DOCUMENTING THE IMPORTANCE
OF WATER FLOW TO EVERGLADES LANDSCAPE STRUCTURE AND SEDIMENT
TRANSPORT IN EVERGLADES NATIONAL PARK
A Research Work Plan and Scope of Work
South Florida Natural Resources Center
Everglades National Park
Daniel L. Childers
Southeast Environmental Research Center &
Department of Biological Sciences
Florida International University
Miami, FL 33199
Department of Earth Sciences
Center for Marine Science
University of North Carolina at Wilmington
In accordance with Cooperative Agreement #1443CA 5280-01-021
between the U.S. Department of the Interior, Everglades National
Park (ENP) and Florida International University (FIU), this
Work Plan and Scope of Work describes specific activities to
be undertaken by FIU and by the University of North Carolina
- Wilmington (UNC-W) as a subcontractor, during the two year
contract period of this project. This Work Plan represents a
large portion of a greater research program that also includes
Dr. Helena Solo-Gabriele at the University of Miami, FL (UM)
and Dr. Sherry Mitchell-Bruker at the SFNRC-ENP. This Work Plan
touches on all aspects of this greater research program, but
where tasks are the responsibility of either Dr. Solo-Gabriele
or Dr. Mitchell-Bruker, we refer the reader to the UM contract
or to Dr. Mitchell-Bruker for details. Close contact will be
maintained with Sherry Mitchell-Bruker, Co-Investigator and
Project Manager at the South Florida Natural Resources Center
(SFNRC-ENP) throughout this time.
Introduction and Background
The Everglades wetland landscape is as
distinctive as it is heterogeneous. This landscape is characterized
by teardrop-shaped tree islands in a wetland mosaic of strand-shaped
sawgrass marsh and deeper slough systems. Increasingly, evidence
shows that this landscape structure is a result of the once
historic, regular flow of water downslope from Lake Okeechobee
to the Gulf of Mexico. The last 100 years of water management
in this system has compartmentalized the landscape and impounded
water, though. In most of the remaining Everglades, flow is
either greatly reduced and sporadic or is non-existent. Analyses
of historical data support the hypothesis that the Everglades
wetland landscape was not only structured by water flow but
that this structure was also historically maintained by regular
water flow. Furthermore, McVoy and Crisfield (2001) hypothesized
that this flow-maintained landscape was actually a dysequilibrium
condition. They argued that deep sloughs would naturally tend
to fill in, probably with organic sediments, while the elevations
of the higher sawgrass strands would be mediated by mean hydroperiod.
Flow thus kept the sloughs scoured, maintaining pathways for
downstream movement of large volumes of water in a non-equilibral
way (McVoy and Crisfield, 2001). The loss of both the "corrugated"
ridge and slough topographic features and of many tree islands
in much of the remaining Everglades point to the validity of
this hypothesis. In this research plan, we propose to quantify
flow regimes around tree islands and in open marsh in Shark
River Slough, ENP. We also propose to experimentally manipulate
flow rates in SRS marshes in order to quantify the importance
of water flow to inorganic sediment and organic matter transport
through this landscape. Both aspects of our research will help
parameterize hydrodynamic and ecological models that will be
used to test the implications of different restoration options
to maintaining and enhancing water flow in the Everglades landscape.
Flows across vegetated marsh surfaces are inherently complex.
Flow energies may be dissipated by friction with the marsh surface
and by wake formation around closely spaced plant stems (Nepf
et al. 1997). The resulting fine scale flow dynamics and turbulent
exchanges are known to affect: 1) oxygen and nutrient exchanges
at the soil-water interface (Escartin and Aubrey, 1995); 2)
microclimate regulation between plant stems (Dade, 1993); 3)
the nutritional environment for suspension-feeding animals (Irlandi
and Peterson, 1991), larval recruitment (Eckman, 1983), and;
4) sediment deposition and retention processes (Leonard and
Luther, 1995). Hydrodynamic exchanges may also influence: 1)
substrate selection by invertebrate larvae (Butman, 1987); 2)
the distribution of meiofauna (Palmer, 1986); 3) the export
of primary (detritus) and secondary production from the marsh
surface to adjacent flowing-water systems (Peterson and Turner,
1994), and; 4) the trophic relationship between protozoan suspension
feeders and their prey (Shimeta et al., 1995). Although flow
regime may impact many different wetland processes, little field
data exist which describe detailed flow behavior through the
emergent vegetated ridge and slough system of the Everglades.
Previous (e.g. Leonard et al., 1995, Leonard 1997) and on-going
(Leonard et al., in review) work in other marsh systems has
shown that fine scale flow structure controls suspended particle
distribution over both vertical and horizontal scales. Flow
patterns resulting from the synergistic interactions of channel
morphology, marsh microtopography, and length of inundation
exert significant control over the spatial distributions of
suspended and deposited particulate matter in many vegetated
systems (Leonard, 1997, Christiansen et al., 2000). Similar
interactions may be responsible for shaping and maintaining
the geomorphic framework of the tree island/ridge and slough
system in the Everglades.
Another complexity in understanding particulate dynamics in
vegetated wetland systems arises from the fact that the vertical
structure of marsh flows may be strongly related to canopy architecture.
Flow profiles collected in Spartina alterniflora and Juncus
roemerianus canopies in Gulf of Mexico marshes and Atriplex
portulacoides marshes in the North Sea indicate that vertical
flow profiles are not uniform and that they deviate from the
logarithmic shaped profile encountered in areas lacking vegetation.
Leonard and Luther (1995) demonstrated that profile shape was
affected by both inter-profile and intra-profile biomass distribution.
Intra-profile baffling potential was maximum at heights where
plant material was most abundant, and inter-profile baffling
potential was greatest at maximum plant densities. Even for
very low velocities (<5 cm sec-1)--such as those expected
for the ridge and slough system-plant stem densities over about
1.25 cm may cause transitional to fully developed turbulence
to occur (based on measured Reynolds Numbers). In many cases,
flow conditions in wetland canopies are neither fully turbulent
nor fully laminar because of plant/flow interactions, (Kadlec,
1990; Leonard and Luther, 1995). Consequently, microturbulence
and flow energy in a vegetated canopy may be most strongly affected
by basal stem diameter and plant canopy architecture. This is
of particular interest to flow through Everglades marshes because
most sloughs that were once either unvegetated or sparsely vegetated
(in the pre-drainage system) are now vegetated by Eleocharis
and Panicum. This change in both plant density and morphology
may thus exert strong control on maximum possible flow rates
as well as whether those flows are laminar or turbulent. And
both flow rate and flow character directly control rates of
both sediment entrainment and transport..
Tree islands are as fundamental to the Everglades landscape,
history, and ecosystem function as the broad marshes that run
among them. Restoration of the Everglades will not be successful
without protecting their place and character. Currently, however,
our knowledge of tree islands is insufficient to predict how
they will respond to changing water management and restoration.
We have ideas about how tree islands form, but don't know how
they develop over time from these beginnings, and what conditions
will enable them to persist. We don't know where they need more
water, where they need less. From the uniformity of their orientation
and shape from place to place in the Everglades, we know that
surface water flow plays a vital role, but at present we lack
functional understanding of how water flow controls the sedimentation
processes that build or maintain tree islands.
The tree island profiled in Figures 1 and 2 is one of thousands
in ENP. The teardrop-shaped landform narrows and extends to
the south-southwest, as do most of its neighbors. The island
has formed above and downstream of a large limestone outcropping.
In the tree island's "head", i.e., above the outcrop
proper, tall trees common to dry or mesic tropical uplands are
rooted in several feet of mineral soil. In its "tail",
temperate trees of shorter stature grow under much more hydric
conditions. The organic soils of the tail are elevated by several
dm above the adjacent marshes, but there is little evidence
of bedrock control. One scenario suggested by this profile is
that differential sedimentation in the lee of the tree island
head has, over many centuries, raised the surface of the tail,
allowing woody plants to survive, albeit in smaller form. Other
alternatives are possible. For instance, several mechanisms
may cause the initial forest in the tree island head to concentrate
nutrients, which may be transported downstream to create a tail
of relatively high productivity in comparison to adjacent wetlands.
Either scenario implicates water flow and material transport
processes as fundamental to landscape character. In this proposal,
we propose to initiate a modeling effort, supported by data
from natural and experimentally manipulated landscapes, that
will allow us to simulate these process under different flow
In May 1999, a large group of research scientists from universities,
federal agencies (including ENP, through the active involvement
of S. Mitchell-Bruker), state agencies (including SFWMD), and
non-governmental organizations (including National Audubon Society)
were awarded funding by the National Science Foundation to establish
a Long Term Ecological Research (LTER) site in Everglades National
Park. For detailed information about the NSF LTER Network, see
www.lternet.edu. This new LTER is known as the Florida Coastal
Everglades LTER Program (FCE LTER; for more information on the
FCE program, see http://fcelter.fiu.edu). Some of the research
being conducted by the FCE LTER program is basic science. However,
much of the LTER research is quite similar to the work we are
proposing here. Water flow is central to the focus of FCE LTER
research, particularly as it relates to the oligohaline regions
of ENP estuaries. Because of this overlap, we propose that this
CESI proposal be viewed as a valuable partnership between the
FCE LTER Program and ENP.
General Goals and
We will quantify the flow regime within
and immediately adjacent to tree islands. Additionally, we will
measure water flow rates in open marshes between tree islands,
and both upstream and downstream of tree islands, focusing on
how the islands themselves modify flow regimes in the landscape.
We hypothesize a positive feedback mechanism whereby tree islands
accelerate local water flow by channeling water around their
higher elevations (the Bernoulli Effect), which leads to maximal
flows along the upstream island flanks that scour sediments
here but deposit them immediately downstream of the islands
(in the "tail" region). In this way, tree islands
actually maintain their structure, and those not "anchored"
in place by geologic features may actually migrate downstream
very gradually. More tree islands in a landscape should thus
relate to higher peak flow rates for a given mass flux of water
and thus a greater potential for maintenance of the islands
in the mosaic.
Water flow is important to the open marsh as well. In addition
to the well-substantiated hypothesis that flow is critical to
maintenance of the [sawgrass] ridge and [deeper water] slough
topography of ENP marshes, we suggest that flow is also the
critical force responsible for particle transport. Everglades
wetlands contain very little inorganic sediment, and most particles
found in these marshes are organic in nature. Furthermore, Everglades
wetlands are characterized by very clear water. The bulk of
particulate organic matter in these marshes takes the form of
flocculent detritus ("floc") found in an unconsolidated
layer just above the soil surface. We hypothesize that 1) water
flow is critical to moving this floc as bedload through Everglades
marshes; 2) the minimum flow necessary to mobilize and move
floc controls the importance of that floc to local ecological
processes, including soil dynamics and plant productivity, and;
3) the downstream transport of floc is the most important organic
matter and nutrient input to Everglades estuaries by the upstream
In a separate project directed by S. Mitchell-Bruker, a model
of deposition, scouring, and transport of sediment through vegetated
shallow waters typical of the Florida Everglades is already
being developed. The simulator considers horizontal flow of
water assuming that water is essentially hydrostatic in the
vertical. A vertically averaged diffusion equation is used for
simultaneous overland and porous-medium flow. The simulator
can be run in either steady, quasi-steady, or fully transient
modes. Sediment is characterized by an arbitrary number of discrete
particle sizes, each with a characteristic particle density
and diameter. Each sediment class is considered independently.
Transport of a sediment class is performed using the advection-diffusion
equation, with settling and scour providing sinks and sources
for each cell. Settling rates are based on the fall velocity
of a spherical particle, while several formulations are provided
to model scour. The initial distribution of suspended sediment
within the column will be based on field measurements. Thus,
a final objective of the work proposed here is to provide flow
measurements and suspended sediment characterization to support
this modeling and to run model simulations.
We have structured both our research and this Work Plan around
research questions. In the case of each question, we discuss
our rationale for this question and briefly present a methodology
or experimental design for answering the question. Before presenting
our specific research questions, however, we discuss our general
field sampling approach (below). For each question, the investigators
responsible for this research objective will be noted. Where
Dr. Helena Solo-Gabriele is the investigator responsible for
a task, we refer the reader to the UM contract or to other documents.
The UM contract supports the participation of Dr. Solo-Gabriele
and a graduate student. Supplies and equipment for all aspects
of the study, including the UM portion, are included in this
General Field Sampling Approach
Baseline data collection will focus on
the wetland mosaic surrounding three major Shark Slough tree
islands: Black Hammock, Gumbo Limbo Hammock, and an unnamed
island referred to as Satin Leaf Hammock (Figures 3 and 4).
All three are subjects of a current study of forest structure
and function ("Everglades National Park tree islands: interaction
of hydrology, vegetation, and soils": Cooperative Agreement
CA 5280-00-017). In that study, M. Ross (with collaborators
Jayachandran and Oberbauer) has established permanent plots
in which plant and soil processes are monitored in hardwood
hammock, bayhead, and bayhead swamp environments, in relation
to background variation in water level (from nearby longterm
recorders), nutrient availability, and other physical factors.
The study also includes a paleoecological component which attempts
to reconstruct the history of the different tree island sub-environments,
and the development of the landform as a whole. A missing element
in the ongoing research is the integration of the study tree
island into the surrounding landscape via sheetflow pathways
and sediment transport processes.
We addressed a number of questions of scale when setting up
our sampling scheme. A key objective of this research is that
our empirical approaches be designed to enhance our modeling
efforts. Our experimental/sampling design centers around transects
around the 3 tree islands discussed above: One transect through
the marsh upstream of each tree island (600 - 900m), one transect
along the island periphery, and one lateral transect from either
side of the island (200 - 300m each). To the extent possible,
we will link the lateral transects to 1 of M. Ross' 3 existing
island transects. The spot measurements of flow discussed below
will be made along transects from tree islands into the marsh.
Flow sampling of the lateral transects will follow an exponential
design, with the shortest intervals between sampling points
at community boundaries (i.e. the tree island-marsh boundary
or the slough-sawgrass boundary). We will establish initial
transect locations and dimensions using field reconnaissance
combined with aerial photography. After the transects have been
established, we will conduct a flow regime reconnaissance along
each transect. From this information, we will determine the
location of the routine flow monitoring stations (discussed
below) on each transect.
With all field work, we will employ the minimum tool approach.
In early phases of the project we will experiment with the feasibility
of accessing sites by airboat, kayak, boardwalks, and/or by
foot to ensure minimum impact. During data collection, sampling
will always begin at the most downstream sites and progress
upstream. Elevation benchmarks will be established using differential
GPS or optical surveying techniques. Installed equipment, wells,
and sampling sites will be surveyed and tied to the benchmark.
Removable PVC poles will be used to mark sampling sites. In
all cases, we will minimize short and long-term impacts on the
system we are studying.
The hypotheses described in the previous section will be tested
with long-term field manipulative experiments in which we actually
modify the flow regime around relatively large areas of marsh,
both reducing/eliminating flow and increasing flow. At each
experimental site, we will have marsh sampling locations both
within the manipulated area and outside of it (as controls).
We will quantify water flow and calculate water flux at both
places. We will quantify floc transport, floc production and
accretion, floc nutrient content, and floc organic content.
We will calculate floc transport, as bulk flux, for each experimental
marsh and relate these flux values to the flow regime. The key
to this type of experimental manipulation is the physical modification
of flow regimes. We will accomplish this using 75 cm high walls
of 12 mil plastic sheeting established as "flow fences"
in the marsh. In the no flow treatments, we will construct walls
that block and divert flow from the downstream experimental
marsh. In the flow enhancement treatments we will construct
walls that concentrate and direct accelerated flow through the
downstream experimental marsh (the Venturi Effect). We have
a great deal of experience with these kinds of flow manipulations.
For example, a graduate student working under D. Childers has
used this method to experimentally reduce water flow around
experimental tree islands in Southern Everglades marshes south
of the C-111 canal for 2 years now. These walls are inexpensive,
require minimal maintenance for long-term deployment, and effectively
reduce flows by 70% or more (see Figure 5). We are thus confident
of the efficacy of this simple yet elegant design.
The model simulations will be based on data from water samples
collected at the tree island and marsh sites, and from characterizations
of suspended sediments in the water column. Particle density
and diameter distribution will be determined through sedimentation
tests, sieving and filtering techniques. In addition, Doppler
flow meters will be used to further characterized the particle
density distribution in the water column, under various flow
regimes. The results of these suspended sediment analyses will
be used to provide input to the sediment transport model. In
addition, measured flow rates will be used to calibrate the
model. Finally, we will model a range of scenarios representing
typical flow regimes in the Everglades, examining model results
to ascertain the role of flow and sediment transport in the
formation and maintenance of tree islands and the ridge and
Specific Research Questions and Methodologies
Primary Research Question 1: What
is the role of ecological pattern and process in controlling
flow in the Ridge and Slough Landscape?
Question 1.1: How does plant community
structure and/or primary productivity control flow regimes?
We will quantify vegetative resistance measuring plant biomass
and biovolume in both slough and sawgrass marsh communities.
These measurements will be made in 1m X 1m plots in both slough
and sawgrass marsh using nondestructive allometric models that
already exist (Daoust and Childers, 1999; L.Leonard and D.Childers).
We will also use intensive flow measurements to identify transition
zones in flow between the slough and sawgrass marsh communities.
We expect these physical/hydrologic interfaces to be points
of minimum sediment/floc transport and maximum deposition in
the sloughs (L.Leonard and H.Solo-Gabriele, see UM contract).
Wind is likely to play an important role in this interaction
between vegetation and water flow, particularly in broad, relatively
open sloughs. We will obtain local wind conditions with a hand-held
anemometer when flow data are collected (H.Solo-Gabriele). Finally,
we will explore the interactions between vegetation and water
flow in more detail using ecosystem manipulations in which we
modify the properties of the plant canopy through clipping to
reduce stem density. If possible, we will carry out these experiments
in conjunction with the flow enhancement manipulation discussed
above. In addition to carefully measuring vertical and horizontal
profiles in water flow [in these canopy manipulations], we will
also measure floc transport, short-term materials deposition,
and long-term changes in soil elevation (L.Leonard and D. Childers).
Question 1.2: How does plant community
structure control materials deposition? We will quantify the
relationship between plant community structure and materials
deposition in the marsh by quantifying surficial sediment accumulation
in the vegetation plots (see Question 1.1; L.Leonard and D.Childers).
In tree islands, we will examine this relationship by quantifying
primary production as well as sediment accumulation, using asediment
trap or marker horizon method (M.Ross).
Question 1.3: How does plant community
structure control topography and soil elevation dynamics? Because
changes in soil elevation are very slow, we view this as primarily
a long-term question. In the marsh, we will quantify long-term
changes in net soil elevation by measuring depth to bedrock
twice each year (wet season and dry season) in the vegetation
monitoring plots. We will also quantify long-term changes in
net soil elevation using the stainless steel pin method, and
we will measure surficial materials accumulation using a marker
horizon approach (D.Childers). In the tree islands, we will
also measure change in net soil elevation (pin method) and surficial
materials accumulation data (marker horizon method; M.Ross).
Question 1.4: What are sources
of sediments and materials in different Ridge and Slough communities?
Questions of sources of organic matter are central to the research
being conducted in the FCE LTER Program. As part of this research,
Dr. Rudolf Jaffé (FIU) has demonstrated the efficacy
of several sophisticated organic geochemical techniques to differentiate
broad patterns of sources of estuarine organic matter. We will
work with Dr. Jaffé to use one of these techniques--molecular
markers (biomarkers)-to characterize the organic geochemical
signatures of materials produced by saw grass marsh, slough
marsh, and tree islands. We will examine both soil cores and
water samples. If we are able to differentiate organic matter
sources among these 3 closely located communities, we will apply
these techniques to floc and sediment samples (M.Ross and D.Childers).
M. Ross will be examining materials deposited on tree island
soil surfaces as part of his ongoing study (working with Pete
Stone, SCDEP), with the same goal in mind (M.Ross). We will
assess the physical characteristics of the suspended materials
using standard qualitative or gross particles composition assessment
techniques (L.Leonard and H.Solo-Gabriele, see UM contract).
Finally, we will attempt to address the importance of periphyton
mat movement (particularly during large wind events) by physically
tagging a minimum of 3 periphyton mats per study slough and
tracking their positions in time and space (D.Childers, H.Solo-Gabriele,
Primary Research Question 2: What
is the role of intra-annual and interannual variability in the
hydrologic regime on the following processes/features?
Question 2.1: On short-term (years)
changes in soil elevation & topography? We will answer this
question with the soil elevation change and sedimentation data
being collected in the marsh (D.Childers) and tree island (M.Ross)
communities. Additionally, if a dry-down occurs, we will quantify
soil microtopography within the vegetation plots (L.Leonard).
Question 2.2: On rates of material
transport and flow? We will address this question through our
measurements of temporal changes in flow and material load (including
particle characteristics and nutrient concentration of suspended
materials) over study period (H.Solo-Gabriele, see UM contract).
We will also monitor surface water and ground water at 4 locations
per tree island site. We will use these data to identify regional
hydrologic gradients, and they will be incorporated into the
modeling efforts (M.Ross, H.Solo-Gabriele, and S.Mitchell-Bruker).
Primary Research Question 3: What is the role of the
flow in maintaining a Ridge and Slough Landscape?
Question 3.1: What is role of
material transport in tree island development and maintenance?
We will use our measurements of change in net soil elevation
(pin method) and surficial sediment accumulation (trap) to address
the maintenance aspect of this. We will use compositional and
textural analysis of the surficial soils to ascertain the recent
depositional/erosional environment, particularly downstream
of tree islands in tails (L.Leonard and M.Ross). The quality
of materials being transported is also important to this question,
so we will determine the characteristics of particulates being
transported via bedload at the tree island-marsh interface (L.Leonard
and H.Solo-Gabriele, see UM contract). We also hypothesize that
major wind events may be an important mechanism of materials
transport when they force "rafts" of periphyton mat
onto tree islands. This episodic deposition of periphyton wrack
may be an important source of material to build tree island
soil elevation. We will address this phenomenon through a combination
of the long-term soil elevation measurements and our periphyton
tagging and tracking experiments (D.Childers, H.Solo-Gabriele,
Question 3.2: What is role of
material transport in marsh [sawgrass ridge and slough] development
and maintenance? As with Question 3.1, we will use compositional
and textural analysis of the surficial soils at sawgrass-slough
interfaces, periphyton mat tagging, and determination of the
characteristics of particulates being transported via bedload
at the slough-sawgrass interface to answer this question (same
responsibilities as Question 3.1). Additionally, our flow manipulation
experiments will allow us to directly address this question
as we use ecosystem manipulations to increase and decrease ambient
flow rates (D.Childers and L.Leonard).
Question 3.3: What is the role
of flow in mediating energy and nutrient exchange among Ridge
and Slough communities? Flow and hydrology will affect energy
and nutrient exchanges both above and below the soil surface.
We will quantify temporal change in nutrient concentrations
in ground water on an intermittent basis along our sampling
transects (S.Mitchell-Bruker). We will attempt to integrate
surface and groundwater measurements by quantifying nutrient
concentrations in pore waters at the tree island-marsh and the
sawgrass-slough interfaces (M.Ross). We will also periodically
sample surface water at 4 transect perimeter stations to quantify
any nutrient gradients that may be a result of island-flow interactions
(D.Childers). Finally, our flow manipulation experiments will
allow us to address this question on a local scale as we use
ecosystem manipulations to increase and decrease ambient flow
rates (D.Childers and L.Leonard).
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