By Will Dunham
WASHINGTON, May 19 (Reuters) – The loss of tidal wetlands – fragile coastal ecosystems continually flooded and drained by the ebb and flow of ocean tides – is accelerating in the United States, with Louisiana hit the hardest, though they are actually expanding in California, according to research based on decades of satellite data.
Researchers said the United States lost about 7.5% of its tidal wetlands – roughly 400,000 acres (162,000 hectares) – from 1985 to 2023, as shown by satellite observations along U.S. coastlines, excluding Alaska and Hawaii.
Gradual changes such as rising sea levels accounted for about 60% of the total losses, the researchers said. But the acceleration in losses, they said, is being driven more dominantly by another factor – increasingly intense and frequent hurricanes and other extreme weather events that can ravage these ecosystems.
Thanks to conservation efforts, human factors such as construction of buildings and roads – offset by tidal wetland restoration efforts – now account for only a small share of losses, about 4%, they said.
“The findings show that existing conservation policies have helped reduce direct human conversion, but they are not sufficient to protect tidal wetlands from accelerating climate-driven pressures,” said Zhe Zhu, a professor of remote sensing and director of the Global Environmental Remote Sensing Laboratory at the University of Connecticut’s Department of Natural Resources and the Environment, senior author of the study published on Tuesday in the journal Nature Communications.
“Essentially, the continuous stressor of sea-level rise pushes the ecosystem to the brink, making it highly vulnerable, and the acute extreme events can act as the fatal trigger for abrupt, large-scale loss,” Zhu said.
Tidal wetlands are coastal ecosystems in the intertidal zone, the area between the highest high tide and the lowest low tide. They include tidal marshes, mangrove forests and tidal flats. Their dense vegetation and muddy soils protect against shoreline erosion and damage from storm surges, among other benefits.
In 2023, the continental United States had about 4.9 million acres (2 million hectares) of tidal wetlands, about 81% of which were tidal marshes, 12% mangrove forests and 7% tidal flats, the researchers said.
“Tidal wetland loss is not happening consistently across the country. The Gulf Coast is losing tidal wetlands at the highest rate due to high relative sea level rise and increasing extreme weather in the region. In San Francisco Bay, however, tidal wetland area is actually increasing. This is thanks to ongoing efforts to restore and protect tidal wetlands and the natural lack of hurricanes,” Zhu said.
California’s tidal wetlands grew by about 16.7%.
Most U.S. tidal wetlands are concentrated along the Gulf of Mexico and Atlantic Coast. At the state level, Louisiana has by far the largest tidal wetland area, and experienced the largest loss, shrinking by 16.6% over the study period. Among other major tidal wetland states, Florida lost about 2.1%, Texas 3.7%, South Carolina 1.6% and Georgia 1.1% over the study period.
The Gulf Coast region as a whole lost about 382,000 acres (154,700 hectares) of tidal wetlands over the study period, with tidal marsh loss dominating this decline. Mangrove forests are expanding their reach, replacing tidal marshes in some parts of Florida, Louisiana and Texas because they are better adapted to handle sea level rise and extreme weather events.
But while the total area covered by mangrove forests remained steady, that statistical stability is misleading, the researchers said.
“Mangroves are expanding northward as winters warm, especially into areas formerly occupied by tidal marsh, but this expansion is offset by severe dieback from hurricanes and extreme freeze events in southern Florida,” said study lead author Xiucheng Yang, a senior research fellow at the University of Victoria in Canada.
The Atlantic Coast experienced smaller losses than the Gulf Coast, but its loss trajectory is accelerating, the researchers said. By contrast, the Pacific Coast has held up better, registering net gains in tidal wetland area.
(Reporting by Will Dunham, Editing by Rosalba O’Brien)
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