Under‐Ice Oxygen Depletion and Greenhouse Gas Supersaturation in North Temperate Urban Ponds.

Stormwater ponds are common features in urbanized landscapes because they enhance flood reduction and nutrient retention. With shallow depths and high inputs of organic matter, these systems can be highly productive with rapid oxygen depletion when thermally stratified or ice‐covered. However, most of our understanding of the biogeochemistry of stormwater ponds comes from the open water period. We explored under‐ice oxygen dynamics in 20 stormwater ponds in Madison, WI (USA) that were ice covered from late December to early March to investigate the drivers of bottom water oxygen saturation and the impact on the accumulation of carbon dioxide (CO2) and methane (CH4). Winter anoxia was driven by ice transmissivity, winter nutrient concentrations, and precedent summer productivity. Oxygen depletion led to overall higher concentrations of greenhouse gases in pond surface waters. This research enhances our understanding of winter pond biogeochemistry and its links to summer productivity. Plain Language Summary: In urban areas, stormwater ponds are common to help manage flooding and filter out nutrients to improve water quality in downstream water bodies. In the summer, urban ponds can experience a loss of oxygen in the bottom water, but less is known about what happens in these ponds in the winter under ice cover. We measured oxygen profiles in 20 ponds during the winter to investigate the drivers of oxygen loss in the bottom waters. We also explored the consequence of oxygen loss by measuring greenhouse gas (carbon dioxide and methane) concentrations in the surface water. Winter oxygen loss was driven by snow and ice clarity, winter nutrients, and previous summer organic matter from either algae or floating plants. Ponds with less oxygen under ice had higher concentrations of greenhouse gases in the surface water. Warmer winters could increase freeze thaw cycles and decrease oxygen under ice or increase oxygen with more instances of open water. Key Points: Urban ponds exhibit a range of under‐ice oxygen regimesWinter anoxia is driven by ice transmissivity, nutrient concentrations, and preceding summer productivityA consequence of under‐ice oxygen depletion is the accumulation of methane and carbon dioxide [ABSTRACT FROM AUTHOR]