Monitoring and Mapping Salinity Patterns in Estero Bay, Southwestern Florida
Michael J. Byrne
U.S. Geological Survey, Fort Myers, FL, USA
Estero Bay is a very long and shallow estuary located about 3 miles south of the
Caloosahatchee River along the southwestern coast of Florida. Abundant and diverse flora and fauna exist within the bay and its watershed including threatened
and endangered species. Historically, sheet flow from several sloughs drained into
Estero Bay. Most flow now is concentrated at several inflow points as a result of
rapid development within the watershed. The principal inflows come from Hendry Creek, Mullock Creek, Estero River, Spring Creek, and Imperial River (fig.
1).
Figure 1. The Estero Bay watershed.
Water exchange with the Gulf of Mexico is restricted by a series of barrier islands, with most exchange through four passes; Matanzas Pass, Big Carlos Pass,
New Pass, and Big Hickory Pass. Monitoring and mapping salinity in Estero Bay
will help determine the effects that altered flows have had on the aquatic health of
the bay.
Baseline data for Estero Bay and its tributaries are needed to provide regional resource managers information to manage future development around the bay and
its watershed. The U.S. Geological Survey has undertaken a study to map salinity
patterns and determine freshwater residence times in Estero Bay. Salinity is the
chemical tracer that will be used to map water movement through Estero Bay and
the information generated from this project will be used in the development of
hydrodynamic models and statistical analysis of salinity variance the bay.
The study began in April 2001 and will end in September 2004. Monthly surfacewater salinity and temperature data are presently being collected throughout Estero Bay. The methods of data collection involve attaching a water-quality sensor
to the stern of a boat just beneath the water surface and logging the precise location of data points using a global positioning system (GPS) receiver. Two boats
are used to collect about 6,000 individual data points are in the estuary over an 8hour period. These data are used to generate the surface-water salinity maps and
for statistical analysis of salinity variability due to stormwater releases through
the Caloosahatchee River.
Salinity results reveal a large volume of water from the Caloosahatchee River
moves a short distance through the Gulf of Mexico and then enters Estero Bay
through Matanzas Pass. This organic-rich water may reduce light penetration and
salinity and increase sedimentation in the northwestern part of the bay. Water entering the northeastern part of the bay travels south and mixes with water from the
Estero River before flowing through Big Carlos Pass. In the south-central bay,
hydrologic exchange is limited and salinities increase. Salinity data in southern
Estero Bay suggest the Imperial River discharges to the south and north in similar
volumes. The Imperial River discharge leaves Estero Bay through Big Hickory
Pass and New Pass.
The salinity maps can be used to assess trends in saltwater movement within the
estuary and improve an understanding of manatee and fish migration. The information from these maps also can be used for hydrodynamic model verification
and to help map seagrass beds.
Michael J. Byrne, U.S. Geological Survey, 3745 Broadway, Suite 301, Fort Myers, FL, 33901, Phone: 239-275-8448, Fax: 239-275-6820, mbyrne@usgs.gov,
Hydrology and Hydrological Modeling