Florida Everglades
(Excerpt from USGS
Circular 1182)
Subsidence
threatens agriculture and complicates ecosystem restoration
The Everglades
ecosystem includes Lake Okeechobee and its tributary areas, as
well as the roughly 40- to 50-mile-wide, 130-mile-long wetland
mosaic that once extended from Lake Okeechobee to
the southern tip of the Florida peninsula at Florida Bay.
Since 1900 much of the Everglades has been drained for
agriculture and urban development, so that today only 50 percent
of the original wetlands remain. Water levels and patterns of
water flow are largely controlled by an extensive system of
levees and canals. The control system was constructed to achieve
multiple objectives of flood control, land drainage, and water
supply. More recently, water-management policies have also begun
to address issues related to ecosystem restoration. Extensive
land subsidence that has been caused by drainage and oxidation
of peat soils will greatly complicate ecosystem restoration and
also threatens the future of agriculture in the Everglades.
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The
Everglades were formed in a limestone basin, which
accumulated layers of peat and mud bathed by
freshwater flows from Lake Okeechobee.
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Natural Flow
Patterns (c. 1900)
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| "The
outline of this Florida end-of-land, within the Gulf of
Mexico, the shallows of the Bay of Florida and the Gulf
Stream, is like a long pointed spoon. That is the
visible shape of the rock that holds up out of the
surrounding sea water the long channel of the Everglades
and their borders. The rock holds all the fresh water
and the grass and all those other shapes and forms of
air-loving life only a little way out of the salt water,
as a full spoon lowered into a full cup holds two
liquids separate, within that thread of rim."
- Majorie
Stoneman Douglas, 1947
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The Everglades ecosystem has, in fact, been badly degraded,
despite the establishment of Everglades National Park in the
southern Everglades in 1947. Prominent symptoms of the ecosystem
decline include an 80 percent reduction in wading bird
populations since the 1930s, the near-extinction
of the Florida panther, invasions of
exotic species, and declining water
quality in Florida Bay, which likely is due, at least in part,
to decreased freshwater inflow.
An area of thick peat soil south of Lake Okeechobee was
designated the "Everglades agricultural area." Farther
south, other areas of peat soils less suitable for agriculture
were designated as "water-conservation areas." These
areas are maintained in an undeveloped state, but a system of
dikes and canals allows water levels to be manipulated to
achieve management objectives that include flood control, water
supply, and wildlife habitat.
During dry periods, the level of Lake Okeechobee drops as
water is released to provide water to the agricultural area, to
canals that maintain ground-water levels in urban areas along
the Atlantic coast, and to Everglades National Park. At other
times, drainage water pumped from the agricultural area is
released into the water-conservation areas, providing needed
water but also undesirable amounts of the nutrient phosphorus.
In recent years, "best management practices" have
helped reduce phosphorus loads from the agricultural area. The
managed part of the remaining Everglades-approximately the
northern two-thirds-now consists of a series of linked,
impounded systems that are managed individually.
Subsidence Clouds
the Future of Agriculture
The Everglades agricultural area
is now mainly devoted to sugarcane, with considerably smaller
areas used for vegetables, sodgrass, and rice. The value of all
agricultural crops is currently about $750 million.
The eventual demise of
agriculture in the Everglades has been predicted for some time
(Douglas, 1947; Stephens
and Johnson, 1951). The agriculture depends upon a relatively
thin, continually shrinking layer of peat soil that directly
overlies limestone bedrock. Agronomists have known for many
decades that peat-rich soils (histosols), which form in
undrained or poorly drained areas, will subside when drained and
cultivated. The causes include mechanical compaction, burning,
shrinkage due to dehydration, and most importantly, oxidation of
organic matter. Oxidation is a microbially mediated process that
converts organic carbon in the soil to (mainly) carbon dioxide
gas and water.
Through photosynthesis,
vegetation converts carbon dioxide and water into carbohydrates.
Under natural conditions, aerobic microorganisms converted dead
plant material (mostly sawgrass root) to peat during brief
periods of moderate drainage. Vegetative debris was deposited
faster than it could fully decompose, causing a gradual increase
in peat thickness. In what is now the Everglades agricultural
area, a delicate balance of 9 to 12 months flood and 0 to 3
months slight (0 to 12 inches) drainage for about 5,000 years,
with sawgrass the dominant species, led to a peat accretion rate
of about 0.03 inches per year. Drainage disrupted this balance
so that, instead of accretion, there has been subsidence at a
rate of about 1 inch per year.
Peat soils may virtually
disappear
Rates of subsidence in the
Everglades are slower than those in the Sacramento-San Joaquin
Delta of California, the other major area of peat-oxidation
subsidence in the United States; in the Delta, average
subsidence rates have been up to 3 inches per year. However, the
pre-agricultural peat thickness was much greater in the Delta (up
to 60 feet) than in the Everglades, where initial thicknesses
were less than 12 feet. The subsidence rates observed in the
Everglades are similar to those observed in the deep peat soils
of the English fens during the past 100 years.
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(Stephens
and Johnson, 1951)
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Two
cross sections through the agricultural area show the
drop in land-surface elevation
caused by the
drainage of peaty soils.
Two
cross sections through the agricultural area show the
drop in land-surface elevation
caused by the
drainage of peaty soils.
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