Establishing baseline freshwater flow to Florida Bay for water management
Robin Bennett, Chelsea Donovan, David Rudnick, and Joel VanArman
South Florida Water Management District, West Palm Beach, FL
Two assessments of the relationship between freshwater flow through the
Everglades and the ecological status of Florida Bay are currently ongoing at the
South Florida Water Management District (SFWMD). The first is a component
of the Comprehensive Everglades Restoration Plan (CERP) – the definition of
“Existing Legal Sources of water”. This assessment establishes the baseline (prerestoration, up to December 2000) input (timing and distribution) of water to
Florida Bay and the extent to which this baseline delivery is beneficial to the
ecosystem. The second assessment is derived from Florida legislation to define
“minimum flows and levels” (MFLs) for “priority” water bodies and wetlands to
ensure that water supply is sufficient to prevent “significant harm” to these
systems.
For both assessments, it is necessary to establish relationships among water
management discharges, watershed hydrologic conditions, freshwater flow into
Florida Bay, salinity conditions in the Bay, and responses of aquatic resources
(species of special interest and habitats). A simplifying assumption has been
made that salinity levels and variations are the primary environmental parameter
in Florida Bay that varies with freshwater flow and levels and also is a primary
driver of biotic condition. Furthermore, for both assessments, it is desirable to
accurately estimate the effect of changing watershed hydrologic conditions on
salinity far from the coast. Since a validated hydrodynamic model is not yet
available for Florida Bay, we employed a statistical approach to estimate
hydrologic-salinity relationships. This analysis was limited to assess salinity
conditions near the northern coast of Florida Bay and within Whitewater Bay.
For the Legal Water Sources assessment, we have taken the following approach:
(1) determine relationships among upstream flow, stage, and Bay salinity regime
(mean, range, frequency of a given magnitude); (2) identify key biological
components that have demonstrated salinity tolerances or optima; and (3) describe
the distribution, abundance, and other life-history attributes for biological
components as a function of salinity regime for various areas of Florida Bay and
Whitewater Bay. We developed regression models to evaluate flow-stage-salinity
relationships. Salinity data were analyzed from eleven of Everglades National
Park’s (ENP) Marine Monitoring Network (MMN) stations. These near-shore
stations are located in close proximity to freshwater outflows from the
Everglades, and thus, are likely to be influenced by upstream water management.
These salinity data were compared against stage data for 5 upstream wetland
stations (P33, P37, EPS, TSB, and CP). Both stage and salinity were examined
against flow data from canals discharging into the Shark Slough, Taylor Slough,
and lower C-111 basins. Stage and flow were calculated for a variety of temporal
periods (individual day, 30d moving average, and 30d moving average with 30,
45, and 60d lags). We also verified regression models by comparing predicted
daily salinity with salinity measured by Florida International University (FIU) for
the corresponding day (FIU data was independent from the MMN input data). In
order to comply with a pre-CERP baseline definition, data from 2001-2002 were
not included in our analyses.
Assessment of the salinity effects on Florida Bay biota was based on literature
and unpublished information on the salinity tolerances and optima for several
taxa. We selected a short list of species representing a range of trophic levels and
habitat preferences (widgeongrass, shoalgrass, pink shrimp, spotted seatrout,
American crocodile, roseate spoonbill). We examined habitat suitability for each
species as a function of salinity regime at each site based on time-series plots and
frequency distributions of salinity. Biotic response to salinity was also estimated
during various climatic events that represented extremes for flows into the Bay
(drought of 1989-1991, high water period of 1994-1995, and high variability of
1999). For each species, the effects of temporal variation of salinity during
critical life cycle periods (e.g., spawning, nesting, and success of
hatchlings/juveniles) were considered.
Salinity in Florida Bay and Whitewater Bay generally showed strongest
correlation with upstream wetland stage, calculated as a 30d moving average
value. Regression models (all p<0.001) were able to account for between 50-75%
of the variability between stage and salinity, and from 41-81% of the relationship
between upstream stage and flow from canal structures (flow relationship was
also strongest when calculated as 30d moving average). Salinity from most
eastern sites was best predicted by P37 stage, while western sites showed
strongest correlation with stage at station P35. Salinity predictions for
independent FIU data were strongest at sites in eastern FL Bay; mean difference
was generally within the standard error of the FIU site-respective mean. Lowest
R2 values were calculated for north-central Florida Bay regression models.
Relationships between flow and salinity were weaker than between stage and
salinity.
Salinities occurring during the 1989-1991 drought were frequently outside the
optimal ranges for the species we examined. Moreover, the unusually low
salinities that occurred during the high water period of 1994-1995 provided a
more beneficial habitat for crocodiles and spoonbill forage fish. These lower
salinities also coincided with the eastward expansion of juvenile seatrout
populations (Thayer et al. 1999). Furthermore, the salinities occurring in Little
Madeira Bay during this high water period were close to the historical levels
calculated by Bjork and Powell (1994). This suggests that, at least for the Taylor
Slough system, such inflows could be beneficial for restoration. For the Legal
Water Sources assessment, we tentatively conclude that baseline flows of
freshwater to Florida Bay are beneficial to the ecosystem, especially if
improvements can be made to the spatial distribution of these existing flows (in
favor of the Taylor Slough basin).
The process of setting a MFL rule for Florida Bay is another important vehicle for
resource protection and water supply planning. Florida Bay is designated as a
MFL Priority Water Body and the MFL rule is due for adoption in 2005. As has
been used for other SFWMD estuaries, a minimum flow (versus minimum level)
will be the target for Florida Bay. A minimum flow is defined by Ch.373.042(1)
F.S. as “the limit at which further withdrawals would be significantly harmful to
the water resources or ecology of the area.” The definition of this point requires
several steps, including (1) identifying resources and functions in the Bay, (2)
selecting appropriate sensitive resources and functions, (3) determining technical
relationships between inflow and resource functions, and (4) defining numeric
criteria that reflect the threshold at which these resource functions have incurred
“significant harm.” Significant harm is defined in Chapter 40E-8 F.S. as the
degree of impact requiring more than two years for the water (or biological)
resource to recover.
The development of MFL criteria for Florida Bay is underway and will be
assisted by other ongoing efforts, including the assessment of Existing Legal
Sources, development of a hydrodynamic model for the Florida Bay and Florida
Keys Feasibility Study, and the definition of restoration targets based on this
Study and other CERP projects.
The definition of Existing Legal Sources of water and the development of MFL
criteria should not be viewed as end points in defining freshwater needs for
Florida Bay. They are critical steps needed to protect existing resources from
significant harm and establish baseline conditions, and thus are important
processes towards a goal of restoration.
Bennett, Robin, Florida Bay and Lower West Coast Division, South Florida
Water Management District, 3301 Gun Club Road, West Palm Beach, FL, 33406,
Phone: 561-753-2400 x4612, Fax: 561-791-4077, rbennet@sfwmd.gov, General