The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
3.11 Thermal characteristics of the spring run and the lower river and use by cold
sensitive species
The list of estuarine species that utilize the lower Hillsborough River is linked to the
populations that inhabit Tampa Bay. Although it is classified as a sub-tropcial estuary,
Tampa Bay is in the northern limits of the geographic range for a number species that
occur on the west coast of Florida. For example, hard freezes can kill or damage
mangroves in upper regions of the bay (Lewis and Estevez 1988) and cause mortality of
a number of cold-sensitive fishes, including the prized gamefish snook (Gilmore et al.
1978; Shafland and Foote, 1983).
By providing flow of water that is largely isothermal year round, Sulphur Springs
provides a thermal refuge in the spring run for cold-sensitive species during winter
fronts. The spring also mediates to some extent the effect on cold fronts on the water
temperatures in the lower Hillsborough River. This is particularly important for the use of
the river by the Florida manatee (Trichechus manatus latirostris), a marine mammal that
is currently on the United States Department of the Interior endangered species list.
During summer months, manatees migrate freely between rivers, bays and other
coastal waters on both Florida coasts, and have been reported as far north as Virginia,
with frequent sitings in Georgia and North Carolina. During winter months the United
States manatee population is largely restricted to peninsular Florida. During the coldest
part of winter, manatees in coastal Florida waters seek areas of warmer water
temperatures, such as near the discharges from artesian springs or electrical power
generation plants. Manatees begin to seek thermal refuge when water temperatures fall
below 20 o C., and are usually not present in an area when water temperatures fall below
15 o C. Manatees that are unable to leave an area with water temperatures of less than
10 o C will likely die within two days (B. Ackerman, personnel communication).
Manatees are periodically observed in the Lower Hillsborough River, especially in or
near the Sulphur Springs Run. Data on use of the river/spring system are available from
the Marine Mammals Program of the Florida Fish and Wildlife Conservation
Commission (FWC). Information gathered from interviews with FMRI staff (B.
Ackerman, personal communication) and data from the FWC photo-identification effort
are summarized below.
Manatee use of lower river and spring run was documented as part of a six-year radiotelemetry
project of manatees that over-wintered at the Apollo Beach power plant on
Hillsborough Bay (FWC written communications). In 1991, forty-seven manatees were
tagged with radio transmitters following their capture in Tampa Bay. The project was
continued until February 1997 when the last radio transmitter was removed. Seven of
the tagged animals, including five males and two females, utilized the Hillsborough
River during the time they were tagged. Only one of the tagged animals was located in
the Hillsborough River during the winter. The remaining animals occurred in the river
during non-winter months. The use of the river is likely related to access to fresh water,
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