Building Marsh Capital

Feb 12, 2020 | Healthy Habitats, Reserves, What We Work For

Reserve scientists in Maryland check a test plot where dredged sediment was applied. NERRS science is helping us understand how this approach can be used to protect our valuable marshes around the country. 

Every business owner knows the importance of building capital if you want to grow. Tidal marshes need capital too, but it’s measured in terms of elevation, not dollars. As rates of sea level rise accelerate, marshes that can’t keep pace will drown. A team assembled by eight National Estuarine Research Reserves (NERR) is advancing the science and practice of placing thin layers of sediment  on the marsh surface as a strategy to help marshes survive in place.

“The approach leverages a marsh’s natural capacity to withstand large, storm-driven sediment deposits and the ability of its plants to grow through new sediment or to colonize patches of bare mud and sand,” says Dr. Kenneth Raposa, the project lead and research coordinator at Rhode Island’s Narragansett Bay Reserve. “It mimics this process through the application of layers of sediment, often dredged material, to the marsh surface.”

Tidal marshes protect water quality, provide habitat for fish and wildlife, buffer against storm surge, reduce erosion, fortify upland property, and—together with other wetlands—provide an estimated $23.2 billion in storm protection annually. Thin-layer placement of sediment (TLP) is a marsh restoration technique that has been applied in Louisiana for decades, and it is being tested in different regions of the United States. Until now, however, there have been few scientific studies on its effectiveness and the factors that influence its success or failure.

The contrast of healthy and degrading marshes in Narragansett Bay, Rhode Island (top) and on California’s Elkhorn Slough (bottom).

Together with coastal managers from federal and state organizations and nonprofits, the NERRS team developed authoritative guidance documents on the use of TLP for enhancing marsh resilience. These include a consensus statement that advocates the testing of TLP as a restoration strategy across diverse geographies, a guide to navigating the permitting process, and universal monitoring metrics and guidance to consider when tracking the success of TLP projects.

The Reserves also conducted coordinated restoration experiments that looked at the impact of marsh elevation and sediment type and thickness on the success of this emerging wetland restoration technique. Greenhouse experiments exploring the effect of different sediment types complemented these field studies.  At three Reserves, researchers also looked at the addition of biochar—plant-based charcoal—to improve soil and plant health.

At each site, the addition of sediment raised marsh elevation, increased sandiness, and affected soil conditions such as acidity, moisture, and oxygen levels. Vegetation recovered quickly at most sites, with plants surviving the thinner and thicker sediment addition treatments. However, one of the most striking findings of the study was that marsh response to TLP varied greatly across marsh communities at different elevations and in different geographic regions.

The NERRS capacity for long-term monitoring across a national system allows Reserves to function as sentinel sites that can provide early warning signals to inform climate change adaptation and other management strategies. It also makes Reserves ideal living laboratories for testing innovative approaches to marsh conservation and restoration—at one site or across a range of diverse tidal marshes.

“As a system, the NERRS offers a rare national platform to conduct coordinated restoration experiments that apply statistical models and analyses across sites and over extended monitoring periods,” says Kerstin Wasson, research coordinator at California’s Elkhorn Slough Reserve and an investigator on the study. “While this capacity is critical to understanding trends in coastal and environmental conditions, it is extremely rare in the field of wetland restoration science.”

This research was made possible by sustained congressional investment in the NERRS approach to collaborative science. By engaging local communities in the research process, NERRS collaborative research projects directly address their needs while advancing estuarine science. Through a national network dedicated to sharing tools and knowledge, local research conducted at one Reserve strengthens all 29 sites and the communities they serve, leaving them better prepared to manage our changing coasts. Learn more about the NERRS approach to collaborative science.

What We Work ForHealthy HabitatsBuilding Marsh Capital