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and characterizes the approach as a "natural flow regime principle" which
assumes that an altered hydrologic regime (i.e., the MFL) "...is still near-natural
in terms of magnitude, frequency, duration and timing of freshwater inflows..."
This approach is not likely to work in Biscayne Bay due to the extreme
Potential Alternative Approaches for MFL Development for Biscayne Bay
Freshwater Flow and Ecological Relationships in Biscayne Bay 5-13
modifications to historical flows (Bellmund et al., 1999, Browder and Wanless
2001, Wanless and Browder 2001, Browder et al. 2001, Serafy et al. 2001).
ECOLOGICAL PRESERVATION
Under circumstances where some anthropogenic alterations to the watershed
and estuary have occurred, but the extent of change has not caused significant
harm, it is possible to manage freshwater inflows by carefully establishing the
existing abiotic and biotic conditions for VEC's and indicator species, and plan
for preserving those indicators in population sizes similar to that which currently
exists by designing a percent-of-flow withdrawal plan. This is similar to what is
currently underway for the Alafia, Little Manatee and Manatee Rivers that flow
to Tampa Bay (Flannery et al. 2002) where commercial and recreationally
important fish species, and their food sources and habitats, are targets for
preservation. Again, the significant modifications to the watershed and the
estuary of Biscayne Bay, and the resulting changes in VECs and indicator species,
do not appear to show promise for successful use of this method (Serafy et al
2001).
REQUIREMENTS FOR PREFERRED FISH COMMUNITIES
Valued fish communities in Biscayne Bay include the snapper-grunt complex
that is most often caught by recreational fisherman, and the “highly prized
gamefishes, such as bonefish, tarpon and snook” (Ault et al. 2001). This former
group of fish is currently over-fished in the Bay, and the latter group is rarely
caught, as is the spotted seatrout which was historically a popular gamefish, but
is now almost non-existent in fisherman's creels. The redfish is simply nonexistent, and nine years of stocking efforts have failed to bring it back (Serafy et
al. 2001).
All of this points to extreme pressure on existing fish populations, and the
reduction or disappearance of some of the historically prized gamefish likely due
to reductions in both the static and dynamic estuarine fish nursery habitat
essential to most of these species (Lewis et al. 1985). Thus these types of habitats
need to be restored in Biscayne Bay, following plans similar to those prepared by
Meeder et al. (2001, 2002). Restoration of tidal creeks and their appropriate tidal
flows was a popular recommendation among interviewees for this project but is
not the subject of this particular effort, but that of CERP, RECOVER, and the
Biscayne Bay Coastal Wetlands project.
Potential Alternative Approaches for MFL Development for Biscayne Bay
Freshwater Flow and Ecological Relationships in Biscayne Bay 5-14
COMMUNITY INDEX
Various indices, usually combining abundance, density and uniqueness of
species composition to create a mathematical formula to describe the "health" or
biotic index of an ecosystem have been proposed (see review in Engle 2000). The
biotic index necessarily integrates multiparameter effects on a particular suite of
species.
Graves et al. (2003) (Estuarine Indicators Workshop, Sanibel) presented the
results of their study of the macrobenthos and water quality along two transects
extending seaward from the shoreline at C-103 and C-2 into Biscayne Bay. The
results were indicative of multi-parameter affects, with changes in the inshore
macrobenthos at the C-103 site, as compared to the C-3 site in spite of very
similar canal discharge rates and presumed salinities. The cause of the
differences was determined to be a ten-fold increase in nitrate nitrogen in the
discharge water from the canal at C-103. The point being, that trying to use a
mixed suite of macrobenthos as an indicator of salinity differences or changes
would not be a good idea, at least in this area of Biscayne Bay. Use of a biotic
index to manage freshwater flows into Biscayne Bay, therefore, does not appear
to be useful at this time.
FOOD WEB SUPPORT
Carbon and nutrient flows can be measured and modeled for estuarine
ecosystems. If a particular source of carbon or nutrients can be linked to the
health of the estuary, or support of a particular VEC or indicator species, and
also tied to freshwater flows, it might be useful to manage freshwater flows. For
Biscayne Bay, however, the level of modeling is at the conceptual stage (Lirman
et al. 2002, Browder et al. 2003) and genuinely identifying where food web
support for Biscayne Bay is controlled by freshwater inflows is still only at the
conceptual stage except for the work of Lorenz (1999, 2000) and Lorenz et al.
(2002) for the food web of Southern Biscayne Bay. Thus this approach does not
appear useful at this time for any of the sub-areas except this one.
SOIL CHARACTERISTICS)
Soil conditions have been used by the St. John River Water Management District
as the primary indicator for establishing Minimum Flows and Levels rules for
many water bodies that are located within their region. Soils that are inundated
and/or saturated for long periods of time develop various characteristics that are
so predictable and measurable that their presence and/or absence is used as an
indicator in identifying wetland boundaries. Because soils are comparatively
slow to respond to changes in water levels, soil conditions are used for wetland
Potential Alternative Approaches for MFL Development for Biscayne Bay
Freshwater Flow and Ecological Relationships in Biscayne Bay 5-15
delineation primarily when there have been changes in vegetative cover or
hydrology.
In the context of MFL rule-making, soils have been used as indicators primarily
in fresh water lakes and ponds, where changes in soil characteristics at a specific
location could be an indication that levels have decreased to the extent that the
existing floral and faunal communities would be subject to harm. The use of
soils as an indicator in estuarine habitats would only be potentially useful in the
transition zone between open-water areas and uplands. Within this gradient,
measurements of soil salinity and/or depths to water during the dry season
could be helpful in ensuring that reduced levels and flows do not cause harm to
existing biota. Because the majority of the wetland/upland transition zone along
the shoreline within the Biscayne Bay project area has been replaced with
seawalls and bulkheads, the use of soil conditions as an indicator of estuarine
conditions would be potentially useful only along the southern portions of the
Bay. Even in these regions, the measurements would need to be taken in areas
west of the mangrove shoreline. Some of these areas are already being
considered for restoration through the Biscayne Bay Coastal Wetlands project.
The major potential drawback to using soils as an MFL indicator is the
comparatively slow response time; by the time there are actual measurable
changes in soil characteristics, the harm to previously-existing flora and fauna