FCE I research focused on biophysical dynamics in the estuarine
ecotone regions of the coastal Everglades. Our central theme and
organizing hypotheses focused on understanding how dissolved
organic matter from upstream oligotrophic marshes interacts with a
marine source of phosphorus (P), the limiting nutrient, to control
estuarine productivity where these two influences meet—in the
oligohaline ecotone. This dynamic is affected by the interaction of
local ecological processes and landscape-scale drivers (hydrologic,
climatological, and human; Fig. 1-1). The central theme of our FCE
I research was that regional processes mediated by water flow
control population and ecosystem level dynamics at any location
within the coastal Everglades landscape. We tested hypotheses along
freshwater to marine gradients represented by landscape transects
in two Everglades drainage basins of Everglades National Park (ENP;
for FCE I site map, see http://fcelter.fiu.edu/research/sites/).
The Shark River Slough transect (SRS) is anchored at a canal inflow
point along the Tamiami Trail and extends through the mangrove
estuary to Florida’s southwest coast. Historically, most of the
water draining the Everglades flowed through this system. The
Taylor Slough/ENP Panhandle transect (TS/Ph) is anchored at two
main canal inflow points, and extends through the oligohaline
ecotone and Florida Bay estuary to the same coastal ocean endpoint.
This is a smaller, more localized drainage basin. Because the
freshwater Everglades is a highly oligotrophic, P-limited system
(Noe et al. 2001), freshwater inflow to both estuaries is very
nutrient-poor. In fact, the source of P to Everglades estuaries is
marine water from the Gulf of Mexico, not the upstream watersheds
(Fourqurean et al. 1992; Chen & Twilley, 1999; others). Because
of this reversal in limiting nutrient source—compared with
“typical” estuaries—we refer to these systems as biogeochemically
“upside-down” (Childers et al. 2006a).
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Figure 1-1: A conceptual simplification of the central
theme and three main hypotheses that drove FCE I research.
The ovals represent key hydrologic, climatological,
ecological, and human drivers. The small rectangles are the
inputs thought to most strongly control ecosystem
productivity in the oligohaline ecotone. H1, H2, and H3
refer to the three central hypotheses of FCE I.
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Data from the 1990s suggested a generalized ecosystem productivity
peak in the oligohaline ecotone region of our SRS transect, where
tidal inputs of marine P meet organic matter-rich inputs from the
freshwater Everglades. We hypothesized no such peak in the southern
Everglades ecotone (our TS/Ph transect) because Florida Bay is so
efficient at sequestering marine P (Fig. 1-2A). This hypothesis
about estuarine productivity, and controls on that productivity,
directed our research in the oligohaline ecotone regions of both
transects, but also required us to learn more about biophysical
dynamics both upstream (freshwater Everglades) and downstream (the
Shark River mangrove estuary and Florida Bay) of the ecotone. Based
on these data and this hypothesis, we focused our FCE I research on
understanding how dissolved organic matter (DOM) from upstream
oligotrophic marshes interacted with a marine source of the
limiting nutrient, phosphorus (P), to control productivity in the
oligohaline estuarine ecotone.
FCE I research tended to show the opposite pattern, however, with
many ecosystem components showing enhanced productivity in the
TS/Ph ecotone, but not in the SRS Figure 1-2: (A) Generalized
landscape-scale patterns of our FCE I hypothesis about how
ecosystem productivity would vary along the SRS (solid line) and
southern Everglades (TS/Ph; dashed line) transects. (B) Generalized
landscape-scale patterns of how ecosystem productivity actually
varied along the Shark River Slough (solid line) and southern
Everglades (TS/Ph; dashed line) transects, based on FCE I
results.ecotone (Fig. 1-2B). Aboveground net primary production
(ANPP) by the dominant macrophytes showed a “wedge” of increasing
productivity towards the marine endmember of our SRS transect (Ewe
et al. 2006; Fig. 1-2B). The same marine- directed increase in ANPP
occurs in Florida Bay (Fourqurean et al. 1992). We saw some
indications of higher ANPP in the oligohaline ecotone along the
TS/Ph transect, which was also contrary to our original hypothesis
(Fig. 1-2; Childers et al. 2006b; Ewe et al. 2006). Water column P
concentrations followed a similar pattern, with unexpectedly high P
in the TS/Ph ecotone during the dry season (Childers et al. 2006a).
Landscape-scale patterns of soil P content along our transects also
followed the pattern shown in Fig. 1-2B rather than our original
hypothesis (Fig. 1-2A; Chambers & Pederson 2006). It is well
known that canal inputs of P are responsible for eutrophication
patterns in Everglades wetlands (Craft & Richardson 1993; Doren
et al. 1997). However, canal inflows influence ENP marshes to a
lesser degree than is seen in wetlands further north (Childers et
al. 2003). Along FCE transects, soil P levels at canalside sites
and marine sites were similar (Chambers & Pederson 2006). At
times, we did observe high periphyton productivity at our TS/Ph
canal sites, but these events occurred at the onset of the wet
season and were short-lived (Iwaniec et al. 2006).
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Figure 1-2: (A) Generalized landscape-scale patterns of our
FCE I hypothesis about how ecosystem productivity would
vary along the SRS (solid line) and southern Everglades
(TS/Ph; dashed line) transects. (B) Generalized
landscape-scale patterns of how ecosystem productivity
actually varied along the Shark River Slough (solid line)
and southern Everglades (TS/Ph; dashed line) transects,
based on FCE I results.
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We have spent considerable effort studying the sources, transport,
and fate of DOM along our FCE transects (Lu et al. 2003; Jaffé et
al. 2004; Maie et al. 2005; 2006a). The leaching of dissolved
organic carbon (DOC) and P from periphyton and senesced mangraove,
spikerush, and sawgrass leaves is primarily a physical process, but
mobilization of nutrients—particularly P—by microbes on the leaves
becomes more important later in the decomposition process (Davis et
al. 2003a, 2006; Maie et al. 2006b; Romero et al. 2005; Rubio &
Childers 2006). The same is true of the seagrass decomposition
process (Fourqurean & Schrlau, 2003). Of the major ecotone
plant litter examined, DOC leaching rates were greatest from
mangrove leaves. Davis et al. (2006) combined their leaching data
with leaf litterfall rates (mangrove) and leaf mortality rates
(sawgrass and spikerush) and showed that this process may be an
important vector for moving soil nutrients—particularly P—into the
water column. Additionally, Scully et al. (2004) found different
rates of physico-chemical processing, photodegradation and
microbial degradation for DOM leached from different vegetation
sources, suggesting the need for a more detailed molecular
characterization of DOM (Jones et al., 2004, 2006).
Our organic geochemical research has shown that Everglades DOM is
more refractory than originally hypothesized (Boyer et al. 2003; Lu
et al. 2003; Jaffé et al. 2004; Maie et al. 2005; 2006c). We have
also begun to understand the importance of detrital organic matter
production and transport to ecotone dynamics (Wood 2005; Leonard et
al. 2006). Jaffé et al. (2001) used a biomarker-based assessment of
sources of particulate organic matter (POM) to the SRS and TS/Ph
estuaries. Their conceptual model suggested different processes
controlled POM mixing in these two systems. Mead et al. (2005)
refined this model by showing that simple end-member models do not
work well in Everglades estuaries. Much of the POM in these systems
is not suspended in the water column, but rather is found as a
flocculent detrital layer above the soil surface (colloquially
referred to as “floc”). Neto et al. (2005) reported that much of
this “floc” was locally produced, which greatly complicates
traditional 2-source allochthonous mixing models. “Floc” also is
the base of aquatic food webs (Williams & Trexler 2006).
Interestingly, fish standing stocks showed an ecotone peak along
both transects, supporting our original hypothesis about
landscape-scale patterns in ecosystem productivity in SRS but not
the southern Everglades (TS/Ph transect; Green et al. 2006; Lorenz
& Serafy 2006).
The dominant disturbances in the coastal Everglades are hurricanes
and fire, which affect the landscape at a range of spatial scales
(Lockwood et al. 2003), and hydrologic extremes (droughts and
floods, mediated or exacerbated by human activities), which
primarily affect animal and upland communities (Trexler et al.
2005). Sea level rise is also a disturbance that has gradual
effects (i.e. “press-type”) rather than event-based impacts (i.e.
“pulse-type”). Long-term peat accretion in the [relatively new]
transgressive mangrove wetlands of the southern Everglades has been
considerably higher (approximately 3 mm yr-1) than the marl soil
accretion by the freshwater marshes they replaced (about 0.8 mm
yr-1; Ross et al. 2000; Gaiser et al. 2006). This rate of peat
production is roughly equal to the rate of eustatic sea level rise
in south Florida. Short-term sediment deposition rates [in the
ecotone] associated with specific storm events can be considerably
higher, however (Davis et al. 2004). For example, the storm surge
from Hurricane Wilma, a Category 3 hurricane that tracked northeast
directly along our SRS transect on October 24 2005, deposited over
3 cm of carbonate mud in the mangrove forests near the Gulf of
Mexico.
Our modeling and synthesis activities have focused on understanding
how ecological pattern and process in upstream freshwater
Everglades marshes affected the composition of water flowing into
the oligohaline ecotone. We have used a “dynamic budget” approach
to simulating dynamics at a given location (Childers et al. 1993a),
and have conceptually linked sites along a given FCE transect with
“ribbon models”. Decadal run output from these models has shown the
highest net P accumulation in sawgrass and wet prairie marshes
nearest to canal inputs (0.15 to 0.34 g P m-2 yr-1), but near
steady-state conditions in oligotrophic interior marshes (-0.02 to
0.06 g P m-2 yr-1) and upper ecotone marshes (0.00 to 0.09 g P m-2
yr-1). These simulated P accumulation rates were similar to
observed estimates from nutrient-enriched (0.40-0.46 g P m m-2
yr-1) and unenriched (0.06 g P m-2 yr-1) marshes in the northern
Everglades (Water Conservation Area 2A; Craft & Richardson
1998). These “ribbon” models also predicted water P concentrations
that were within the range of observed values for all areas except
interior oligotrophic sawgrass marsh. Simulations generated
relatively high water P concentrations in these marshes (10 to 50
µg P L-1); however, when the models were modified to account for
potential exchanges of waterborne and detrital P between
communities, model predictions for both sawgrass and wet prairie
marshes improved considerably. These results highlight the need for
further investigations of the biophysical mechanisms affecting
water and detrital P dynamics in oligotrophic interior marshes,
especially exchanges of P between communities and ultimately
downstream to ecotone marshes (see Childers 2006 for a more
detailed synthesis of FCE I findings).
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Master's thesis, Florida International University.
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