CARBON DYNAMICS IN IMPOUNDED AND RESTORED WETLANDS: LINKAGES TO WETLAND HYDROLOGY AND ELEVATION

Over the past century, intense management of coastal wetlands has resulted in area losses between 30-80% across New England states. Hydraulic management, either intention or incidental, have led to impoundment, drainage, or other land-use modification of many former coastal wetlands. Such modifications of tidal hydrology have negative impacts on coastal wetland carbon storage and elevation. Draining wetlands lowers the water level, exposing buried organic material to oxygen, resulting in loss of both stored carbon and associated elevation of the marsh. Additionally, converting salt marsh habitat to another ecosystem disconnects the natural feedbacks between sea-level rise and platform elevation, leaving coastal wetlands with a reduced capacity to respond to future changes. In this study, we determine carbon storage and accretion rates across the diverse ecosystems currently found in the impounded and drained former salt marshes of the Herring River estuary (Cape Cod National Seashore, MA, USA). Since diking over a century ago, freshwater ecosystems, including Phragmites Australis, Typha sps., and forest and shrub areas replaced former salt marsh habitat. Each of these ecosystems has unique carbon burial rates and thus projected elevation trajectories. Ultimately, drained and impounded former marshes in the Herring River system do not store carbon at rates (70-180 g C/m²/y) that match adjacent healthy salt marshes responding to sea-level rise (160-250 g C/m²/y). Wetland systems, such as the Herring River, that continue to have altered hydrology are not resilient to predictions of future sea-level rise with accretion rates of 0 to 4.5 mm/year, and thus represent coastal landscapes at risk of flooding and, in the long-term, ecosystem loss. We compare currently impounded wetlands to formerly impounded wetlands that have had reestablished connection to the sea. These sites respond rapidly, which ecosystem shifts back to salt-tolerant species, and quickly build elevation (up to 15 mm/y) and rapidly store carbon (350 g C/m²/y) during the re-equilibrium period, indicating that hydrologic restoration is successful at returning this key ecosystem service.