Laying it on thick: Ecosystem effects of sediment placement on a microtidal Rhode Island salt marsh

Abstract

Heightened recognition of impacts to coastal salt marshes from sea-level rise has led to expanding interest in using thin-layer sediment placement (TLP) as an adaptation tool to enhance future marsh resilience. Building on successes and lessons learned from the Gulf and southeast U.S. coasts, projects are now underway in other regions, including New England where the effects of TLP on marsh ecosystems and processes are less clear. In this study, we report on early responses of a drowning, microtidal Rhode Island marsh (Ninigret Marsh, Charlestown, RI) to the application of a thick (10–48 cm) application of sandy dredged material and complimentary extensive adaptive management to quickly build elevation capital and enhance declining high marsh plant species. Physical changes occurred quickly. Elevation capital, rates of marsh elevation gain, and soil drainage all increased, while surface inundation, die-off areas, and surface ponding were greatly reduced. Much of the marsh revegetated within a few years, exhibiting aspects of classic successional processes leading to new expansive areas of high marsh species, although low marsh Spartina alterniflora recovered more slowly. Faunal communities, including nekton and birds, were largely unaffected by sediment placement. Overall, sediment placement provided Ninigret Marsh with an estimated 67–320 years of ambient elevation gain, increasing its resilience and likely long-term persistence. Project stakeholders intentionally aimed for the upper end of high marsh plant elevation growth ranges to build elevation capital and minimize maintenance costs, which also resulted in new migration corridors, providing pathways for future marsh expansion.

Beneficial use of sediments to restore a Chesapeake Bay marsh island

“…Increased acceptance and widespread use of NI approaches requires data-driven analysis of NI project performance (Davis et al, 2021). Currently, published monitoring data are available for a handful of projects across the U.S. for which dredged sediments were used for restoration of coastal habitats (McAtee et al, 2020;Douglas et al, 2021;Davis et al, 2022;Puchkoff and Lawrence, 2022;Raposa et al, 2022). While the practice of using dredged sediments to create coastal habitats is not new (dredge material-created islands are a common feature along many navigation channels), most historical projects were implemented with the primary goal of convenient disposal of dredged sediments rather than creation of specific habitats.…”