Contrasting CO ₂ Fluxes Among Arctic Pond Types Are Driven by Differences in Carbon Lability, Terrestrial Inputs, and Benthic Respiration

Small freshwater tundra ponds are hotspots for greenhouse gas release into the atmosphere. As the Arctic warms, thermokarst processes drive changes in tundra pond geomorphology, with implications for their contribution to carbon cycling in these regions. To understand how these morphological changes affect pond biogeochemistry, we compared low‐centered polygonal (LCP) ponds and thermokarst trough (TKT) ponds that are formed through ice‐wedge permafrost thaw. We found substantially higher levels of carbon, nutrients, and algal biomass in TKT ponds as compared to LCP ponds located in the Arctic Coastal Plain near Utqiaġvik, AK. These differences were likely driven by TKT ponds receiving increased terrestrial inputs due to discrete thermokarst events and slumping, lateral inflow of carbon and nutrients from surrounding soils, higher lability of TKT carbon, and greater rates of benthic respiration. Together, these factors create ponds that are more productive and greater sources of CO 2 to the atmosphere than LCPs. During the peak growing season (July‐August), CO 2 uptake from macrophyte‐dominated habitat in LCP ponds compensates for CO 2 release from open water, making LCP ponds net CO 2 sinks (−4 ± 3 gCO 2 /m 2 /d), whereas TKT ponds are CO 2 sources (3 ± 6 gCO 2 /m 2 /d), despite TKT macrophyte‐dominated habitat also being a site of CO 2 uptake. The net atmospheric greenhouse gas contribution of these ponds will likely be altered with high latitude warming, as air temperature influenced factors including vegetation cover, precipitation, water temperature, and thermokarsting alter rates of CO 2 flux from tundra ponds.