Sea level Rise and Carbon Cycle Processes in Managed Coastal Wetlands

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By Woods Hole Coastal and Marine Science Center May 10, 2023

The U.S. Geological Survey is working to inform decision makers at Federal land management agencies, including the U.S. Fish and Wildlife Service, the National Park Service, state government agencies and private entities, on the role of coastal wetlands in climate change mitigation and adaptation. This includes assessing opportunities for ecosystem restoration to enhance soil carbon sequestration, reduce methane emissions, and to increase carbon and alkalinity export to the coastal ocean. This comprehensive knowledge of carbon cycle processes is necessary to support the integration of coastal ecosystems within programs that reduce and inventory greenhouse gas emissions, including development of programs for greenhouse gas monitoring, reporting, and verification (MRV).

In this project, USGS scientists are studying how human modifications of coastal hydrology affect wetland resilience, migration, hazard mitigation, ecosystem condition, and carbon cycle processes. This knowledge will help guide decisions made by federal, state, and private land management organizations.

Why is this work important?

Photo of a tidally restricted wetland in the Herring River Estuary, Cape Cod, MA — Tidally restricted wetland in the Herring River Estuary, Cape Cod, MA

Wetlands can capture large quantities of carbon dioxide from the atmosphere and store it in their soil and plants—a process known as carbon sequestration. In fact, they are such significant carbon sinks that they can store carbon that has accumulated over hundreds to thousands of years.

Persistence of these habitats in the 21^st century depends on relatively near-term decisions regarding transportation infrastructure and other responses by society to sea level rise hazards. Maintenance and new deployments of roads and coastal defense structures, such as seawalls and dikes, can interfere with coastal hydrology, causing drainage or impoundment, and prevent tidal wetlands from migrating in response to sea level rise. When coastal wetlands are drained, impounded, or restricted, it jeopardizes the quality and existence of these vital ecosystems.

For centuries, in the U.S. and globally, there has been ongoing deployment of human-built structures that change coastal hydrology, including dikes, berms, levees, ditches, and seawalls. Additionally, transportation networks, including roads and railroads, cause widespread incidental tidal restriction in coastal wetlands. In a first set of national-scale analysis, we have estimated that more than 2.5 million acres of tidal wetlands in the conterminous United States are impounded, tidally-restricted, or drained behind infrastructure. In response to climate change and the accelerating rate of sea level rise, deployment of dikes is expected to increase in the coming decades, with potential for substantial additional impact on wetland ecological condition, on their ability to migrate landward in response to sea level rise, and ultimately the persistence of these vital ecosystems.

Coastal wetlands and mangroves that have been damaged or destroyed can lose their carbon sequestration capacity and the large quantities of carbon stored there can be released into the atmosphere. Improved management of salt marshes and mangroves, leading to enhanced conservation and restoration, is therefore a crucial climate change mitigation strategy—providing both enhanced carbon dioxide removal and avoided methane emissions.

graphic with 4 images of soil cores, tidal restrictions, and marshes — The soil core (top) was collected from Bass Creek, Yarmouth, MA, which was restored in 2008. From this soil core, scientists recreated the elevation of the marsh surface over the past 100 years, as well as how quickly elevation changed. The marsh peat also holds clues to the source of the carbon stored over the past century, an important ecosystem service that coastal wetlands provide. Restoring tides to this area brought the return of salt marsh vegetation, and enhanced soil accretion and carbon storage.

What we do?

The USGS and colleagues will develop functional maps, models and predictions for the existing and future intertidal landscape. Specific tasks include:

As a part of the NPS Herring River restoration project in the Cape Cod National Seashore, scientists are analyzing the blue carbon value of dike removal and wetland restoration. They are creating site scale models to predict groundwater and surface water hydrological changes in response to sea level rise, biogeochemical and vegetation responses that drive soil elevation change, changes in soil and vegetation carbon stocks, methane emissions and lateral export of carbon to the coastal ocean.

Learn more about the project!

Quantify environmental and geomorphic drivers of lateral carbon export to the ocean to inform numerical model development as well as provide data for coastal carbon budgets. Although not well known, tidal wetlands remove large amounts of carbon dioxide from the atmosphere, and export the carbon to the coastal ocean. Developing the first predictive model of carbon exports from marshes will provide a tool for predicting future changes in coastal carbon budgets with continued sea level rise.

More on the project!

person in yellow hat and blue waders kneels on ground and measures depth of hole in ground

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a sensor sits halfway out of the ground with two tubes sticking out of it surrounded by grass

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Assess the net ecosystem carbon budget of wetlands in Louisiana. Wetlands are undergoing a variety of restoration and management actions with the goal to provide tools for managers to plan for future resiliency. As a part of the Deepwater Horizon oil spill restoration effort, the carbon capture of flora, fauna and soils associated with coastal wetland restoration will be evaluated to determine what methods create more resistant marshes with high rates of carbon sequestration.

More on the Gulf Spill Restoration Project.

Explore drivers of coastal wetland accretion in the face of rising seas and human impacts to understand how burial of organic matter, sediment, and geomorphic evolution of salt marshes is tied to ongoing resilience and associated climate benefits.

green grassy marsh with taller brown grass in the background

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Two people stand in marsh with tube partially in the ground to core marsh

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Person in a lab splitting a core taken from a wetland

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This work is being conducted for current, future, and potential intertidal space throughout the conterminous United States (CONUS) and will include site-, state-, and regional-level analyses.

2 people standing in a green salt marsh under a cloudy sky

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Partners, stakeholders, and end users for this project include the U.S. Fish and Wildlife Service, the National Park Service, state governments and private land managers, and non-governmental organizations. The USGS will continue to actively engage with them to ensure project objectives help meet their specific needs CONUS-wide.