The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Lastly, the relationship between the indicator and flow should be quantifiable, so that
change in the indicator can be expressed and used in determining the minimum flow
rates.
The District determined there are three management goals important to the
establishment minimum flows for Sulphur Springs, which are: (1) minimize the incursion
of high salinity water from the river into the upper spring run; (2) maintain low salinity
habitats in the Lower Hillsborough River; and (3) provide a thermal refuge for manatees
in the Lower Hillsborough River. The rationale and ecological indicators corresponding
to for each of these goals are presented below. This discussion is based on the
information presented in Chapter 3 concerning the relationships of flow from Sulphur
Springs to the ecology of the spring run and Lower Hillsborough River.
4.5 Goal 1 – Minimize the incursion of high salinity river water into the upper
spring run.
As discussed in Section 3.4, salinity data indicate that waters from the Lower
Hillsborough River do not move into the upper spring run under normal flow conditions
(no withdrawals). However, as flows are reduced by withdrawals, relatively high salinity
water (up to 17 ppt) from the river can move into the upper spring run. Data collected by
the FDEP and the FWC indicate that high salinity values that occurred in the upper
spring run during the drought of 2000-2001 had a major impact on the
macroinvertebrate community there, as a number of freshwater species were lost from
the spring run. With the resumption of normal flows in late 2001, 2002 and 2003, a
number of these species reappeared in the spring run, accompanied by increases in
overall species richness, evenness, and diversity. A number of salt-tolerant species that
persisted in the spring run during the drought also showed marked increases in
abundance with the resumption of normal flows. Many of the species that rebounded
with normal flows are important fish food organisms (isopods, amphipods, aquatic
insects). Based on these findings, a goal of the District's minimum flow evaluation for
Sulphur Springs is to minimize the incursion of high salinity river water into the upper
spring run. This goal should provide suitable conditions for many of the freshwater taxa
that have been found in the spring run, and maintain the high species richness and
diversity that was observed during periods of normal flow.
When there are no incursions of river water, salinity values in the upper spring run
typically vary between 1 and 3 ppt on a seasonal basis. Given this salinity range, the
upper spring run would not be classified as fresh water (