studies suggest that CH 4 oversaturation in surface waters can be sustained, at least partly, by methanogenesis occurring under oxic conditions. Although we are gaining a better understanding of the extent and drivers of oxic CH 4 production (OMP) in oceanic and lake environments, the existence and variability of OMP in rivers and streams remain unknown. Here, we present experimental evidence for the occurrence and a large variability of OMP rates in a tropical river network. The positive correlation between chlorophyll a concentration and OMP rates and reduction of OMP during the experimental inhibition of photosynthesis establishes a clear link between OMP and photosynthesis. At the same time, a general decrease of the OMP rates with increasing total phosphorus (TP) concentration and the correlation between stable carbon isotopic (δ 13 C–CH 4 ) values of the OMP-derived CH 4 and TP suggest the likely involvement of P-availability as well in regulating the OMP rates. While our estimation suggested a minor contribution of the OMP in the CH 4 cycling of the studied tropical system, we show that the OMP in the fluvial environment may be highly sensitive to the current and future changes in algal and nutrient dynamics.