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South Florida Wetlands Ecosystem: Biogeochemical Processes in Peat
U.S. Department of the Interior
U.S. Geological Survey
SOUTH FLORIDA WETLANDS ECOSYSTEM:
BIOGEOCHEMICAL PROCESSES IN PEAT
The South Florida wetlands ecosystem is an environment of great
size and ecological diversity (fig. 1 ).
The landscape diversity and
subtropical setting of this ecosystem provide a habitat for an
abundance of plants and wildlife, some of which are unique to
South Florida. South Florida wetlands are currently in crisis,
however, due to the combined effects of agriculture,
urbanization, and nearly 100 years of water management. Serious
problems facing this ecosystem include (1) phosphorus
contamination producing nutrient enrichment, which is causing
changes in the native vegetation, (2) methylmercury contamination
of fish and other wildlife, which poses a potential threat to
human health, (3) changes in the natural flow of water in the
region, resulting in more frequent drying of wetlands, loss of
organic soils, and a reduction in freshwater flow to Florida Bay,
(4) hypersalinity, massive algal blooms, and seagrass loss in
parts of Florida Bay, and (5) a decrease in wildlife populations,
especially those of wading birds.
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Figure 1. Major regions of the South Florida wetlands
ecosystem including the Everglades Agricultural Area (EAA), the
Water Conservations Areas (WCA's), and Everglades National Park
(ENP). Sampling sites for the Biogeochemical Processes project
are shown in Water Conservations Areas 1 and 2A and along the
Hillsboro Canal. (Click on image for full-sized version.)
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This U.S. Geological Survey (USGS) project focuses on the role of
organic-rich sediments (peat) of South Florida wetlands in
regulating the concentrations and impact of important chemical
species in the environment. The cycling of carbon, nitrogen,
phosphorus, and sulfur in peat is an important factor in the
regulation of water quality in the South Florida wetlands
ecosystem. These elements are central to many of the
contamination issues facing South Florida wetlands, such as
nutrient enrichment, mercury toxicity, and loss of peat.
Many important chemical and biological reactions occur in peat
and control the fate of chemical species in wetlands. Wetland
scientists often refer to these reactions as biogeochemical
processes, because they are chemical reactions usually mediated
by microorganisms in a geological environment. An understanding
of the biogeochemical processes in peat of South Florida wetlands
will provide a basis for evaluating the effects on water quality
of (1) constructing buffer wetlands to alleviate nutrient
contamination and (2) replumbing the ecosystem to restore natural
water flow. The results may also suggest new approaches for
solving problems of contamination and water quality in these
wetlands. A second focus of this project will be on the
geochemical history of the South Florida ecosystem. Peat is a
repository of the history of past environmental conditions in the
wetland. Before effective action can be taken to correct many of
the problems facing these wetlands, we must first study the
biogeochemistry of the peat at depth in order to understand
whether current problems are the result of recent human activity
or are part of a long-term natural cycle. Coordination with other
(USGS) projects for South Florida is ongoing. These projects are
studying the biological history of the ecosystem by using pollen
and shells buried in the peat, together with procedures for
dating the peat at various depths, to develop an overall
ecosystem history model, with emphasis on the last 100 years.
Project Goals
- Examine biogeochemical processes in peats controlling
nutrient and sulfur cycling in wetlands.
- Determine the rates of these biogeochemical processes for
inclusion in ecosystem models.
- Examine the influence of nutrient and sulfur geochemistry in
peats on mercury cycling and bioaccumulation.
- Determine the effects of differences in peat organic
structure on nutrients, sulfur, and mercury.
- Develop a model of ecosystem history from studies of
geochemical history and collaboration with paleoecology and
geochronology groups.
Peat of South Florida Wetlands
Large areas of South Florida wetlands (fig. 1)
are underlain by peat. Peat consists of the partially decomposed
remains of plants that grew in the wetlands. Waterlogged
conditions in wetlands foster the accumulation of peat by
excluding atmospheric oxygen from the soil and retarding the
decomposition of dead vegetation. The oxygen-free or anoxic
condition of most peat is a characteristic that plays an
important role in the recycling of nutrient elements (carbon,
nitrogen, and phosphorus) and in the transformation of mercury to
its toxic methyl mercury form. Water management practices over
the last 100 years have greatly altered the natural water flow of
South Florida wetlands, resulting in more frequent drying in some
areas. As peats dry, they are exposed to the atmosphere,
resulting in oxidation and loss of the peat. Drying can also lead
to the release of nutrients and other chemical species stored in
the peat when it is later rewetted. Analytical methods for
determining the forms of organic compounds are being used to
examine evidence of peat oxidation in areas subject to
drying. Results from a study in a water impoundment area (Water
Conservation Area 2A, WCA 2A) show that in some areas the surface
peat is currently being oxidized. The geochemical record in peat
cores is also being examined for evidence of past episodes of
peat oxidation.
Other geochemical methods are being used to examine differences
in peat types in various regions of the South Florida
ecosystem. Significant differences have been observed in the
organic structure of peats from the Water Conservation Areas
(WCA's) and some parts of Everglades National Park (ENP),
possibly because of differences in duration of wet periods and
amount of nutrient input. These differences may indicate the
capability of the peat to react with elements of environmental
concern such as phosphorus, nitrogen and mercury.
Biogeochemical Processes in Peats of South Florida
Sampling methods
Samples of peat and the water contained within the peat (pore
water) are obtained for analysis by using a plastic coring tube,
which takes a continuous cylindrical sample of peat from the peat
surface down to about 1.5 meters depth. The peat is
extruded from the coring tube and divided into 5-centimeter
sections, which are stored for later analyses. Often, only pore
water is obtained by coring the peat using a plastic tube with
sampling "port-holes" along its length. In these cases,
water is squeezed out of the peat through the holes and into
syringes using a specially designed apparatus that
applies piston pressure to the top and bottom ends of the peat in
the coring tube.
Phosphorus and Nitrogen
Phosphorus is an element of particular concern in South Florida
wetlands. High concentrations of dissolved phosphorus in canals
draining the Everglades Agricultural Area (EAA) are discharged
into South Florida wetlands. In the wetland marsh, phosphorus may
be adsorbed by surface peat or utilized by plants and later
incorporated in the peat after the plant dies or sheds
leaves. Studies by this project and by others indicate that
phosphorus accumulates in peats near canal discharge areas at
rates of 100-1000 times greater than in peats remote from canal
discharge in WCA 2A (fig. 1). High
concentrations of phosphorus in surface peats near canal
discharge areas may be responsible for a dramatic shift in the
nature of the wetland vegetation. The native sawgrass, which is
adapted to a low-phosphorus environment, is being replaced by
cattails, which outcompete the sawgrass under high-phosphorus
conditions.
But what is the ultimate fate of phosphorus after deposition in a
wetland peat? How much is recycled within the wetland, how much
is exported to other uncontaminated areas, and how much is locked
away in the peat? At what rate is phosphorus recycled? These are
some of the important biogeochemical questions that this project
is addressing. Some of the more important biogeochemical
processes governing the behavior of phosphorus are shown in figure 2. Phosphorus is deposited in the
peat largely as organic phosphorus in dead plant debris. As this
plant debris is transformed into peat, the phosphorus is released
into water in the pore spaces of the peat. The dissolved
phosphorus in the pore water is available for uptake by aquatic
plants rooted in the peat, uptake by microorganisms in the peat,
or diffusion or migration back
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Figure 2. Simplified biogeochemical cycles for phosphorus (left)
and sulfur (right) in organic-matter-rich sediments of South Florida. P,
phosphorus; CH3Hg +, methyl mercury ion.
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into the surface water. Concentrations of phosphorus (in the form of
phosphate), and nitrogen (in the form of ammonium) dissolved in pore water
(fig. 3) often greatly exceed concentrations found in the surface water.
By measuring the change in concentration between surface water and pore
water, and with a knowledge of
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Figure 3. Dissolved phosphate and ammonium concentrations in
pore water from sites in WCA 2A. Concentrations are in micrograms per liter
(µg/L) and depths are in centimeters (cm).
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peat accumulation rates, calculations can be made to estimate the rates
at which these elements are recycled, and the rates at which they may diffuse
or migrate back to the surface water. This kind of quantitative rate information
is key to understanding the long-term effects of nutrient contamination
of wetlands from agricultural or urban runoff, to predicting the long-term
effectiveness of constructed buffer wetlands for protecting wetland conservation
areas.
Sulfur and Mercury
Sulfur is another element of major concern in South Florida wetlands, primarily because of its role in the cycling of mercury. Many of the important biogeochemical processes in peat involving sulfur are shown in fig. 2. In the surface water of wetlands where dissolved oxygen is present, sulfur exists primarily in its oxygenated ionic form as dissolved sulfate. Sulfate can enter the wetland from canal discharge, rainfall, or ground-water discharge. In most freshwater wetlands,sulfate in surface water is present at low concentrations. However, areas
of the South Florida wetlands ecosystem affected by phosphorus contamination
also have high sulfate concentrations. The high sulfate concentration may
originate from agricultural soil amendments used in the EAA. One goal of
this project is to determine the source of sulfate in the wetlands by using
sulfur isotope analyses. Isotopes are different forms of an element having
the same number of protons in the nucleus but different numbers of neutrons.
The different isotopes of an element have slightly different chemical and
physical properties and can accumulate at different rates. The ratios of
abundance of two isotopes of an element (expressed as d34S values in the
case of sulfur) are often used in geochemistry as "fingerprints"
to determine the origin of chemical species or processes affecting them.
We expect sulfate originating from fertilizer to have significantly different
d34S values from sulfate originating from rainwater or ground water. The
stable isotopic ratios of the major forms of sulfur also reflect sulfate
concentrations and rates of sulfate reaction to form sulfide (fig.
2).
Probably the most important biogeochemical
process involving sulfur in oxygen-free sediments is sulfate "reduction,
" the process by which sulfate is changed to sulfide. Under oxygen-free,
"reducing" conditions in peat, certain bacteria (sulfate-reducing
bacteria) use sulfate in their metabolism and produce hydrogen sulfide
as a byproduct. Hydrogen sulfide is a chemical form of sulfur with a very
distinctive "rotten egg" odor. It is very reactive with dissolved
metals and will quickly form highly insoluble metal sulfides in the peat
when present in sufficient concentrations. Mercury in peat pore water will
readily react with hydrogen sulfide to form highly insoluble mercuric sulfide,
which poses little health hazard buried as a solid in the peat. However,
sulfate-reducing bacteria are thought to be involved in the production
of toxic methylmercury (CH3 Hg+) as a byproduct of the sulfate reduction
process (fig. 2).
Ongoing studies are being conducted to determine trends in sulfate concentration
and d34S values in surface water from the WCA's and their adjacent canals.
These trends are being related to location, season, nutrient enrichment,
rates of sulfate reduction, and, ultimately, to methyl mercury production
in the South Florida wetlands ecosystem.
Continuing Studies of Biogeochemical Processes in Peats of South Florida
Project work in South Florida
wetlands in 1994 and 1995 primarily emphasized reconnaissance studies of
peat and pore-water biogeochemistry throughout the ecosystem, with an emphasis
on work along a known nutrient enrichment gradient in WCA 2A (fig.
2).
In 1996, collaborative co-sampling was begun with USGS and other scientists
who are focusing on the biogeochemistry of mercury. Current studies also
include the effects of seasonal changes in water retention and rainfall
on the exchange of nutrients and sulfur between surface water and sediments.
We have recently expanded our studies into Everglades National Park, with
emphasis on Taylor Slough and Shark River Slough, areas of major water
transport to Florida Bay (fig.2). Laboratory studies designed to verify
interpretations of the results of field observations are planned for future
years.
Anticipated Project Schedule:
1994- Reconnaissance sampling in Water Conservation Areas (WCA) and Everglades National Park (ENP); biogeochemistry of peat and pore water.
1995- Focused study of peat and pore-water biogeochemistry in WCA 1 and 2.
1996- Study of temporal variation in pore-water biogeochemistry in WCA 2 and 3 and buffer wetlands; Study of sediment and pore-water biogeochemistry in Taylor Slough transect (ENP).
1997- Continued pore water and sediment studies in WCA 1, 2, and 3 and Taylor Slough; begining of sediment and pore-water studies in Shark River Slough; beginning of laboratory simulation studies of sulfur-mercury interactions and nutrient cycling.
1998- Continued studies in Shark River Slough; completion of studies in WCA; continued laboratory simulations.
1999- Completion of studies in Shark River Slough; completion of laboratory simulations; completion of synthesis and final reports.
Planned Products
- USGS open-file reports.
- Synthesis articles for publication in scientific journals.
- Data tables and rate constants for use in ecosystem models.
- Presentations at scientific meetings and at collaborating agencies.
- GIS (geographic information system) maps of sediment and pore-water
geochemical parameters.
Collaboration and Partnerships
This project is closely coordinated
with a number of other USGS projects in South Florida and with university
projects, including projects studying mercury biogeochemistry and bioaccumulation,
geochronology of peats, trace-element and mercury accumulation in peats,
palynology and paleoecology, dissolved organic carbon, and hydrology. This
coordination involves planning field work, co-sampling, and exchange of
data, information, and ideas. Coordination and information exchange with
the scientific staff and management of the South Florida Water Management
District have also been key components of this project since 1994. The
district provides access to Water Conservation Areas, logistical support,
and information that guides the selection and timing of sampling. Information
exchange and coordination of sampling activities have also been conducted
with the U.S. Fish and Wildlife Service for work in WCA 1, the National
Park Service for work in Everglades National Park, the Florida Department
of Environmental Protection for mercury studies, and the U.S. Department
of Agriculture for work in the Everglades Agricultural Area.
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For more information contact:
William H. Orem
U.S. Geological Survey
956 National Center
Reston, VA 20192
Telephone: 703-648-6279
E-mail: borem@usgs.gov
Related information:
SOFIA Project: Geochemistry of Wetland Sediments from South Florida
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