Pond excavation reduces coastal wetland carbon dioxide assimilation

Coastal wetlands comprise important global carbon sinks; however, anthropogenic disturbance accompanied with accelerating sea level rise threaten their continued survival. In this study, we quantified habitat disturbance to salt marshes in Barnegat Bay, New Jersey, resulting from the construction of ponds for mosquito control. Geographic object-based image analysis of high-resolution four-band aerial imagery revealed that over 7,000 ponds were constructed in the marsh complex with pond densities as high as 290 ponds per km(2). Physical disturbance from pond creation and sediment dispersal extended to over 17% of the bay's tidal wetlands. By tracking recolonization of vegetation, we estimated that it took 5 years for 51% vegetation recovery and 10 years for 69% recovery, with complete recover (100%) not expected for more than 50 years. This suggests that efforts to extend the lifespan of drowning coastal wetlands through sediment additions might disrupt carbon dioxide assimilation, as effects of disturbance persist. Focusing on greenhouse gas exchange, our work found that areas of marsh vegetation contribute to carbon assimilation (-42 g C . m(-2) . year(-1)), while ponds and areas of bare peat created by pond excavation were associated with carbon emissions (44 and 125 g C . m(-2) . year(-1), respectively). These results suggest that the conversion of wetlands to ponds-which is a significant driver of coastal wetland loss worldwide-may convert coastal wetlands from greenhouse gas sinks to sources. Additionally, quantifying the area of vegetation within a marsh (vs. bare ground or open water) is important for quantifying their greenhouse gas mitigation function. Plain Language Summary Coastal wetlands are important for our planet as they remove carbon dioxide from the atmosphere. Our goal was to determine how the construction of ponds in wetlands-which is common worldwide to support expanding aquaculture-affects the ability of marshes to take up carbon dioxide. We estimated the number and extent of ponds constructed for mosquito control in coastal wetlands in Barnegat Bay, New Jersey. Additionally, we measured greenhouse gas exchange from the ponds, surrounding intact marsh, and in bare areas at several sites across the growing season to estimate the total carbon uptake of the whole marsh. We found that over 7,000 ponds have been dug for mosquito control in New Jersey and that pond density is as high as 290 ponds per km(2). We also found that the ponds constructed in wetlands reduce the ability of wetlands to remove carbon dioxide from the atmosphere.