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Freshwater Flow and Ecological Relationships in Biscayne Bay 2-19
Central Biscayne Bay
The Central Biscayne Bay sub-region extends from the Fisher Island/Virginia
Key area on the north, southward to the Cutler area. Although the barrier island
of Key Biscayne is present in the northerly portion of this sub-region, the absence
of barrier islands to the south make this sub-region the most well-flushed area of
Biscayne Bay. Freshwater inputs, which enter the west side of the Bay through
the Coral Gables Waterway and Snapper Creek, result in estuarine salinities (17-
32 ppt (FIU, 2002)) along the west shoreline. Salinities increase toward the east,
and are marine on the east side of the project area. This salinity gradient,
together with a westerly shoreline that includes mangroves and other naturallyoccurring vegetation provides habitat for a comparatively high diversity of flora
and fauna.
Seagrasses are a major component of the benthos, and where seagrasses are not
continuous, hardbottom is interspersed with seagrasses. Seagrasses are
primarily dense beds of turtle grass in the east, but shoalgrass, paddle grass and
manatee grass are also present, primarily in the western areas. The
southernmost extent of the known range of Johnson’s seagrass is present in the
northern portion of this sub-region. Designated critical habitat for Johnson’s
seagrass ends in this sub-region, with the 250 45’ North latitude line (which
crosses through Virginia Key) serving as the southern boundary for the critical
habitat area (Figure 8).
Attached macroalgae are abundant within this sub-region, particularly on
hardbottom and/or in eastern portions.
The combination of dense seagrass beds and grassbeds interspersed with
hardbottom create ideal habitat conditions for a variety of crustaceans and fishes.
This portion of Biscayne Bay sustains a major commercial pink shrimp
(Farfantepenaeus duorarum) fishery. Reported landings increased greatly from
1990 (<100,000 lbs) through 1999 (> 600,000) pounds (Harper et. al., 2000).
Aerial surveys and telemetry tracking have documented the presence of West
Indian Manatees within this sub-region (DERM, 1995). These monitoring efforts
have also documented the presence of manatees in manatee-accessible areas of
the Coral Gales waterway, Snapper Creek and the Kings Bay/Cutler area. The
extent to which fresh water and/or comparatively warm water during the
winter, are the attracting features for manatees is not known, however the
presence of manatees in these waterways at different times of the year suggest
that both fresh water and warm water may be attracting features to varying
extents throughout the year.
Biscayne Bay Characteristics
Freshwater Flow and Ecological Relationships in Biscayne Bay 2-20
Loggerhead turtles, green turtles, and leatherback turtles are known to occur
within this sub-region, although nesting habitat for these species in this subregion is exceedingly small. These species are designated as threatened
(loggerhead) and endangered (green and leatherback) by both the State of
Florida and federal resource agencies.
South-Central Biscayne Bay
The South-Central sub-region extends from the Cutler area on the north to the
Turkey Point – Cutter Bank area on the south. Several features along the western
shore of the Bay differentiate this sub-region from others to the north and south.
These features include: 1) the presence of a mangrove community for almost the
entire length of the sub-region, and 2) the presence of several major canals (C100, C-1, C-102, Military Canal, and C-103) that discharge fresh water into the
Bay. The majority of this sub-region is also within the boundary of Biscayne
National Park, the boundary of which is shown in relation to the Biscayne Bay
Aquatic Preserve on Figure 9. The National Park extends from the landward
extent of the red mangrove forest on the west to the 60-foot bathymetric contour
(approximately 14 miles) on the east. A natural off-shore barrier island
community consisting of Ragged Keys to the north and Elliot Key to the south
extends in a north northeast direction varying from approximately three miles to
4.5 miles east of the western shore.
Natural resources in this sub-region of the Bay are extremely diverse, although
the more intensive scientific investigations that have occurred as a result of the
national park status may partially contribute to the higher level of knowledge
and awareness of the biota in this region when compared with other areas of the
Bay. Seagrasses and mixed seagrass – hardbottom communities are present
throughout this sub-region. Although shoalgrass, paddle grass and manatee
grass are present, primarily in the western areas, extensive beds of turtle grass
are the primary grassbed communities. Johnson's seagrass has not been
documented to occur in this sub-region. It is widely believed that the increasing
salinities that have resulted from reductions in fresh water inflows (both surface
and sub-surface) into this region of Biscayne Bay during this century have
resulted in changes in the submerged aquatic vegetation community; namely
that turtle grass, which prefers comparatively high salinities, has gradually
replaced shoalgrass and perhaps wigeon grass that prefer comparatively lower
salinities, in sizable portions of this sub-region.
Graduate-level research presently being conducted by Danielle Mir-Gonzalez, a
student at Florida International University, involves mapping the spatial
distribution of the different species of seagrasses along the western shore of this
Figure 9
Boundaries of Biscayne Bay Aquatic Preserve and Biscayne National Park
Biscayne
National Park
Source: Draft Biscayne Bay Aquatic Preserve Management Plan, MetroDade Department of Environmental Resources Managment
Biscayne Bay Characteristics
Freshwater Flow and Ecological Relationships in Biscayne Bay 2-22
portion of Biscayne Bay. Her research also includes monitoring of sub-surface
freshwater inputs into the Bay through the installation and monitoring of
seepage wells. As soon as these data become available, they should be obtained
and analyzed, as the results of these efforts will likely be helpful in identifying
the amounts of freshwater input that will be necessary to prevent harm to these
submerged aquatic vegetation communities. Modeling will likely be necessary,
using the results of the Mir-Gonzalez research, to determine the timing and
volumes of fresh water flows that will be necessary to maintain a salinity
envelope that will prevent harm to the existing seagrass community or change
the community as a preference by managers to a more estuarine character.
As in the Central region, above, the combination of dense seagrass beds and
grassbeds interspersed with hardbottom create ideal habitat conditions for a
variety of crustaceans and fishes. This portion of Biscayne Bay sustains a major
commercial pink shrimp fishery. Reported landings increased greatly from 1990
(<100,000 lbs) through 1999 (> 600,000 pounds) (Harper et. al., 2000). Shrimp are
valuable both for the economic value of the fishery and for their contribution to
the food web. Many species of fish that are valuable either commercially or
recreationally feed on pink shrimp. And although sustaining an economically