Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Abstract

Freshwaters are important sources of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH₄ and CO₂ flux from a small shallow pond during a period of 4 months. Mean CH₄ flux and surface water CH₄ concentration were 8.0 [3.3–15.1] ± 3.1 mmol m⁻² day⁻¹ (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO₂ flux was 1.1 [−9.8 to 16.0] ± 6.9 mmol m⁻² day⁻¹. Substantial diel changes in CO₂ flux and surface water CH₄ concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH₄ emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO₂ flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO₂ consumption by plants was higher relative to CO₂ production by respiration during warm sunny days. Interestingly, CH₄ fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH₄ and CO₂ summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH₄ emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.