Vegetation community changes along the Loxahatchee River in southeastern Florida:
examination along a salinity gradient
John G. Zahina
South Florida Water Management District, West Palm Beach, Florida
The Loxahatchee River Watershed, located in northeastern Palm Beach County was
historically connected as part of the Greater Everglades Watershed. Subsequent
construction of canals and highways created hydrologic barriers between the Loxahatchee
River headwaters and the Everglades Water Conservation Areas. Restoration efforts are
presently looking at options that will reestablish hydrologic connections across these
barriers to provide means to transfer water from Lake Okeechobee to the Loxahatchee
River. In recognition of the historical connection of the Loxahatchee River drainage to
the Everglades, the boundaries of the North Palm Beach Comprehensive Everglades
Restoration Project have recently been extended to include this critical watershed as part
of the greater Everglades restoration effort.
The Loxahatchee River is located in southeastern Florida and flows into the Atlantic
Ocean through the Jupiter Inlet (Figure 1). The Loxahatchee is regarded as the last free
flowing river in southeast Florida and 7.5 miles of the Northwest Fork were designated as
Florida’s first Wild and Scenic River in 1985. In addition, different portions of the river
and estuary are also designated as an Aquatic Preserve, Outstanding Florida Waters, and
a state park. Downstream segments of the Northwest Fork floodplain contain a dense red
mangrove forest, while the upper segment contains one of the last vestiges of native
cypress river-swamp within southeast Florida. Over the past century, downstream
floodplain wetlands that were once dominated by swamp hardwoods and bald cypress
have changed to mangrove-dominated swamp. This change in vegetation is believed to
have occurred because of salinity intrusion into freshwater areas of the river, caused
primarily by alteration of the watershed and river by human activities.
Protection of the remaining freshwater floodplain community and potential restoration
efforts require that sufficient freshwater conditions be provided. In order to understand
the impact of salinity on the distribution of freshwater floodplain swamp vegetation, a
relationship between salinity conditions (magnitude, duration and frequency of exposure)
and distribution of “indicator species” was examined. The current distribution of
floodplain vegetation was assessed by conducting field surveys along the Northwest Fork
from 2000-2002 (see Figure 2 for location of survey sites) and long-term salinity
conditions for some of these survey sites were calculated using a salinity model and field
data.
Field vegetation surveys included both semi-quantitative and quantitative methods to
characterize plant community composition and number of species, as well as the height,
stem diameter, and canopy diameter for dominant canopy species. A recording of the
number of adults, stump sprouts, saplings and seedlings provided a measure of the
reproductive success of key species at each site. The results of these surveys were highly
correlated to distance from the Jupiter Inlet, the primary source of salinity in the
Northwest Fork. Examination of these trends was useful as a means to describe relative
salinity tolerances among a wide range of species in the absence of such information in
the scientific literature. These data were also used to determine the point where the
freshwater floodplain swamp was impaired (i.e. reproduction absent or reduced, canopy
area reduced, etc.) and which species may be good indicators of salinity stress.
Long-term salinity sampling data were not available for all vegetation sites surveyed. In
order to estimate a long-term salinity time series, a hydrodynamic/salinity model
(utilizing RMA-2 and RMA-4 programs, and flow-salinity relationships for the
Northwest Fork) was used to simulate salinity conditions at several upstream river sites
for a 30-year period of record using existing measured flow data. Continuously sampled
field data (1997-2000) from several of the vegetation survey sites were used to calibrate
and verify the model. Model output was also compared to previous field data and
regression models of studies developed for the Northwest Fork. Output from the model
was used to calculate a unique value, Ds/Db, for each vegetation monitoring site. The
Ds/Db value is obtained by dividing the duration of a salinity event (Ds) by the time
duration between salinity events (Db) at a site, using a threshold value to define a salinity
event. The Ds/Db value allowed three discrete salinity factors relative to salinity
exposure (magnitude, duration, and frequency of salinity exposure) to be represented by a
single value.
Use of this method allowed the determination of relative tolerance of different freshwater
swamp species to salinity exposure and provided a tool to correlate salinity exposure to
floodplain swamp species composition. Application of this method may be useful in
other freshwater-saltwater transitional areas that are currently being considered for
restoration or enhancement where little salinity tolerance data for species of interest is
available.
Figure 1. Location of the Loxahatchee River along the SE Coast of Florida.
Figure 2. Location of Vegetation Survey Sites along the Northwest Fork of the
Loxahatchee River
Zahina, John G. Water Supply Department Mail Stop 4350, South Florida Water
Management District, West Palm Beach, FL, 33406, 561-682-2824, jzahina@sfwmd.gov