Anthropocene Survival of Southern New England’s Salt Marshes
Watson, E., K. Raposa, J. Carey, C. Wigand, AND R. Warren. Anthropocene Survival of Southern New England’s Salt Marshes. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 40(3):617-625, (2017).
The research project, SHC 3.3.11, Wetlands and Nitrogen in Narragansett Bay Watershed, focuses on understanding components of sustainability, nutrient and other co-stressor effects, and decision-making beneficial to communities. The manuscript is a preface for a set of 7 manuscripts in a special Focus Issue of Estuaries and Coasts examining impacts of sea level rise on salt marshes in southern New England. Recent studies suggest that southern New England salt marshes are disappearing rapidly in this region, leading to reductions in habitat quality, plant diversity, carbon sequestration, erosion resistance and coastal protection. Results from the studies suggest that current inundation patterns have reduced the persistence of salt marsh plants and altered the diversity of coastal salt marsh habitats. Together the set of articles conclude that accelerated sea level rise is incompatible with the long-term survival of coastal salt marshes in southern New England. Salt marshes are coastal wetlands that lie at land margins, protecting human populations from inundation by the sea and buffering marine ecosystems from anthropogenic runoff. However, salt marshes are vulnerable to a changing climate, including accelerated sea level rise. To date, scientific attention in southern New England has not been focused on monitoring the extent and vulnerability of coastal wetlands, nor on detecting changes over time. In this Focus Issue, the objectives were to establish contemporary loss rates for coastal wetlands in Rhode Island and Connecticut, and second, to evaluate the role of inundation in current marsh loss patterns. These results suggest that southern New England salt marshes are already experiencing deterioration and fragmentation in response to sea level rise, and may not be stable as tidal flooding increases in the future. Collectively the studies point towards an extremely uncertain future for the Anthropocene survival of these valuable ecosystems. Finally, two manuscripts in the Focus Issue describe first, the development and implementation of a marsh vulnerability index, and second, describe climate adaptation actions and approaches to be field-tested as possible methods to counter the adverse effects of sea level rise on coastal wetlands.
In southern New England, salt marshes are exceptionally vulnerable to the impacts of accelerated sea level rise. Regional rates of sea level rise have been as much as 50 % greater than the global average over past decades, a more than fourfold increase over late Holocene background values. In addition, coastal development blocks many potential marsh migration routes, and compensatory mechanisms relying on positive feedbacks between inundation and sediment deposition are insufficient to counter inundation increases in extreme low-turbidity tidal waters. Accordingly, multiple lines of evidence suggest that marsh submergence is occurring in southern New England. A combination of monitoring data, field re-surveys, radiometric dating, and analysis of peat composition have established that, beginning in the early and mid-twentieth century, the dominant low-marsh plant, Spartina alterniflora, has encroached upward in tidal marshes, and typical high-marsh plants, including Juncus gerardii and Spartina patens, have declined, providing strong evidence that vegetation changes are being driven, at least in part, by higher water levels. Additionally, aerial and satellite imagery show shoreline retreat, widening and headward extension of channels, and new and expanded interior depressions. Papers in this special section highlight changes in marsh-building processes, patterns of vegetation loss, and shifts in species composition. The final papers turn to strategies for minimizing and coping with marsh loss by managing adaptively and planning for landward marsh migration. It is hoped that this collection offers lessons that will be of use to researchers and managers on coasts where relative sea level is not yet rising as fast as in southern New England.