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generally related to water clarity and quality, substrate,
salinity levels, and variability. Syringodium filiforme
and H. wrightii are common in the northern bay, where
salinities are lower and water clarity is diminished due
to high freshwater discharge combined with a low
flushing rate. Significant mixed Thalassia/Syringodium
beds also exist in North Biscayne Bay. Thalassia is
most prominent in central and south Biscayne Bay
where salinities are higher and more stable and nutrient levels are lower overall.
The distribution of seagrass species and other benthic flora and fauna in the western nearshore area of
central and southern Biscayne Bay is influenced by
both canal discharges and submarine ground-water
seepage (Kohout and Kolipinski 1967, Meeder et al.
1997, 1999). Presence or absence of Thalassia often
is an indication of distinct zones where ground-water
influence is substantial (Thalassia absent) or insignificant (Thalassia present). Along a transect from 25 to
300 m from shore, Meeder et al. (1997, 1999) found
the maximum ground-water seepage about 200 meters
from shore. The amount of ground-water seepage and
its influence has been diminished by the general lowering of the water table in Miami-Dade County (Parker
et al. 1955) to facilitate development in wetlands. Sealevel rise also reduced ground-water seepage to Biscayne Bay by reducing the hydraulic gradient, or difference between the water table and sea level at the
coast, which, according to Darcy’s Law, drives
ground-water flow in an unconfined aquifer (Chow
1964).
Where sediment depths and currents are appropriate,
seagrass species generally follow a pattern of zonation
from west to east (Ruppia, Halodule, Thalassia, Syringodium) correlated with general salinity gradients
and salinity fluctuation (Lirman and Cropper 2003).
The freshwater inflows (surface and ground) occurring
along the shoreline are critical in maintaining this zonation and benthic diversity. The altered salinity patterns that resulted in concentration of surface-water inflows into canals and reduced ground-water seepage
likely affected competition among seagrass species,
changing this zonation and making it less defined. Results from a hydrodynamic simulation model comparing canal inflows versus distributed inflow indicate
that the canal scenario produces higher overall salinity
in the nearshore zone than the distributed inflows (i.e.,
to simulate flow through the historical creeks (Brown
2003). Channelization of the Miami River might have
had a similar effect as construction of the South Miami-Dade canals that shortcircuited the historic creeks.
Analysis of sediment cores from southern Biscayne
Bay indicates that it has become more saline and less
variable in the last 100–200 years (Wingard et al.
2003). Seagrass composition in these areas has been
documented to vary between Ruppia, Thalassia, and
Halodule, or mixtures of Halodule and Ruppia or Halodule and Thalassia, depending on salinity regime.
Mangrove Functionality and Herbaceous Wetlands
Coastal wetlands are highly productive habitats that
provide nursery, foraging, and refuge areas for many
bird, fish, and invertebrate species. In addition, these
coastal wetlands help maintain water and habitat quality by filtering sediments and nutrients from inflowing
waters. Biscayne Bay’s remaining mangroves and associated herbaceous wetlands, including nearshore
freshwater wetlands, have lost much of their ecological
function because fresh water has been diverted away
from coastal feeder streams and creeks into drainage
canals. Restoration of both brackish and freshwater
wetlands and coastal creeks on the western shore of
Biscayne Bay is important to the success of bay restoration and, therefore, is defined as an indicator of
success. In the southern part of the western bay, water
management and watershed development activities to
date have caused saltwater intrusion and led to an encroachment of scrub mangroves on former freshwater
wetland. Freshwater wetlands are a vital component of
the coastal wetland system, and their loss is undesirable, even when replaced by salt-tolerant species like
mangroves. The presence of a system of coastal wetlands integrated by the inflow of freshwater from upstream and, to varying degrees, by tidal exchange, is
essential to the restoration of a fully functional Biscayne Bay ecosystem.
EXHIBIT 7
Browder et al., Biscayne Bay CEM 859
Benthic Communities
Benthic organisms such as mollusks, attached fauna,
and infauna provide essential ecological and biological
functions in the bay and can influence the quality of
the environment. The benthic community is the basis
for development of high quality habitat that will support diverse fish and motile invertebrate populations.
Degradation or loss of benthic communities will diminish the ability of the bay to maintain the mosaic
of conditions that support high habitat diversity and
productivity. Benthic communities are depauperate
within the dredged canals and channels of the drainage
system that empty into the bay. These channels provide poor habitat because of their depths, near vertical
banks, low dissolved oxygen, and reduced water transparency (DERM 2005b). In addition, they are frequently redredged, disturbing the bottom sediments,
and are regularly sprayed with herbicides. The present
operation of water-control structures (opening and
closing automatically according to upstream and
downstream water level) causes discontinuous freshwater flows that result in localized extreme salinity
variability that is unsuitable habitat even for estuarine
organisms (Serafy et al. 1997).
Pink Shrimp, Blue Crabs, Stone Crabs, and Oysters
Juvenile pink shrimp immigrate to Biscayne Bay
from offshore spawning grounds each year and settle
in the seagrass beds close to the mainland shoreline
near freshwater inputs. Pink shrimp seem to prefer a
salinity range of 20–35 parts per thousand (ppt) (Pattillo et al. 1997), but survival and growth have been
tied to temperature and salinity (Browder et al. 1999),
with an optimal salinity for juvenile growth at 30 ppt
(Browder et al. 2002). This species would be expected
to benefit from an expansion in estuarine habitat in the
western bay. Pink shrimp’s ecological characteristics
and economic value, together with the background of
knowledge about this species in South Florida, make
it an appropriate biological indicator of change in
freshwater inflow quantity, timing, and distribution.
Furthermore, pink shrimp constitute the most significant commercial fishery in Biscayne Bay (Berkeley
1984). A commercial pink shrimp live-bait fishery has
operated in Biscayne Bay for many years, and a more