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marsh species. Historically, fresh surface water flowed to
the bay through natural sloughs, rivers, and wetlands, and
fresh groundwater flowed through the Biscayne Aquifer
(Parker et al. 1955; Kohout and Kolipinski 1967; Buchanan
and Klein 1976) and seeped into the bay along the coast and
through the bay bottom. At the southern end of the Miami
Rock Ridge, freshwater wetlands in the Biscayne Bay watershed occupied extensive coastal marl prairies that were east
of the ridge where it curves westward away from the coast.
Prior to drainage and land development, these marshes were
dominated by sawgrass (Cladium jamaicense), spike rush (Eleocharis cellulosa), and other freshwater graminoids, including
grasses and herbs (Gaiser and Ross 2004). The hydropattern
depended on direct precipitation, surface flow from the Everglades through breaks in the ridge called “transverse glades,”
or rising groundwater when the regional water table was high
(Wanless 1976). In a study of the paleoecology using soil cores
along the coast, Gaiser and Ross (2004) showed a change from
freshwater swamp forest to mangrove forest and a change in
salinity from approximately 6.5 ppt to 12.5 ppt.
The transverse glades and freshwater forests and wetlands
drained into a transition zone of coastal creeks and a mix of
herbaceous freshwater-brackish wetlands, tidal marshes, and
mangrove forests on the edge of the coast. These supported
the bay ecosystem by spreading freshwater inflow, absorbing excess nutrients, and providing habitat - including critical nursery habitat - for fish and shellfish, as well as feeding
habitat for wading birds. Local rainfall, groundwater seepage,
overland sheet flow, and small coastal rivers fed water to the
bay all along its mainland margins. The flow of freshwater
through the forested wetlands and freshwater marshes provided dynamic storage of water and a buffer to reduce the amplitude of rainfall runoff events, and moderated the transition
from freshwater inflows to estuarine and marine conditions
in the bay (Wanless 1976; Browder and Wanless 2001). Egler
(1952) documented the distinct vegetation bands parallel to
the coast prior to construction of major drainage canals. He
concluded that changes in vegetation composition were a
result of increases in salinities between freshwater wetlands
and saline wetlands near the coast. The area was still largely
dominated by graminoid freshwater wetland, followed by
dwarf mangrove scrub, and a narrow band of coastal mangroves in the 1940’s and early 1950’s. Kohout and Kolipinski
(1967) found a distinct biological zonation of flora and fauna
in nearshore Biscayne Bay based on salinity in the area around
the old Cutler power plant north of the old Burger King headquarters site. Ishman et al. (1998) documented increasing
salinity with distance offshore in pre-drainage Biscayne Bay.
They also compared historical salinities from 1850-1900 to salinities in 1996. Gaiser and Ross (2004) also documented the
historic salinity gradients in the remnant coastal wetlands. Average annual mean pre-drainage salinities were less than 2 ppt
and freshwater marshes and associated forest units covered
90% of the area, with only a narrow band of mangroves along
the coastline. Today that same area has a mean salinity of 13.2
ppt, and 95% of the area is covered by mangroves. Estuarine
conditions, which were maintained by freshwater inflow, provided a broad band of habitats for organisms having a variety
of salinity needs.
Western Bay Zone. Many different estuarine species flourished in the western part of Biscayne Bay. Miami-Dade County was at the core of the American crocodile ( Crocodylus acutus) geographic range in the U.S. (Kushlan and Mazzotti 1989),
with the coastal wetlands along the western shore of Biscayne
Bay providing important habitat. Oyster reefs and associated
fauna could be found in the bay (Smith 1896; Meeder et al.
2001). A large number of fish species could also be found in
Biscayne Bay, with Smith (1896) listing 95 fish taxa.The presence of several species of drum (family Sciaenidae), which
prefer brackish conditions, and the description of some of
these species as either abundant or common, is particularly
revealing of more estuarine conditions over 100 years ago (Serafy et al. 2001).
Recent paleoecological research conducted by the U.S.
Geological Survey provided insight into pre-drainage conditions and general trends in bay salinities since 1900 (Ishman
et al. 1998; Wingard et al. 2003, 2004). Results from sediment
cores showed a long-term trend of increasing salinity at virtually all sites studied in Biscayne Bay, particularly since 1900. In
the vicinity of Black Point, these data indicated that mid-level
mesohaline conditions existed prior to 1900, tending toward
more polyhaline conditions after that time. Sediment core
data indicated that, since 1950, salinities have increased to
mostly polyhaline conditions at these sites. Middle Key and
Manatee Bay sites to the far south (Barnes Sound area) are
farthest from the influence of Everglades transverse glades
drainage, but are instead under the influence of the eastern
margin of the Taylor Slough drainage system. Sediment cores
from these two sites indicated conditions were predominantly
freshwater (0 ppt) until about 1900. After that, oligohaline
and low-end mesohaline conditions appeared, possibly in
response to construction of the Flagler railroad, initial land
drainage activities, and the construction of roadways (Wingard et al. 2004). The final shift occurred around the 1960’s
to mid-1970’s when marine conditions (30-40 ppt) appeared
in the sediment record, as evidenced by the remains of invertebrate species that can tolerate high salinities in areas that
were once estuarine. Clearly, since 1900, a significant amount
of freshwater from the historic Everglades drainage has been
diverted, converting what were once fresh-to-brackish, estuarine habitats in the nearshore areas to marine conditions with
the loss of estuarine productivity and function.
8 South Florida Natural Resources Center Technical Series (2006 1)
Current Conditions
Hydrology has been altered by regional drainage, canal construction and operation, and urban development, as well as by
construction of roads, levees, and other hydrologic barriers
to surface flow. The bay currently receives freshwater inflow
as canal flow, with only minor overland flow, and occasional
groundwater seeps (Kohout and Kolipinski 1967; Buchanan
and Klein 1976; SFWMD 1995). Freshwater flow to the Card
and Barnes Sounds section comes only through moderateto-light discharges from the C - l l l Canal, and from overland
runoff from extensive freshwater and coastal wetlands contiguous with the mainland shoreline of these two basins.
Coastal Mangrove Zone. The L31E levee and canal run parallel to the coast about 2 km inland and intercept the local subbasin drainages and transverse glades that used to discharge
water to southern Biscayne Bay. As a result, a narrow strip of
isolated coastal stream remnants and coastal fringe mangrove
forest, most of which is within the park, is disconnected from
the regional hydrology. The loss of freshwater inflows has resulted in the expansion of the fringing mangrove forest to the
eastern side of the levee system (Ross et al. 2000; Gaiser and
Ross 2004).
Currently, the hydrologic conditions within the CMZ are
dependent upon canal stages and local rainfall. Although there
are no current freshwater deliveries to this area, water levels in