A freshwater, saltwater tug-of-war is eating away at the Everglades

The boardwalk at Pa-hay-okee Overlook is a brief, winding path into a dreamworld in Everglades National Park. Beyond the wooden slats, an expanse of gently waving saw grass stretches to the horizon, where it meets an iron-gray sky. Hardwood tree islands — patches of higher, drier ground called hammocks — rise up from the prairie like surfacing swimmers. The rhythmic singing of cricket frogs is occasionally punctuated by the sharp call of an anhinga or a great egret.

And through this ecosystem, a vast sheet of water flows slowly southward toward the ocean.

The Everglades, nicknamed the river of grass, has endured its share of threats. Decades of human tinkering to make South Florida an oasis for residents and a profitable place for farmers and businesses has redirected water away from the wetlands. Runoff from agricultural fields bordering the national park causes perennial toxic algal blooms in Florida’s coastal estuaries.

But now, the Everglades — home to alligators and crocodiles, deer, bobcats and the Florida panther, plus a dizzying array of more than 300 bird species — is facing a far more relentless foe: rising seas.

South Florida is ground zero when it comes to sea level rise in the United States. By 2100, waters near Key West are projected to be as much as two meters above current mean sea level. Daily high tides are expected to flood many of Miami’s streets. The steady encroachment of saltwater is already changing the landscape, killing off saw grass and exposing the land to erosion.

Against this looming threat, Everglades ecologists and hydrogeologists are racing to find ways to mitigate the damage before the land is reclaimed by the ocean, irrevocably lost.

Sea level rise is a global problem, but coastal water management in South Florida faces some particular challenges, as a 2014 National Climate Assessment report noted. Growing urban centers need access to freshwater, flat topography encourages ponds of water to linger, and porous limestone aquifers are particularly vulnerable to encroaching saltwater. Storm surges occasionally drive seawater far inland, compounding the problem.

“We can’t ignore it anymore,” says Shimelis Dessu, a hydrogeologist at Florida International University in Miami. When it comes to water management needs in South Florida, ecological conservation has tended to be low on the list, compared with human and agricultural needs, Dessu says. Now, sea level rise is forcing people to think differently. “The ocean is no longer an external thing,” he says. “It’s already in the house.”

Draining the swamp

Florida’s tug-of-war over water has a long history.

In the 1800s, settlers first began draining the land to make way for agriculture and communities. Water management in the state began in earnest in 1948, when the U.S. Congress authorized the Central and Southern Florida Project for Flood Control and Other Purposes.

That project was meant to control flooding along the Kissimmee River and Lake Okeechobee, in the south-central part of the state. During the rainy months in summer and fall, the river and the broad, shallow lake often overflowed, flooding surrounding areas. The spillage would travel slowly southward across southern Florida in a broad sheet and eventually drain into Florida Bay, an open water body between the mainland and the Florida Keys. During the journey, some of the water would seep into the ground, replenishing the Biscayne Aquifer, a limestone layer that underlies much of the southeastern part of the state.

But the recurrent flooding made the land uninhabitable and farming impossible. So with Congress’ 1948 authorization, the U.S. Army Corps of Engineers built a complex system of levees, canals and reservoirs to control the floods and channel water away from farmlands south of Lake Okeechobee and from growing population centers. Three large “water conservation areas” were constructed to collect and store water during high rainfall events and release it in times of drought. The remaining wetlands — encompassing about half of their original area — were enclosed into two protected areas, Everglades National Park and Big Cypress National Preserve.

Such an intensive overhaul of South Florida’s water cycle led, perhaps inevitably, to new problems. Reducing the amount of freshwater that naturally heads south into the Everglades proved destructive to the habitats of plants and animals. Wading bird populations, for example, shrank by 90 percent over the last century. Diverting the water away from its natural overland course also meant less water was available to replenish the Biscayne Aquifer, which provides drinking water to 3 million people.

Agriculture is big business in Florida; the state’s exports total more than $4 billion each year. But fertilizer from the agricultural regions pollutes waterways feeding into Lake Okeechobee, causing algal blooms in the lake. Regulated discharges from the lake to control flooding shunt polluted water to the east, west and south, causing periodic algal blooms on the coasts and in Florida Bay.

Hoping to undo some of the damage, Congress approved a 35-year, $10.5 billion project in 2000 to send more freshwater south into the river of grass. That project, the Comprehensive Everglades Restoration Plan, or CERP, remains the largest hydrologic restoration project ever undertaken in the United States.

CERP has shown signs of success. The National Academies of Sciences, Engineering and Medicine, which evaluates the progress of Everglades restoration every two years, reported in 2016 that freshwater flow through the Everglades has indeed increased since the project began. And in some areas, groundwater levels and vegetation are beginning to return to how they looked before the extensive water management began.

But the academy’s 2016 report also pointed to a glaring problem. Researchers know a lot more about the effects of climate change now than they did in 2000. Without accounting for these effects, particularly rising sea levels, the restoration plan will not be able to meet its intended goals: restoring the wetlands and buffering inhabited areas against Florida’s intensely fluctuating hydrologic cycle.

Losing ground

Over the last half-century, the freshwater-saltwater transition zone in the Everglades has moved inland by at least a kilometer, due both to rising sea levels and to the reduction of freshwater flow through the Everglades. Some scientists call this inland shift of saltwater the Anthropocene Marine Transgression, a nod to the fact that humans are ultimately responsible for the rising seas and freshwater management.

In part, it’s a simple problem of water pressure. Freshwater flowing down off the land, or in belowground aquifers, pushes toward the sea. If that tap is slowed to a trickle and freshwater pressure is reduced, the seawater meets less resistance and can drive farther inland. It’s a problem many coastal communities around the world have faced when overdrawing from coastal aquifer wells: Removing too much freshwater at once allowed seawater to sneak in and poison the well. Add rising sea levels to the mix, and the low-lying Everglades face a double hit of saltwater intrusion above ground and below.

Because it is underground, the saltwater intrusion zone is not visible on a map. “But you can see the legacy effect … above ground,” says wetland ecologist Stephen Davis of the Everglades Foundation, a nonprofit group based in Palmetto Bay, Fla. “The salinity periodically knocks back the plant community.”

Hardest hit is the ubiquitous saw grass. Saw grass is hardy stuff; it is resistant to wildfires and thrives even in nutrient-poor soil. But saltwater is another matter. In 2000, a team of scientists surveyed the southernmost portion of the Everglades from the air. The researchers noted odd pockmarks dotting the land — bare patches where the saw grass had died. “Some of these landscapes look like Swiss cheese,” Davis says.

Thick, organic peat soil is the building block of many wetlands, including the Everglades, says Fred Sklar, director of the South Florida Water Management District’s Everglades division, based in West Palm Beach. But peat soil is fragile: Too little freshwater and it dries up. And worse, the combination of dwindling freshwater and increasing saltwater inundation is a one-two punch, “a kind of turbo boost, allowing the soil to break down,” Davis says. Chemical or biological changes within the peat soil — scientists aren’t sure exactly what — then trigger a sudden collapse. Soil elevation drops rapidly, exposing the roots of the saw grass, which eventually die.

The bare patches of ground are the most visible scars of saltwater intrusion, but the extent of the damage is probably much greater than is visually apparent, Davis says. Storm surges from hurricanes such as 2017’s Irma, along with king tide events, the highest high tides of the year, can push saltwater several kilometers inland. As a result, many regions that look fine to the eye are destabilizing beneath the surface, on the verge of collapse, he says.

Widespread peat collapse could be devastating to the Everglades on two fronts. Maintaining the elevation of the soil is a bulkhead against seawater intrusion; the collapsed areas become zones of open water. And peat-filled wetlands represent a vast carbon sink — a region where far more carbon dioxide is absorbed through photosynthesis than is released through respiration. Losing the soil effectively changes the region from a place that stores carbon to one that adds carbon dioxide to the atmosphere, fueling climate change.

Researchers don’t yet know how quickly land is subsiding in the Everglades. But research suggests that even slightly salty waters could cause the soil to sink at a “potentially staggering” rate, Davis says, dramatically increasing how quickly rising seas will be able to reclaim land. Biologist Sean Charles of Florida International University infused plots of saw grass–bearing peat soil with brackish water (still much less salty than seawater). In just one year, soil elevation in the salty plots sank by almost three centimeters, while the soil in the freshwater plots held its elevation or increased slightly.

A tale of two field sites

There’s a second boardwalk at Pa-hay-okee, which gets its name from a Native American word for “grassy waters.” Unlike the visitors’ overlook, getting to this platform requires a short, gutsy slog across a few meters of open wetland, possibly under the watchful gaze of an alligator.

That expanse is an intentional deterrent, says Benjamin Wilson, a wetland ecologist at Florida International University. This boardwalk isn’t meant for visitors; it’s for scientists, who built it as part of a long-term research study to try to understand what, exactly, causes peat soil collapse.

About a 20-minute drive to the south, a sister field site near West Lake is hidden behind a forest screen of salt-tolerant mangroves, their roots entangled and exposed, their branches creaking eerily. The two sites sit on either side of the saltwater intrusion zone: Pa-hay-okee is still largely fresh, but West Lake is brackish.

The first phase of the project, led by wetland ecologist Tiffany Troxler of Florida International University, was to figure out where the peat is most vulnerable to sea level rise, now and in the future, using existing well data, geologic maps and computer simulations of sea level rise. The second step — and the reason for studying the paired sites — examined how salinity changes might affect the peat soil and saw grass. “And then we should have a better idea of where saw grass is going to be, and where peat collapse may occur in the future,” Troxler says.

Alongside the boardwalk, the team embedded a dozen Plexiglas tubes right into the marsh. The chambers, each about half a meter in diameter, are open at the bottom and top, but can be twisted open or closed to allow the water to flow freely through them, or to temporarily sequester the chambers from the rest of the wetland.

Many factors can alter soil chemistry. Reduced freshwater flow can dry out the soil briefly, exposing it to oxygen. And seawater seeping up from the phosphorus-rich limestone aquifer below the wetlands brings in an extra supply of the nutrient, which is otherwise in short supply in the Everglades.

Once a month for four years — during wet and dry seasons — team members visited the chambers at both sites, closing them and dosing them with cocktails composed of different amounts of saltwater and nutrients.

“It was fun,” Wilson says cheerfully. Despite the muddy slog, team members chose not to wear full-body waders. “We’re lucky to be in South Florida, where the water never really gets cold.” Then, he pauses. “Well, it can get really miserable,” he acknowledges after a few seconds. Although they didn’t wear waders, the researchers covered up in long-sleeved shirts and pants, even in the summertime, and shielded their faces, despite the stifling heat. “Do you want 100 mosquitoes in your face, or do you want to be sitting in 95-degree humidity, not being able to breathe with these masks on?” he asks rhetorically.

This sometimes grueling work yielded results, as the team tracked how different factors might affect the saw grass ecosystem and peat collapse. Specifically, the researchers assessed changes in how much carbon dioxide the soil released into the atmosphere as a result of added salt and phosphorus, and also tracked changes in saw grass root growth.

A change in microbe activity was another possible culprit in soil collapse. So microbial biologist Shelby Servais of Florida International University examined whether the saltwater increased microbial growth, which could in turn speed breakdown of organic material. It didn’t happen. “What we found is that, in general, salt exposure suppresses activity of the microbial community.”

Even saltwater inundation — by itself — may not be causing the soil breakdown, Wilson says. What really seemed to matter was how dry the soil was to begin with, before saltwater was added. When the soil was already wet, adding more salt had no effect on how much carbon dioxide the soil released to the atmosphere, the team found. But when the researchers added salt to dry soil, carbon dioxide spiked. The team also noticed that saw grass plants grew fewer roots.

A third phase of the peat soil project is now getting underway. The researchers will precisely track where soil elevation has dropped, and by how much. The team will plunge a rod into the ground all the way to the bedrock and use pins attached to the rod to measure elevation changes over time. From that, Troxler says, “you can get an idea of whether [soil creation in] the wetlands is keeping up with sea level rise.”

Race against the rise

What should planners do if, as some simulations suggest, sea level rise is already outpacing the efforts by state and federal authorities to restore freshwater flow through the Everglades?

Dessu and colleagues took a close look at freshwater management efforts side by side with projections of sea level rise. “We have some control over the freshwater management. The other side, the sea level rise, we don’t have any control over,” he says.

The researchers had about 16 years’ worth of data on changing ecology in the wetlands, including information about the transitions of freshwater saw grass to salt-tolerant mangroves, loss of tree islands and proliferations of water- and nutrient-loving cattail plants. The team analyzed these changes, as well as changes in salinity and nutrients measured in wells in the region, to observe which areas had become saltier over time.

Then, Dessu says, the researchers examined freshwater management practices. Since 1985, South Florida water managers have been gauging how much freshwater to release from the water conservation areas based on the amount of rainfall that fell 10 weeks earlier. In the dry season, that delay is a problem, the team reported in April in the Journal of Environmental Management.

“By the time the flow is delivered, it’s two months too late,” Dessu says. The study concluded that the state’s water managers should consider not just how much water to send down into the Everglades, but when, exactly, would be the best time to do it. “That actually was kind of a surprise,” says Florida International University hydrogeologist René Price, a study coauthor.

Managers can use the difference between measured freshwater level and seawater level to decide when best to deliver a plug of freshwater to maintain enough water pressure to help push seawater back, Dessu says. It’s a kind of Band-Aid fix — one that won’t solve the long-term problem of saltwater encroachment into the wetlands, but may at least ameliorate its immediate effects, he adds.

The future of the Everglades

Such fixes are, perhaps, the story of Everglades restoration. In fact, restoration is a misnomer, Sklar notes. “It’s not really possible to bring back the past.”

Rehabilitation is more to the point. In March, Sklar and other South Florida water managers proposed an ambitious plan that could increase the overall flow of freshwater to the Everglades. The plan centers around the construction of a vast new water reservoir that would collect much of the fertilizer-polluted water from Lake Okeechobee to keep it from running to the coasts where it stimulates algal blooms. Within the reservoir, the water would be scrubbed, then sent to the wetlands. If the U.S. Army Corps of Engineers approves the project, it will become part of legislation headed to Congress in the fall for approval, Sklar says.

The Everglades water managers are walking a tightrope, juggling the needs of residents, farmers and business leaders who want a say about where the water goes. Conservationists in Florida understand this all too well. “If we made it all about climate change and sea level rise, there are those that wouldn’t be receptive,” Davis says. “So we talk about … issues like water supply and making the system more drought resilient.”

“Let’s face it,” he adds. “Science is incredibly important in shaping Everglades restoration projects, but it’s politics that gets the projects authorized and ultimately built.” But he notes that researchers still have many questions about how best to save the Everglades. For example, Davis says, scientists are just beginning to examine whether increasing freshwater flow can even save the saw grass.

Too much freshwater might, in fact, be a cure that’s worse than the disease. “There are models out there that show if we continue to release more freshwater to stem the tide of saltwater, it will end up just flooding the Everglades,” Price says, pointing to the push and pull. “We want to save the freshwater system, but how much flooding can it stand?”

In fact, the best hope for Everglades rehabilitation may be the mangroves. The gnarled, salt-tolerant trees are a visible sign of how the ecosystem is already changing, as they steadily march into regions vacated by freshwater saw grass.

Mangroves colonize new areas as their seeds wash inland. When the seeds settle into a spot, the plants can begin to grow, rapidly producing an abundance of fine roots — the primary component of peat soil. The trees can’t prevent all inundation or save the freshwater plants, but they may, at least, be able to keep the soil in place.

But as with so much in the Everglades, it’s a question of timing, Price says. Mangroves can’t move in if the soil is already completely gone. The trees need enough sediment to establish a foothold. Once established, however, mangroves can build up soil quickly, perhaps even at a pace that matches sea level rise.

“If they don’t, the peat collapse will take over,” Price says. “And it’ll just turn to open water.”

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This story appears in the August 18, 2018 issue of Science News with the headline, “Everglades on the Edge: Scientists wrestle with how to fight the effects of sea level rise.”