High Intra-Seasonal Variability in Greenhouse Gas Emissions From Temperate Constructed Ponds

Inland waters play a major role in global greenhouse gas (GHG) budgets. The smallest of these systems (i.e., ponds) have a particularly large-but poorly constrained-emissions footprint at the global scale. Much of this uncertainty is due to a poor understanding of temporal variability in emissions. Here, we conducted high-resolution temporal sampling to quantify GHG exchange between four temperate constructed ponds and the atmosphere on an annual basis. We show these ponds are a net source of GHGs to the atmosphere (564.4 g CO2-eq m-2 yr-1), driven by highly temporally variable diffusive methane (CH4) emissions. Diffusive CH4 release to the atmosphere was twice as high during periods when the ponds had a stratified water column than when it was mixed. Ebullitive CH4 release was also higher during stratification. Building ponds to favor mixed conditions thus presents an opportunity to minimize the global GHG footprint of future pond construction. Ponds are an important contributor to global greenhouse gas emissions, but there is still much uncertainty associated with global emissions estimates. To clarify this uncertainty, we investigated seasonal patterns of greenhouse gas emissions from four constructed ponds in a temperate region of the northeastern United States. We found that methane made up most of pond greenhouse gas emissions on an annual basis, leading the ponds to be a net source of greenhouse gases to the atmosphere. Methane and carbon dioxide exchange between ponds and the atmosphere followed clear seasonal patterns, with highest rates of methane release to the atmosphere during warm summer months. Ponds consumed carbon dioxide during warm months when aquatic plants were growing rapidly. We also found high variability in methane and carbon dioxide emissions over weekly time periods that was associated with whether the water column in the pond was mixed or stratified. When ponds were stratified, methane emissions were higher than during mixed conditions, possibly due to low availability of oxygen near sediments where methane is produced by micro-organisms that require low oxygen conditions. These results provide a path forward to better estimating pond greenhouse gas emissions at a global scale. Constructed ponds are a net source of global greenhouse gass to the atmosphere on annual basisCO2 and CH4 exchange between ponds and the atmosphere show strong seasonal trends, with CH4 highly variable within seasonSummertime diffusive CH4 release dominates annual emissions budget