mangroves, their roots entangled and exposed, their branches
creaking eerily. The two sites sit on either side of the saltwa-
ter 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 break-
down of organic material. It didn’t happen. “What we found is
that, in general, salt exposure suppresses activity of the micro-
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 under
way. 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
Soil slump Beneath the land surface, saltwater and freshwater
struggle for dominance in the Everglades. Saltwater is gaining the upper
hand, as sea levels rise and drive the brackish zone farther inland. The
saltier water causes peat soil to collapse and saw grass to die, leaving a
pockmark pattern of brackish pools of water in the wetlands.
1. Saw grass grows in the freshwater zone. Mangrove trees thrive in the
coastal brackish water.
2. Rising sea levels or hurricane-driven waters push saltwater farther
inland. Soil collapses and saw grass dies.
3. More soil collapses around the dead saw grass, leaving a pool
of brackish water.
Freshwater saw grass marsh
Saw grass dieback
or sea level rise