Publication

With sea level rise accelerating and sediment inputs to the coast declining worldwide, there is concern that tidal wetlands will drown. To better understand this concern, sources of sediment contributing to marsh elevation gain were computed for Plum Island Sound estuary, MA, USA. We quantified input of sediment from rivers and erosion of marsh edges. Maintaining elevation relative to the recent sea level rise rate of 2.8 mm yr(-1) requires input of 32,299 MT yr(-1) of sediment. The input from watersheds is only 3,210 MT yr(-1). Marsh edge erosion, based on a comparison of 2005 and 2011 LiDAR data, provides 10,032 MT yr(-1). This level of erosion is met by <0.1% of total marsh area eroded annually. Mass balance suggests that 19,070 MT yr(-1) should be of tidal flat or oceanic origin. The estuarine distribution of C-14 and C-13 isotopes of suspended particulate organic carbon confirms the resuspension of ancient marsh peat from marsh edge erosion, and the vertical distribution of C-14-humin material in marsh sediment is indicative of the deposition of ancient organic carbon on the marsh platform. High resuspension rates in the estuarine water column are sufficient to meet marsh accretionary needs. Marsh edge erosion provides an important fraction of the material needed for marsh accretion. Because of limited sediment supply and sea level rise, the marsh platform maintains elevation at the expense of total marsh area. Plain Language Summary Tidal marshes in the Plum Island Sound estuary have been gaining elevation over the past 100 years at about the same rate as sea level rise (SLR), but there is concern that they will drown and disappear if rates of SLR increase substantially due to CO2 emissions and climate change. What are the sediment sources enabling elevation gain? Rivers were not the primary source, providing less than 10% of elevation gain needs. Marsh edge erosion is much more important-providing over 30% of needs. We estimate that the remainder comes from the ocean or erosion of tidal flats. If the ocean is a major source, these marshes might be able to maintain elevation throughout the 21st century even if SLR greatly accelerates. However, if erosion of tidal flats is the primary source of sediments, the future outlook is less favorable because the more tidal flat erosion increases, the more edge erosion will also increase. The net result will be a loss of marsh area in this system. The dynamics we found for Plum Island Sound are likely to be occurring globally, as declining sediment inputs from rivers, and increasing rates of SLR due to climate change are worldwide phenomenon. Loss of marshes will have a major impact on the resilience of coastal communities landward of the marshes and the productivity of most coastal fisheries.