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Brandt 1995). The model targets juvenile crocodiles
because studies indicate that this life stage requires
lower salinities due to osmoregulatory limitations
(Mazzotti and Dunson 1984). This model shows that
salinity between 0 and 20 ppt provides the most suitable habitat, 20–40 ppt provides intermediate suitability, and 40 ppt is least suitable. Applying the model
to Biscayne Bay suggests that restoring freshwater
flow to the coastal wetlands would benefit crocodiles,
especially along the western shore in the central and
southern regions. Most of this area is currently unsuitable for juvenile crocodile habitat. Restoration efforts will include redirecting flow from conveyance canals through coastal mangrove wetlands and maintaining flow into the beginning of the dry season.
Manatee
Endangered West Indian manatees occur throughout
Biscayne Bay but are most frequently observed in tributaries and nearshore seagrass beds. Manatees are
present year-round and are most abundant in winter,
when more than 130 have been counted on a single
day (Mayo and Markley 1995). Biscayne Bay seagrass
meadows provide important foraging habitat for manatees wintering at warm water discharges (power
plants) in Broward County, and the bay is a significant
seasonal migratory corridor. Thus, the total number of
EXHIBIT 7
Browder et al., Biscayne Bay CEM 861
animals using the Biscayne Bay area is likely to be
greater than the maximum number observed on any
given day.
Manatees utilizing the bay are part of the larger Atlantic region ‘‘subpopulation,’’ which includes those
animals ranging along the Atlantic coast from southern
Georgia to the Florida Keys and including the lower
St. John’s River. Atlantic coast manatees undertake
seasonal, intraseasonal, and daily migrations or movements (Deutsch et al. 2003). Radio-telemetry studies
and tracking or resighting of known scarred individuals have shown that manatees may travel hundreds of
kilometers seasonally, moving to southeast Florida or
unnatural sources of warmer water. Tracking studies
of animals in the Biscayne Bay area also suggest a
general diurnal pattern, with animals resting in canals
and sheltered basins during the daytime and moving
into bay areas to feed in late afternoon and evening
(C. Beck, United States Geological Survey, pers.
comm.). Although wide-ranging, manatees demonstrate a high degree of site fidelity, yet they also show
individual patterns, flexibility, differential use of sites
over time, and adaptation to changing conditions,
moving among warm water refuges, freshwater sources, and feeding sites. The general distribution of manatees is strongly linked to fresh water; they more frequently occupy areas where freshwater sources are
readily available (O’Shea and Kochman 1990, Mayo
and Markley 1995, LeFebrve et al. 2001, Deutsch et
al. 2003). Changes in timing and volume of freshwater
delivery could affect manatee distribution, particularly
in south Biscayne Bay.
Adult annual survival rates for manatees in the Atlantic subpopulation have been estimated at 88.7–
92.6%, a lower rate than has been estimated for other
regions (Langtimm et al. 1998). Due to uncertainty in
population model estimations, it is not possible to determine with statistical confidence whether the Atlantic
population has been stable, decreasing, or increasing
in recent years; however, annual manatee mortality in
the Atlantic region remains high and appears to be
increasing at a greater rate than optimistic estimates of
population growth (USFWS 2001). Although many
manatees have been killed or injured in Biscayne Bay
by vessel collisions, the leading known cause of manatee in death in Miami-Dade County is crushing or
entrapment in water-control structures (Mayo and
Markley 1995). Thus, changes in operation of these
structures may directly affect survival of individuals
using the Biscayne Bay area and stability of the Atlantic subpopulation.
Wading Birds
Wading birds are being used as biological indicators
throughout the region because of their close association with hydropattern. The islands, tidal flats, and
coastal wetlands of Biscayne Bay provide valuable
habitat for wading birds. Frequently used nesting sites
occur at Greynold’s Park near the northern bay, in the
Arsenicker Keys in the southern bay off Turkey Point,
and on small islands off Key Biscayne and Virginia
Key (Browder personal observation). Tidal flats and
coastal wetlands of the bay provide important feeding
habitat for wading birds that nest nearby. For example,
roseate spoonbills (Ajaia ajaja Linnaeus) that nest in
the Tern Keys of northeastern Florida Bay feed in
mangrove creeks and herbaceous wetlands of southern
Biscayne Bay (Card and Barnes Sound areas), as well
as those of Florida Bay. Wood storks (Mycteria americana Linnaeus) that nest in the southern Everglades
also feed in wetlands of southern Biscayne Bay. The
natural pattern of seasonal variation in water stages
alternately produces and concentrates forage fish for
wading birds. A more natural seasonal variability in
water stages in relation to the rainfall pattern will not
only produce and concentrate fish for wading birds but
also support favorable salinity conditions for estuarine
fish and macroinvertebrates downstream in Biscayne
Bay.
ECOLOGICAL EFFECTS: CRITICAL LINKAGES
BETWEEN STRESSORS AND ATTRIBUTES/
WORKING HYPOTHESES
In the Biscayne Bay Conceptual Ecological Model
(Figure 2), relationships between the five stressors and
the ecological attributes discussed above are depicted
in the four diamond-shaped modules representing
pathways of effects. Most of the ecological attributes
are directly affected by salinity patterns/coastal wetlands, water quality, or sediment/water column contamination. These are determined by the stressors according to the relationships depicted in the ‘‘effects
pathways’’ modules (the diamonds in Figure 2). The
discussion of these effects pathways is followed by a
discussion of hypothesized linkages between the ecological attributes and these effects pathways, including
physical impacts (depicted in its own ‘‘effects pathways’’ module, fourth diamond in Figure 2). Physical
impacts include effects of dredging, water-management control structures, and fishing gear.
Salinity Patterns/Coastal Wetlands