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Rising Sea Levels, Coastal Development’s Effect on Gulf Coast Wetlands

October 3, 2016

As coastal development along the Gulf Coast continues to expand, tidal saline wetlands could have difficulty adjusting to rising sea levels.

Tidal Saline Wetland migration map of the U.S. Gulf Coast.
This illustration shows county-level barriers and opportunities for landward migration of Tidal Saline Wetlands under alternative sea-level rise scenarios. (a) Relative percent of areas available for migration. (b to d) Relative ratio of areas where migration is prevented by current urban land (b), by future urban land (c), and by levees (d). Counties with crosshatching are expected to have less than 1 km2 of area available for migration. USGS Graphic. 

Tidal saline wetlands along the U.S. Gulf of Mexico coast, such as mangrove forests, salt marshes, and salt flats, face survival challenges as sea levels rise rapidly and development along coastlines continues to grow. But, a recently published U.S. Geological Survey study shows there is hope for some of these at-risk Gulf coast wetlands.

In the study, which was conducted from 2012 - 2015, the authors considered the potential for landward movement of coastal wetlands under different sea-level rise scenarios. They also considered the impact of barriers to wetland migration due to current and future urbanization and examined how existing conservation lands, such as parks and refuges, might accommodate expected landward migration.

“Tidal saline wetlands along the northern Gulf of Mexico are abundant, diverse, and vulnerable to sea-level rise,” said Nicholas Enwright, USGS researcher and lead author of the study. “Our findings provide a foundation for land managers to better ensure there is space for future wetland migration in response to sea-level rise.”

Without areas for these wetlands to move to, people and wildlife will lose the beneficial functions they provide.

As the name implies, the water levels within tidal saline wetlands are dictated by tidal fluctuations and this unique environment is dominated by plant and animal species that live in salt water. These wetlands provide positive benefits for nature and society by serving as habitats for fish and wildlife, improving water quality, lessening flooding impacts, supporting coastal food webs, protecting coastlines, as well as providing recreational opportunities.

Tidal saline wetlands are an important and distinct ecosystem, yet the increase in human development along coastlines is placing a burden on the wetland’s ability to adapt to rising sea levels. 

“In response to sea-level rise, coastal wetlands have historically moved across the landscape,” said Michael Osland, USGS Research Ecologist and co-author of the study. “However, coastal barriers can prevent wetland migration.” 

Historically, these wetlands have been able to adjust to small fluctuations in sea level through natural processes that can increase their elevation in their current area. They have also been able to migrate inland when rapidly rising seas have overcome their ability to keep up. However, as coastal development increases – such as buildings, roads, parking lots, and flood-prevention infrastructure like levees and seawalls – the paths wetlands could take for inland migration may become blocked. But, the study does illustrate that proper urban planning could benefit these at-risk ecosystems.

“Although we found migration barriers due to urban development and levees in many areas, we also identified a tremendous amount of land in our study area that is available for future landward migration,” said Osland. “However, it’s also important to recognize that migration will occur at the expense of adjacent upriver and upslope ecosystems.”

The study indicates that three counties in Florida and six parishes in Louisiana (Collier, Monroe, and Miami-Dade counties in Florida; Assumption, Cameron, Lafourche, Terrebonne, St. Mary and Vermilion parishes in Louisiana) account for the majority of the total area available for landward migration. In other areas, higher coastlines and ridges limit the relative area available for landward migration.

Migration barriers from current and future urban development are most likely to occur around Tampa, Florida, extending along the coastline toward Fort Myers, Florida. Barriers also exist in leveed areas, concentrated in South Florida near the Everglades, in south Louisiana, and in eastern Texas. Some of the undeveloped areas within these levee areas could be made available for landward migration through carefully planned levee breaching, which could give these wetlands room to expand.

Coastal resource managers will now have the information outlined in this study as an additional tool to help them develop conservation strategies that will enable these wetlands to better adapt to future change.

“We are hoping that this study will stimulate regional conservation and land-use planning discussions regarding landward migration corridors for tidal saline wetlands,” said Enwright. “Migration corridors can be incorporated into landscape conservation design plans.”

Funding for this study was provided by the U.S. Fish and Wildlife Service, the USGS Ecosystems Mission Area, and the work was conducted in cooperation with the four Gulf Coast Landscape Conservation Cooperatives.

To read the full study, which was published in the Ecological Society of America’s journal, Frontiers in Ecology and the Environment, click here.

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