USA: Rising sea levels could accelerate Florida bay mangrove loss
USGS scientists look to the past to learn about the future of Florida mangroves
Florida has lost much of the mangrove forests that once bordered its coasts and estuaries to development and sea-level rise. Current rates of sea-level rise combined with increasing climate variability could accelerate the loss of mangrove-lined coastlines, according to new USGS research published in the journal Nature Communications. These findings will help resource managers understand whether mangrove-forested islands and shorelines can be maintained as the climate changes and sea levels rise and how water management may help mitigate these changes.
“From previous studies, we know that the mangrove zone in South Florida shifted inland as sea level rose and estuaries covered what was previously mangrove forest during the late Holocene,” said USGS research geologist Miriam Jones, the study’s lead author. “This new research is one of the most detailed studies to pinpoint how fast the transition from mangrove forest to estuary happened during that time.”
The USGS research comes at a time when state and federal government agencies and many other partners are undertaking one of the world’s largest and most ambitious ecosystem restoration efforts: to help recover the health and natural productivity of the Greater Everglades ecosystem, which includes Florida Bay.
Mangroves provide important natural services that benefit people and wildlife. They act as buffers against storm surges and rising seas, help reduce erosion and protect coastal communities. They also serve as nesting sites for birds, act as critical habitat for endangered and threatened species and provide sheltered nursery areas for commercially valuable fish and shellfish. They are an important plant community in Florida Bay, the shallow, salty water body that lies between the Florida Keys and the state’s southern tip, mostly within the boundary of Everglades National Park.
Working on islands in Florida Bay, USGS scientists studied how environmental conditions during the mid to late Holocene — the geologic period from 5,000 years ago through the present day — affected Florida Bay’s mangroves. In this low-lying region, the border between land and sea moves over time in response to cyclical forces like rising or falling seas, or it can change rapidly due to storms.
The researchers found that starting about 3,600 years ago and ending 2,800 years ago, seas were rising at the rate of about 0.67 millimeters per year, the equivalent of about 2.6 inches every 100 years. That is about four to six times slower than today’s rate of sea-level rise. At that slower rate, mangrove shorelines rapidly changed into open water estuaries, the study found.
Scientists tied the demise of the mangroves during the late Holocene to a period of increased droughts and storms driven by changes in long-term ocean-atmosphere circulation patterns.
“Our findings highlight the role of storms and drought on mangrove survival,” Jones said. “A warmer climate could not only result in higher sea levels, but it could also lead to more intense droughts and storms and possibly increase the rate of mangrove loss. This is particularly true if these stresses are prolonged or repeated.”
To reconstruct changes in the area, USGS scientists collected sediment and peat cores from islands in Florida Bay, then analyzed them to see what plants and animals were present at different times in the past. In sediments from the late Holocene, they found that certain types of mollusks, such as snails and clams, and other creatures that normally live in estuaries were found in mangrove environments instead. The researchers say it is likely that intense storms during this time moved these organisms away from the open, salty waters that are their natural habitat and into the mangrove zone.
The scientists identified the types of pollen found in the sediment cores to reconstruct where mangrove trees once grew and when they disappeared. They also analyzed stable carbon isotopes, which have subtle structural differences, to determine whether the sediments contained residues of freshwater or saltwater plants.
Mangroves are considered resilient trees, often capable of keeping pace with high and accelerating rates of sea-level rise. But changes in climate and intense weather, such as droughts and hurricanes, can cause these plant communities to shift or disappear, resulting in lasting changes to the coastlines they protect. “Seeing mangrove loss during the late Holocene, when the climate was cooler and rates of sea-level rise were much lower than today, indicates that in the future, there could be much more stress on this important plant community as the climate continues to warm and rates of sea-level rise accelerate,” said Jones.
Today’s loss of mangroves is compounded by the region-wide water management system built in South Florida between the late 19th and mid-20th centuries, which has reduced the natural flow of water through the Everglades to Florida Bay and other coastal bays. Drier conditions can slow or stop the natural buildup of organic peat soils like those in the Everglades. Lessened water flow can also produce drought conditions that cause the peat soils to collapse, undercutting the mangroves. Storms also play a role. They can damage the trees, temporarily raise seawater levels, and carry sediment inland where it can smother the above-ground roots that provide mangroves with oxygen.