The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
species (10 to 13) were collected during periods of no flow and normal flow; while low
numbers of species (5 to 7) were also collected during periods of no flow and normal
flow.
The presence of fishes in Sulphur Springs may be related to flow and water quality
conditions in the Lower Hillsborough River, but such relationships are only speculative.
During periods of no flow from the dam, salinity in the spring run is generally
considerably lower than in the river. This may attract some species to the spring run, but
factors such as the temperature, clarity and dissolved oxygen in the spring run might be
equally as important. Conversely, it could be argued that during high flows, suitable
habitat becomes more available in the lower river, thus not concentrating fish in the
spring run.
It can be concluded that the spring run provides a valuable low salinity refuge for
freshwater species during periods of no flow from the Hillsborough River dam.
Freshwater species, such as largemouth bass and bluegill, are often collected in the low
salinity areas of tidal creeks and rivers where they can feed on the food resources found
there. A review of the effects of salinity on freshwater fishes in the coastal plain of the
southeastern U.S. is provided by Peterson and Meador (1994). Elevated salinity can
influence fish behavior, physiology, growth, or reproduction. However, the salinity
values that are generally reported for these effects are well above the background
salinity that occurs in Sulphur Springs under normal flows, and even during some
periods of salinity incursion from the river. Acknowledging that salinity tolerances vary
considerably between fish families, Peterson and Meador state that many freshwater
species can withstand extended exposures up to 9 ppt salinity, and tolerate brief
exposures at higher values. They furthermore state that some studies suggest that most
freshwater fishes cannot reproduce in salinities greater than 3-4 ppt, but few studies
have addressed this issue.
It is unlikely that salinity incursions in the spring run observed during this study
jeopardized the survivability of the estuarine/marine fish that inhabit the spring run.
However, the prolonged high salinity values (8-16 ppt) that resulted from the large
withdrawals from Sulphur Springs during the 2000-2001 drought likely caused the
spring run to be unsuitable habitat for most freshwater fishes found in the Lower
Hillsborough River. However, salinity incursions of lesser magnitude or duration, such
as occurred during the fall of 2001 and spring 2002 (Figure 3-5), may not have
jeopardized the survivability of these species or possibly their reproduction.
The use of the spring run by fishes will be dependent not only on salinity, but also on
food resources available in the run. Given the findings for invertebrates presented in
Section 3.7, the species composition of the invertebrate community is likely more
sensitive to changes in salinity in the low range than the fish fauna. It is unclear,
however, if such a community shift in the invertebrates would result in less total prey for
fishes. The potential biological effects of salinity incursions into the spring run resulting
from different flow rates from the spring are considered in Chapter 5.
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