Luk, Sheron, Eagle, Meagan J., Mariotti, Giulio, Gosselin, Kelsey, Sanderman, Jonathan, and Spivak, Amanda C.
Salt marsh ponds expand and deepen over time, potentially reducing ecosystem carbon storage and resilience. The water filled volumes of ponds represent missing carbon due to prevented soil accumulation and removal by erosion and decomposition. Removal mechanisms have different implications as eroded carbon can be redistributed while decomposition results in loss. We constrained ponding effects on carbon dynamics in a New England marsh and determined whether expansion and deepening impact nearby soils by conducting geochemical characterizations of cores from three ponds and surrounding high marshes and models of wind‐driven erosion. Radioisotope profiles demonstrate that ponds are not depositional environments and that contemporaneous marsh accretion represents prevented accumulation accounting for 32%–42% of the missing carbon. Erosion accounted for 0%–38% and was bracketed using radioisotope inventories and wind‐driven resuspension models. Decomposition, calculated by difference, removes 22%–68%, and when normalized over pond lifespans, produces rates that agree with previous metabolism measurements. Pond surface soils contain new contributions from submerged primary producers and evidence of microbial alteration of underlying peat, as higher levels of detrital biomarkers and thermal stability indices, compared to the marsh. Below pond surface horizons, soil properties and organic matter composition were similar to the marsh, indicating that ponding effects are shallow. Soil bulk density, elemental content, and accretion rates were similar between marsh sites but different from ponds, suggesting that lateral effects are spatially confined. Consequently, ponds negatively impact ecosystem carbon storage but at current densities are not causing pervasive degradation of marshes in this system. Plain Language Summary: Ponds are natural features of salt marshes but their expansion may be an indicator of ecosystem deterioration because they impede the marsh's ability to keep pace with sea‐level rise and remove decades of buried soil carbon. The water filled holes created by ponds represent volumes of marsh soil carbon that are missing due to prevented accumulation or lost through erosion and decomposition. These loss pathways have different implications for coastal carbon cycling as eroded soils can be redeposited elsewhere while microbial decomposition represents permanent loss. We used geochemical and modeling approaches to assess how much of the carbon missing from ponds can be attributed to prevented soil accumulation, erosion, and decomposition as well as whether ponds reduce the integrity of the surrounding marsh. We estimate that these processes represent 32%–42%, 0%–38%, and 22%–68%, respectively, of soil carbon missing from three ponds in a New England salt marsh. The range of potential erosion losses reflect differences in fetch and wind‐driven waves used in the models. Decomposition was calculated by subtracting the contributions of prevented accretion and erosion from the volume of missing carbon and, while the range is large, losses normalized over time are comparable to previously measured respiration rates. Comparisons of soil properties and composition between the ponds and surrounding marsh demonstrate that the effects of expansion are confined to within a 10 m perimeter. Consequently, in this system, ponds represent net losses from the carbon budget and at current densities are not causing pervasive degradation of the marsh. Key Points: Salt marsh ponds deepen and expand over time but their effects are localized and do not result in deterioration of the surrounding marshThree processes account for soil carbon missing from deepening ponds: prevented deposition, erosion, and decompositionEroded soils may be redistributed and retained within the marsh, while decomposition represents carbon loss [ABSTRACT FROM AUTHOR]