The influence of mixing on seasonal carbon dioxide and methane fluxes in ponds

Inland waters are important sources of the greenhouse gases carbon dioxide (CO₂) and methane (CH₄). Ponds have amongst the highest CO₂ and CH₄ fluxes of all aquatic ecosystems, yet seasonal variation in fluxes remain poorly characterized, creating challenges for accurately estimating annual emissions. Further, ponds can exhibit a range of mixing regimes, yet the impact of mixing regimes on gas emissions remains unclear. Here, we assessed annual dynamics of CO₂ and CH₄ in four temperate ponds (Minnesota, USA) that varied in mixing regimes. The ponds ranged from annual sinks to sources of CO₂ (−1 to 15 mol m⁻² yr⁻¹) and were all significant sources of CH₄ (4.3–8.2 mol m⁻² yr⁻¹), with annual fluxes in CO₂ equivalents of 1.8–4.1 kg CO₂-eq. m⁻² yr⁻¹. Mixing regimes impacted CO₂ and CH₄ dynamics, as stratified periods were associated with more anoxia, greater accumulation of gases in the bottom waters, higher emissions of CH₄, and lower fluxes of CO₂. Ponds with stronger summer stratification also had increased CO₂ and CH₄ fluxes associated with fall turnover. Overall, the two ponds with the strongest stratification had higher annual fluxes (2.6, 4.1 kg CO₂-eq. m⁻² yr⁻¹) compared to the two ponds that more frequently mixed (1.8, 2.2 kg CO₂-eq. m⁻² yr⁻¹).