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the bay provided under CERP (USACE and SFWMD
1999).
Sediment/Water Column Contamination
Processes linking ecological attributes to stressors
are depicted in the first diamond in Figure 2. Community composition, distribution, and health of macrobenthic, infaunal, and demersal organisms can be affected by the presence of toxic substances in sediments. Potentially toxic pollutants, such as metals and
organic chemicals, usually have low water solubility
and tend to bind to particulate material and accumulate
in sediments (Seal et al. 1994, Long et al. 2000). Most
contaminants in Biscayne Bay sediments occur in
highest concentrations in conveyance canals, rivers,
streams, and marinas, and the lowest concentrations
are along the central north-south axis of the bay (Corcoran et al. 1983, Alleman et al. 1995). Trace metals
and synthetic organic contaminants, such as some pesticides and polychlorinated biphenyls (PCBs), are
found in higher concentrations in Miami River and
Wagner Creek sediments than in any other area in the
State of Florida (Schmale 1991, DERM 1993, Seal et
al. 1994). Other canals that have high levels of sediment toxicity include Little River (C-7), Black Creek
(C-1), and Military Canal (USEPA 1999, Miami-Dade
County Department of Environmental Resource Management, pers. comm.).
Relationship of Sediment/Water Column Concentration to Toxicity. Recent studies (Long et al. 2000,
2002) showed that contaminant levels in Biscayne Bay
sediments were slightly below the national average,
but toxicity levels (based on biological assays) were
slightly above. These studies supported earlier findings
that contamination and toxicity were most severe in
several conveyance canals and a few natural tributaries, and that sediments from the open basins were less
toxic than those from the adjoining canals and tributaries. In more open waters of the bay, chemical concentrations and toxicity were generally higher north of
Rickenbacker Causeway than south of it. However, a
section of southern Biscayne Bay showed remarkably
high toxicity that could not be attributed to any of the
substances analyzed in sediments. Evidence suggests
that mixtures of some metals and synthetic organic
chemicals were likely contributors to toxicity observed
in the lower Miami River. For example, an amphipod
survival test showed a high degree of correspondence
with a gradient of general chemical contamination in
the river and adjoining reaches of the bay. Because
contaminants are conveyed to the bay through tributaries and ground-water flux, changes in distribution
or sources of fresh water or ground-water stages may
affect the fate, amount, and pattern of contaminants
introduced. This could increase water- column and
sediment contaminant levels (or toxicity), increase
ecological exposure, and ultimately affect sensitive
species and, perhaps, overall secondary productivity or
diversity.
Seagrass Habitat
Relationship of Seagrass Abundance and Distribution
to Salinity Patterns and Water Quality. Processes
linking the bay’s ecological attributes to stressors are
depicted in the second diamond in Figure 2. Seagrass
and benthic communities require a consistent (both in
range and variability) salinity regime and appropriate
water quality (sufficient but not excessive nutrients and
sufficient light for photosynthesis). Abundance, distribution, and composition of seagrasses will be determined, in part, by modifications of salinity patterns
and water quality. Changes in composition and areal
coverage of seagrasses will affect habitat quantity and
quality with respect to breeding, refuge, and feeding
areas available for dependant invertebrate and vertebrate species. Diversion of part of the canal flow from
a ‘point source’ to more ‘diffuse’ delivery through
coastal wetlands and creeks will approximate reconstruction of freshwater flow to the bay from the Everglades through historic pathways (i.e., the historic
freshwater coastal creeks, as many as 40 of which interdigitated with tidal creeks prior to development).
EXHIBIT 7
864 WETLANDS, Volume 25, No. 4, 2005
This is expected to reduce sediment resuspension and
nutrient concentrations in the water delivered to the
bay and improve water clarity. This could lead to expansion of seagrass cover in the nearshore areas where
sediment depths are adequate and may improve local
water clarity by inhibiting sediment resuspension.
Mangrove Functionality and Herbaceous Wetlands
Relationship of Freshwater Inflow and the Boundary
between Mangrove and Herbaceous Wetlands. The
relationship of mangrove functionality to stressors is
depicted in the third diamond of Figure 2. Diversion
or reduction of freshwater inflow has caused a loss of
the many small creeks that furnished freshwater to the
bay and has diminished the degree to which mangroves support a healthy, diverse epiphytic community
and provide habitat for both sport fish and their prey.
Alteration of freshwater inflow has caused a shift in
the boundary between the mangrove and herbaceous
wetland and the inland migration of the landward
boundary of the ‘‘white zone’’ (Ross et al. 2000). The
white zone is a band of low productivity at the ecotone
between brackish and freshwater wetlands. Recent
studies in the wetlands of Barnes and Card Sounds
(see Figure 1 for location) indicate that the boundary
of the white zone has moved inland by an average of
1.5 km since 1940, and the white zone is expanding
(Ross et al. 2000). The most significant changes to the
white zone boundary and width occur in areas cut off
from freshwater sources by canals or roads (especially
east of U.S. Highway 1). Low productivity of the
white zone may be primarily the result of wide seasonal fluctuations in soil salinity and moisture content
due to reductions in freshwater inputs from upstream
sources (Ross et al. 2002). CERP’s restoration of a
more natural freshwater flow across the coastal wetlands should reduce the areal extent of the white zone
and shift its inland boundary seaward. Reestablishing
flow across a broader front through re-created coastal
freshwater creek systems should also restore full mangrove functionality. Exotic vegetation has replaced the
white zone in some areas but is not a substitute for
natural herbaceous wetland, and the exotics may have