Laying it on thick: Ecosystem effects of sediment placement on a microtidal Rhode Island salt marsh
Raposa, K., M. Bradley, C. Chaffee, N. Ernst, W. Ferguson, T. Kutcher, Rick McKinney, K. Miller, S. Rasmussen, E. Tymkiw, AND C. Wigand. Laying it on thick: Ecosystem effects of sediment placement on a microtidal Rhode Island salt marsh. Frontiers in Environmental Science. Frontiers, Lausanne, Switzerland, 10:939870, (2022). https://doi.org/10.3389/fenvs.2022.939870
Climate change stressors, in particular, accelerated sea level rise are having adverse effects on coastal salt marshes, including marsh fracturing and drowning. The high loss rate of marsh acreage during the past decade has prompted resource managers and conservation groups to develop climate adaptation plans to enhance coastal marsh resilience against sea level rise. One possible technique to build resilience to accelerated sea level rise is to place clean, sandy sediment on the marsh surface to build elevation capital. In this study the beneficial re-use of dredged sediment (26,000 cubic meters)from a boating channel was placed (10 – 48 cm thick) on a nearby coastal drowning marsh in southern New England (Ninigret marsh, RI). Much of the marsh revegetated within a few years of adding dredged sediment. 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. A thick addition of dredged sandy sediment might be best for future climate adaptation projects where the goals are to restore high marsh species, build elevation capital, and establish a migration corridor for salt marsh habitat. For any sediment placement project, determining the targeted range of elevations and the corresponding thickness of added sediment is one of the most important aspects of project design and it ultimately depends on many factors including added sediment type, tide range, existing marsh elevation and plant distributions, project goals and desired lifespan of the new marsh. Our study demonstrates that adding a thick layer of sandy sediment is a viable option for some drowning coastal marshes, but note that adding thinner layers may be a more appropriate option under certain conditions.
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.
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