Anthropocene Survival of Southern New England’s Salt Marshes

Abstract

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 four-fold 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 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 upwards 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.

Sea-Level Rise Tipping Point of Delta Survival

“…Their upper threshold is similar to the .5-6 mm y À1 tipping point suggested in Figures 2 and 3. Also, Fujimoto et al (1996) estimated that the limit of organic accretion for Pacific mangroves was between 2 and 10 mm y À1 , and Watson et al (2017) declared that the future of RSLR had already arrived in New England salt marshes where recent SLR rates over the last 19 years were 4 to 6 mm y À1 and salt marsh losses over the last 3 to 4 decades were 0.4 to 0.44% y À1 . The rate of delta collapse later this century may be much faster than that of delta initiation during the mid-Holocene.…”

Salt Marsh Dynamics in a Period of Accelerated Sea Level Rise

“…One important caveat of this approach, however, is that marshes accreting at the same rate of RSLR are not necessarily maintaining their areal extent through time. Indeed, horizontal processes such as marsh edge retreat (Mariotti & Fagherazzi, 2010) and channel widening (Deegan et al, 2012; Mariotti, 2018; Watson et al, 2016) could erode marshes independently of their vertical accretion. Furthermore, the formation and enlargement of ponds could cause marsh loss despite that the vegetated marsh platform keeps pace with RSLR (Mariotti, 2016).…”

High Spatial Resolution Remote Sensing for Salt Marsh Mapping and Change Analysis at Fire Island National Seashore

“…Historically, salt marshes have displayed high rates of loss due to land reclamation and disturbances such as mosquito ditching [3,4]. Currently, salt marshes along the mid-Atlantic coastal region of the United States are at risk of loss due to sea level rise (SLR), eutrophication, nutrient enrichment, sediment availability, tidal range, and herbivory and human disturbances [5][6][7][8][9][10][11][12]. Recent [44].…”