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• Direct benefit to spawning habitat for sea trout, adult
habitat for forage species (mojarras, silver perch),
and conditions that foster a healthy and diverse seagrass community that can be sustained in a zone that
is subject to freshwater runoff.
• Indirect benefit to nursery habitat for important juvenile fish species (productive seagrass beds and
an extensive brackish water refuge from marine
predators).
A In the late wet season (September-October), the CMZ
should be oligohaline (0-5 ppt), and the WBZ should average less than 20 ppt.
• Direct benefit to juvenile crocodiles that have a physiological requirement for low salinity conditions.
• Direct benefit to important forage fish species in
coastal mangroves that do best at oligohaline to
mesohaline conditions (such as sheepshead minnow,
gold-spotted killifish).
Crocodile, juveniles i ~ f
Seatrout, juveniles f ^
Mojarras f I
Silver perch, juveniles i i
Pink shrimp, juveniles f i
Eastern oyster r................. I
Widgeongrass i i
0 5 10 15 20 25 30 35
Figure 5. Optimal salinity ranges (units in ppt) for Biscayne National Park ecosystem indicators.
Ecological and Hydrologic Targets for Western Biscayne National Park 13
• Indirect benefits to all species that consume forage
species that support upper trophic levels in the mangrove zone, including wading birds, mammals, and
crocodiles.
* Salinity changes should be gradual and reflect changes in
' hydropattern that approximate a natural system. All vegetation, fish, and invertebrate species benefit from gradual
changes in salinity that avoid physiological stress.
* Salinity gradients should always consist of lowest salinities near the coast and highest salinities in the ocean.
Restoration of freshwater inflows to reach the appropriate salinity should result in a highly productive SAV community that
will meet the more general ecological targets outlined earlier.
From Salinity to Freshwater Flow Targets
A salinity metric is more than a sensitive link to the biological environment. Salinity also provides an excellent metric
linking to the dynamic coastal environment because it is an
accurate and integral measure of the net results of the total
freshwater inputs, mixing rate of marine and freshwater flows,
wind mixing, net evaporative losses, and amount of tidal exchange. During the dry season when there is little or no flow
through the coastal structures, the salinity metric will still be
applicable while a flow metric may not provide any additional
useful information (it might be zero).
The freshwater inflows needed for maintenance of target
salinities can be calculated in a number of ways. A factor to
consider in the calculation of freshwater flows is that a water
volume budget, as proposed by draft Guidance Memorandum 4, is not as well constrained in an estuary with open
boundaries since it is difficult to quantify the outgoing/incoming marine flows with an accuracy near that of the incoming
freshwater flows (at this time the National Park Service does
not have long-term time series of inflows/outflows through
the Safety Valve and the ABC Creeks).
Estimates of Target Flows
Ideally, a computer-based simulation that provides estimates
of the spatial and temporal salinity distributions under various conditions would be used to arrive at target freshwater
flows. A verified hydrodynamic model of Biscayne Bay that is
forced by observed atmospheric and marine inputs and that
is coupled with a hydrologic model to provide surface water
and groundwater inputs would be such a tool. Though tools
like this are under development, at this date an operational
tool for salinity prediction is not available. Therefore, we used
several alternative approaches, including statistical models,
dynamic box models, and static volumetric estimation based
on analytic estimates of water budgets and the balance of advective/diffusive processes. These methods provide a range of
freshwater flow quantities within the WBZ. The limitations
and advantages of each are discussed and reasonable approximations of freshwater flow quantity are provided.
Much of the available information on flow is based on current canal discharges, which do not mimic natural conditions
either in spatial distribution or in timing. The desired persistent salinity gradient oriented parallel to the coastline can
only be maintained by the steady flow of waters away from
the coast and all along the coastline, as would be provided
by a coastal freshwater/brackish marsh (this phenomenon is
explained more fully in Appendix A). A constant freshwater
flux is likewise desired at the historic river and creek mouths
in order to maintain the estuarine salinity targets and avoid
ecological damage that is similarly caused by cessation of
flows or large pulses of freshwaters. Pulsed discharges of large
volumes of freshwater are typical following large rain events
and often result in locally low salinities near canal discharge
points. The desired spatial and temporal distributions apply
to all of the target flows derived in this section.
RECOVER Target Estimates. There are a number of RECOVER performance measures that apply to specific areas of
Biscayne Bay for which flow or salinity targets have been developed by the Southern Estuaries Sub-team. For the purpose
of estimating target flows for Central to Southern Biscayne
Bay, SE-6 is applicable. SE-6 specifies a persistent salinity gradient parallel to the southern coast of Biscayne Bay at 250 m
and 500 m from shore by meeting oligohaline to mesohaline
nearshore targets, and it was estimated by Meeder et al. (2002)
that about 470 K acre-ft/yr are required to meet these salinity requirements. Alleman (2003) arrived at a similar figure of
325 K acre-ft/yr for SE-6 targets from a historical data analysis, which lends support to the range of this estimate. In addition, SE-8 stipulates persistent flows of 15 K acre-ft/yr out of
Snapper Creek and into Central Bay to maintain the ecosystem found at the creek mouth. Thus the RECOVER total for
these target flows for South Bay and nearby waters is 485 K
acre-ft/yr. The estimate for the 10,000 acre WBZ will be much
greater because it is 5,800 acres larger.
Until the emergence of any applicable results from the
desired hydrological models, which are coupled to a range of
inflow conditions, some alternate performance measures and
targets can be developed that test the utility of and/or supplant
the RECOVER targets. Rough estimates of target flows across
wet and dry seasons may be gleaned from simple calculations
of the flows required to maintain a persistent salinity gradient
parallel to the coastline, and no periods of hypersaline conditions. These are minimal targets but they must be achieved first
in order to reach other, more voluminous, restoration target
flows. The flows needed to achieve these restoration targets