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month
Figure 13. Average monthly flows as estimated from 2x2 Alt7r
(blue). Red bars represent the average monthly flows that comprise the beneficial flow volumes for southern Biscayne Bay's
10000 acres of SAV (month 1 is January).
also that the salt concentration will be an integral measure of
various effects with inherent time scales from days to many
weeks, and length scales from meters to several kilometers.
These effects, as we have already seen, are often difficult to
estimate, span orders of magnitude in size, and are tricky to
model at the fine scales required in Biscayne Bay.
Projected Volumetric Mean Salinities in a 1km-wide Coastal Zone
1986 1988 1990 1992 1994 1996 1998 2000 2002 2004
year
Figure 14. The daily time series of projected salinities in southern Biscayne Bay within a 1 km-wide coastal zone (driven by
measured flows = green, modeled flows Alt7r5e = blue) with
the 10,000 acre seasonal volumetric target (red) and 3,200 acre
target (yellow).
20 South Florida Natural Resources Center Technical Series (2006:1)
DISCUSSION
Prior to the significant changes in the freshwater flow patterns
in south Florida caused by the creation of a water control system in the early 20th century, Biscayne Bay was a true estuarine system. Large amounts of freshwater in the form of both
surface and groundwater were present throughout most of
the year and supported a wide range of flora and fauna. When
these freshwater sources were diminished and their distribution altered by water management practices, the vegetation in
the bay, as well as the juveniles of many fish and invertebrate
species, were adversely affected and the ecosystem in the bay
changed drastically. The ecosystem that exists today in Biscayne Bay is largely marine in nature, as the volume, timing,
and distribution of freshwater flows are insufficient to maintain an estuarine environment over ecologically-significant
temporal and spatial scales. In keeping both with the Everglades restoration efforts and the NPS mandate to preserve
unimpaired the nation’s natural resources within the parks,
this document provides ecological and physical targets for desired conditions in Biscayne National Park, and attempts to
quantify the existing freshwater flows that are necessary for
the protection of fish and wildlife within the park.
The spatial focus of the discussion of ecologic targets includes the Coastal Mangrove Zone (CMZ) of mainland Biscayne National Park in this report and the adjacent Western
Bay Zone (WBZ). Because the CMZ receives no water deliveries currently, the spatial focus for estimation of target flow
deliveries is on the WBZ within Biscayne National Park - the
10,000 acre area along the western shoreline which contains
the portion of the ecosystem that most benefits from freshwater flows. The shallow waters of the WBZ contain thousands
of acres of seagrasses as well as a fringing mangrove forest.
The desired condition, or overarching goal, for the western
zone of Biscayne National Park is the existence of stable estuarine conditions that persist through the dry season, to be
achieved through more natural timing and distribution of
freshwater flows. These stable estuarine conditions support
a productive, diverse benthic community based on seagrass.
These conditions will also support endangered species and
sustain productive nursery habitat for local and regional fishery resources.
The appropriate restoration area to consider was discussed
in this document. The existing RECOVER performance measures focus on a narrow (500 m) strip of coastline that encompasses 3200 acres of park waters. The alternate approach used
here is to focus on existing geomorphological information
to define an area of soft bottom suitable for seagrasses. This
habitat in the WBZ includes roughly 10,000 acres of park area.
This larger region was chosen as the target area for stable estuarine conditions because it is based on bay geomorphology,
a factor that is fundamental to bay ecology.
The ecological targets for the WBZ were based upon an
approach that includes the benthic community, endangered
species, and important fishery resources in the western bay.
Because seagrass is important nursery and growth habitat
for indicator species, a fundamental resource management
and restoration goal is to maximize coverage by SAV beds
at sustainable levels. Under appropriate salinity and water
quality conditions, it is expected that this area will support
excellent SAV growth where sediment and water depth are
appropriate for such growth. One explicit restoration target
is an increase in the vitality and diversity of the WBZ seagrass
community, with wigeon grass as the dominant SAV species
at the mangrove edge within the nearshore ecotone and shoal
grass becoming co-dominant with turtle grass through much
of the rest of the WBZ. Another explicit target is the restoration of the community of seagrass-associated fauna that have
been largely extirpated from South Bay,and the enhancement
of habitat for others, such as crocodiles and pink shrimp
that will likewise benefit substantially from the target salinity
conditions.
These ecological targets require freshwater flows that produce mesohaline conditions throughout most of the year at
the bottom of the bay, with salinities ranging from 5 to 20 ppt
over the soft bottom areas of the WBZ that have the substrate
necessary to sustain SAV. In particular, measured dry season
salinities (November through May) should not exceed 30
ppt anywhere in the zone. The ecological and salinity targets
that link mesohaline conditions and associated seagrass and
faunal communities for this area are not currently being met
because current freshwater deliveries are insufficient in terms
of quantity, timing, and distribution.
Using draft CERP Guidance Memorandum 4 methodology, freshwater flows that either contribute to reaching, or
reach, the above salinity ranges in the WBZ can be considered
“beneficial.” To quantify these beneficial flows, the implicit
flow targets, the existing flows, and the specific restoration
area of concern need to be clearly defined.
Simple volumetric estimates to reach these salinity goals in
the 10,000 acres of the tidally-driven system result in a target
annual flow of 1.1 Macre-ft/yr, with dry and wet season variations. Other types of flow target estimates - diffusive, empirical, semi-empirical - discussed in this document fall within
this range as well. In the absence of any robust hydrological
modeling results which could reduce the range of estimates,
the volumetric estimate will suffice as a flow target for comparison against the existing flows. Future work should focus
on hydrological modeling results that will not only help refine
the volumetric estimates, but also provide information concerning the expected spatial and temporal distribution of the
freshwater flows.
The existing flows are comprised of the managed water
flows through the control structures at the end of the canals
that empty directly in or adjacent to the WBZ; groundwater