Coastal wetland adaptation to sea level rise: Quantifying potential for landward migration and coastal squeeze

Abstract

Coastal wetland ecosystems are expected to migrate landwards in response to rising seas. However, due to differences in topography and coastal urbanization, estuaries vary in their ability to accommodate migration. Low‐lying urban areas can constrain migration and lead to wetland loss (i.e. coastal squeeze), especially where existing wetlands cannot keep pace with rising seas via vertical adjustments. In many estuaries, there is a pressing need to identify landward migration corridors and better quantify the potential for landward migration and coastal squeeze. We quantified and compared the area available for landward migration of tidal saline wetlands and the area where urban development is expected to prevent migration for 39 estuaries along the wetland‐rich USA Gulf of Mexico coast. We did so under three sea level rise scenarios (0.5, 1.0, and 1.5 m by 2100). Within the region, the potential for wetland migration is highest within certain estuaries in Louisiana and southern Florida (e.g. Atchafalaya/Vermilion Bays, Mermentau River, Barataria Bay, and the North and South Ten Thousand Islands estuaries). The potential for coastal squeeze is highest in estuaries containing major metropolitan areas that extend into low‐lying lands. The Charlotte Harbor, Tampa Bay, and Crystal‐Pithlachascotee estuaries (Florida) have the highest amounts of urban land expected to constrain wetland migration. Urban barriers to migration are also high in the Galveston Bay (Texas) and Atchafalaya/Vermilion Bays (Louisiana) estuaries. Synthesis and applications. Coastal wetlands provide many ecosystem services that benefit human health and well‐being, including shoreline protection and fish and wildlife habitat. As the rate of sea level rise accelerates in response to climate change, coastal wetland resources could be lost in areas that lack space for landward migration. Migration corridors are particularly important in highly urbanized estuaries where, due to low‐lying coastal development, there is not space for wetlands to move and adapt to sea level rise. Future‐focused landscape conservation plans that incorporate the protection of wetland migration corridors can increase the adaptive capacity of these valuable ecosystems and simultaneously decrease the vulnerability of coastal human communities to the harmful effects of rising seas.

Sea-level driven land conversion and the formation of ghost forests

“…Our review suggests that widespread sea-level driven submergence of low-lying land will continue in the 309 future, even under scenarios of coastal population growth and large-scale defense of urban areas. 310 However, land conversion will largely take place on privately owned land 82,86 , where landowner 311 attitudes and adaptation efforts suggest local resistance 70 . We therefore pose the following 3 questions 312 to guide future research and land management decisions: 313 314 1.…”

“…Research in the last 5 years has identified 315 and mapped large barriers to wetland migration such as urban land and publically owned levees at 316 regional scales 77,[80][81][82] . However, the majority of vulnerable land is located on private property in rural 317 areas 38,82,87 . Future research should investigate the efficacy of local and privately maintained barriers 318 such as berms, ditches, and secondary roads, and the probability and consequences of barrier failure.…”

“…Analysis of topography and land use across the entire U.S. Gulf Coast indicate that 39,000 km 2 of land is 270 vulnerable to submergence under a 1.2 m sea level rise, and that barriers projected under population 271 growth will prevent conversion in an additional 6,000 km 2 77 . This work highlights that there are strong 272 spatial gradients in both opportunities and barriers to migration within the Gulf Coast region, such that 273 the absence of land conversion in highly urbanized areas may result in large reductions in local wetland 274 area 77,82 . Nevertheless, these analyses have three fundamental implications at the regional scale.…”

“…Other work 220 identifies additional caveats. For example, the response of low-lying land to sea level rise will vary both 221 within and across regions 19,[80][81][82] , where regions with steep upland topography and anthropogenic 222 barriers to migration may see near complete loss of marshes 81 . In places where marshes persist, the 223 proportion of flood-tolerant vegetation types will increase 78,79,81 and newly created wetlands may 224 themselves be vulnerable to sea level rise 10 .…”

“…6a). This region 267 contains approximately 50% of U.S. saline wetlands 75 , high variability in human population densities and 268 rates of relative sea level rise, and the most extensive flood protection system in the United States 77,82,93 .…”