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be used for this purpose are the Coral Gables Waterway, the upper Miami
River and Little River.
Valued Ecosystem Components
Freshwater Flow and Ecological Relationships in Biscayne Bay 4-29
Other Aggregation Areas
In addition to the known attractants of fresh water and warm water, localized
conditions are known to attract manatees at several specific locations.
Locations within the MFL project area include: 1) an extensive seagrass bed
located on the northwest side of Virginia Key; and 2) a portion of the Black
Creek marina.
Although it is not specifically designated as ‘Aggregation Area’, manatees
have been documented to forage in the seagrass beds in South Dumfoundling
Bay and between the Port of Miami and Rickenbacker Causeway. It is
thought that the manatees that forage in these areas may be individuals that
are attracted to the warm water discharges from the Port Everglades Power
Plant (Broward County), and which move southward to these areas to feed.
Because manatees forage primarily on seagrasses, and because the presence,
absence, distribution and density of individual seagrasses are somewhat
dependant on salinity, manatees may potentially be affected by modifications
to the delivery of fresh water into Biscayne Bay. However, because the likely
effect of even a total cessation of freshwater inflows would be a shift in the
species composition of seagrasses (replacement of manatee grass, shoalgrass
and paddle grass with turtle grass) and not a significant overall reduction in
seagrass biomass, reductions of inflows would not likely result in adverse
impacts to manatee foraging habitat.
The MPP identifies a number of causes of manatee mortality and identifies
corrective measures that are to be implemented to address these causes.
Because flood gates, where manatees had gotten crushed when automatic
sensors directed flood gates to close, were a major component of manatee
mortality (46% during the period from 1974 through 1994) (DERM, 1995),
direct attention was paid to re-designing the structures to reduce manatee
mortality. Because collisions with vessels was the second highest known
cause of death (27% during the period from 1974 through 1994), the County
adopted a variety of zones where boat speeds were to be restricted. Vessel
speed restriction zones are found in five of the six MFL-designated subregions of Biscayne Bay; there are no restriction zones in the Southern Bay
sub-region. The location of these zones is shown on a variety of brochures
(e.g., Boating and Angling Guide to Biscayne Bay) and distributed free to the
public.
Valued Ecosystem Components
Freshwater Flow and Ecological Relationships in Biscayne Bay 4-30
West Indian Manatees have been identified by USFWS (1999) as an indicator
species for aquatic habitats, including seagrasses and mangroves in the South
Florida ecosystem. Because seagrasses and mangrove habitats provide areas
for foraging, calving, resting and mating, the presence, abundance and
distribution of manatees are indicators of the health and vitality of these
systems.
Johnson’s Seagrass
Johnson’s seagrass (Halophila johnsonii) is a small, rare aquatic plant that is
known only to occur along the east and southeast coast of Florida from
Brevard County to Biscayne Bay. It was designated as a threatened species in
1998 pursuant to the federal Endangered Species Act, as amended. It is not
currently protected under State of Florida statutes, rules or regulations,
although Section 581.185 F.S. ‘provides for the automatic listing as a state
endangered species of “all species determined to be endangered or
threatened pursuant to the federal ESA of 1973” (NMFS 2002). A federal
recovery plan for this species has been developed and was approved by the
National Martine Fisheries Service, National Oceanic and Atmospheric
Administration in 2002. Critical Habitat was designated for this species in ten
distinct areas. The largest of these areas encompasses a significant portion of
Biscayne Bay, and includes portions of the MFL Snake Creek/Oleta River,
Northern Biscayne Bay and Miami Rover/Government Cut Sub-regions.
Johnson’s seagrass is one of twelve species of Halophila, a genus that is
distributed in warm-temperate and tropical waters worldwide (NMFS 2002).
It is one of three species that is documented to occur in Biscayne Bay, and can
be differentiated from the other species by having pairs of 2-5 cm long, linear
leaves that extend upward from a rooted rhizome that is located just below
the sediment surface. Johnson’s seagrass somewhat resembles paddle grass,
Halophila decipiens, with which it will occur (pers. observations) and was
relatively recently recognized as a separate species. It is the first marine plant
to be listed pursuant to the Endangered Species Act, and because it is a recent
addition and because its Recovery Plan was very recently developed,
information concerning the abundance and distribution of this species is very
up-to-date.
Johnson’s seagrass has been documented to occur only on the east coast of
Florida, being found in coastal lagoons along approximately 200 km of
coastline from approximately Sebastian Inlet (Brevard County) to Biscayne
Bay, including areas of the Indian River Lagoon, Lake Worth Lagoon and
Biscayne Bay. Based upon this limited range, Johnson’s is thought to be the
most spatially-restricted species of seagrass in the world (NMFS 2002). The
Valued Ecosystem Components
Freshwater Flow and Ecological Relationships in Biscayne Bay 4-31
largest known areas of Johnson‘s seagrass within this range are in the Indian
River Lagoon and Lake Worth.
Within this range, Johnson’s seagrass grows in a patchy, non-contiguous
distribution in water depths from within the intertidal zone to maximum
depths of approximately 3 m (Kenworthy, 1993; Virnstein et. al., 1997). Due
to its presence in very shallow waters, and even being exposed at extreme
low tides, it is thought to be tolerant of moderate desiccation and wide
temperature ranges. Although it is reported to be more commonly found in
monotyic patches, it may also be present with shoal grass and manatee grass.
Relatively little work has been done to identify the salinity, temperature
and/or water quality preferences and tolerances of Johnson’s seagrass.
Research to date, however suggests that Johnson’s seagrass survives in at
least salinities from 15-43 ppt, and that its range may be greater (NMFS,
2002). It also grows in areas of varying water clarity, from comparatively
turbid areas to clear-water areas near inlets, where currents are fast and
strong.
In spite of its comparatively small size, studies indicate that Johnson’s
seagrass serves a food source for other organisms, as a refuge, habitat and
nursery for wildlife species, assists in sediment stabilization and therefore