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are beneficial for fish and wildlife and define conditions that
cause impairment of natural resources. The daily mean salinity for the CMZ should fall within the 0-5 ppt in the late wet
season (October and November) and average less than 20 ppt
annually. A natural gradient of salinity should exist between
Ecological and Hydrologic Targets for Western Biscayne National Park 11
the mangrove zone and the eastern edge of the WBZ throughout most of the year. Although the extent and character of
the gradient is influenced by a variety of factors including
the rate of freshwater arrival, evaporation, and tidal mixing,
water management operations should not result in any unnatural extremes in salinity and should approximate natural
inflows. The WBZ should have salinities between 5 and 20 ppt
throughout the year, allowing for only short-term excursions
(10 days or less) to higher salinities (not exceeding 30 ppt)
as a result of water management. Longer hypersaline events
because of intercepted coastal inflows must be avoided, and
sustained periods of relatively low salinity are considered essential for sustaining key estuarine species. Unnatural pulse
discharges of freshwater that cause rapid and significant decreases in salinity that damage benthic habitats must also be
avoided.
Summary of Ecological Targets
The productivity and richness of the estuarine communities of
Biscayne National Park have significantly diminished, as have
those throughout Biscayne Bay, as a result of channel creation
and the diversion of water from the natural systems in south
Florida. The quantity of freshwater, the seasonal timing of
inflows, and the distribution along the coast have been significantly altered, profoundly affecting the historic estuarine
nature of the western portion of the bay. The alteration of the
hydrology of south Florida has resulted in the near complete
loss of estuarine habitats from the bay, including Biscayne National Park, diminishing the ecological and economic values
of this portion of the greater Everglades ecosystem.
Additional water losses that may result from future demands for water supply and flood control will exacerbate
existing impacts. Freshwater inflows during the dry season
months are particularly important for protection of fish and
wildlife resources, when local rainfall no longer compensates
for evaporation in moderating salinity. Not only has the system
become more marine in salinity, but the increased frequency
and duration of hypersaline events adversely impacts even
marine biota, most of which have very narrow salinity and
temperature tolerances.
At present, the water that is released to Biscayne National
Park provides marginal estuarine benefits, far short of those
that would result from a more natural, historic hydropattern.
The timing of current water releases differs dramatically from
a natural pattern, with large freshwater pulses during the wet
season that drive salinities too low, and minimal flows in the
dry season that allow salinities to greatly exceed the pre-drainage environment. The location of current water deliveries
also differs from a natural pattern, in that the CMZ no longer
receives any substantial overland flow, and water inputs to
the WBZ arrive as point discharge from canals. In short, the
quantity, timing, and delivery of water arriving at Biscayne
National Park is not adequate to provide stable estuarine
conditions. Therefore, from a biological perspective, all the
water that is currently provided is necessary for Biscayne National Park and the bay, and as such, is required for protection
of fish and wildlife in Biscayne National Park. Furthermore,
the data clearly indicate that substantially increased flows of
freshwater will be required to begin the process of ecosystem
recovery for the CMZ and WBZ.
12 South Florida Naturai Resources Center Technical Series (2006:1)
HYDROLOGIC TARGETS FOR BISCAYNE
NATIONAL PARK
For the purposes of monitoring and maintaining a physical
environment that will support the SAV-dependent ecological
targets discussed above, it is necessary to develop metrics that
represent that physical environment.
Two pragmatic metrics exist for the physical conditions
needed to reach the target ecological conditions for Biscayne
National Park: 1) measurement of salinities in the estuarine
zone and 2) quantification of the flows themselves through
the coastal structures.
Though quantification of flows is easily attained, how
these flows influence the salinity distributions throughout
the WBZ is a complex physical question that depends on
currents, winds, vertical and horizontal shear, insolation, and
mixing rates, among other variables. The coastal flows to Biscayne Bay are largely managed and are a calculated parameter
in current water management planning tools. We explored the
link between these managed freshwater flows and the salinity
in the WBZ using a variety of estimations.
Salinity
In several ways, salinity is the best metric to use as a base for
the calculation of flows needed to produce the target ecological conditions. Evidence of the requirements of a number of
species presented in Section 2 demonstrates that salinity is a
key habitat factor for the bay ecosystem.
Figure 5 summarizes the optimal salinity ranges for Biscayne National Park ecosystem indicators, including primary
producers, primary consumers, and predators. The majority
of these indicator species prefer salinities between 5 and 20
ppt. Based on this observation and taking into account that
other species (such as seatrout and oysters) may require periods of time with slightly higher or lower salinities, we propose
the following salinity targets for the CMZ and WBZ of Biscayne National Park.
* At no time should measured salinities exceed 30 ppt. This
will be particularly critical to measure in the dry season,
from November to March.
• Direct benefit to important fishery species with a life
cycle stage that is well-suited to estuarine salinities,
fish that serve as sport fish forage, post-larval juvenile
shrimp, and oysters that rely on brackish water as a
refuge from marine predators.
• Indirect benefit to recreational and commercial fish
species that rely on the forage base produced by estuarine conditions, such as adult sea trout, snapper,
and grouper fish stocks.
A From March through August (late dry season - early wet
season), average monthly salinities should range between
15-25 ppt in the WBZ.