Minimum Flows for the Tampa Bypass Canal, Tampa - Southwest ...

May 15, 2005 Draft
At the outset of the evaluation, several ecological resources believed to be significant to
McKay Bay/TBC were identified for evaluation. Those included oysters, birds,
vegetation, benthic macroinvertebrate communities, fish and invertebrates. In the
absence of clear breaks in resource/inflow relationships, an a priori goal of less than a
fifteen percent loss of resource was established. In addition, a low flow threshold (LFT)
of 30 cfs corresponding to the lowest 10 percent of baseline flows was initially chosen
for evaluation.
Of the resources chosen for evaluation, only the fish and invertebrate response to inflow
proved useful. The oyster evaluation revealed that even under zero discharge from S-
160, the salinity in McKay Bay/TBC remains within optimal limits for oysters. In
essence, McKay Bay/TBC will assume the salinity of East Bay (which in turn assumes
the salinity of Hillsborough Bay etc) that is non-threatening for oysters.
The dominant vegetative structures in McKay Bay/TBC are mangroves. Contemporary
research into mangrove ecology subscribes to the belief that the salinity in the water
column is relatively unimportant to the health of mangroves. Of far greater importance
is the salinity in the soil, which mangroves have been shown to regulate.
It was determined that birds were also a poor choice of resource related to inflow partly
because of their mobility. Successful connections to freshwater inflow will likely be
through qualitative relationships to vegetation or prey abundance.
Useful quantitative relationships between McKay Bay/TBC benthic infauna and
freshwater inflow have yet to be published, although community gradients related to
salinity have been noted. Circumstantial evidence indicates that hypoxia is a major
factor in shaping the TBC community, but relationships quantitatively linking abundance,
diversity or other community metrics with flow were unavailable at the time of the
evaluation. The McKay Bay/TBC literature describing the role of salinity in shaping the
benthic community is conflicting and inconclusive at this time.
The response of fish and invertebrates to varying inflow proved to be a useful tool for
the MFL evaluation. Coupling the results of over 2,000 sampling episodes with the
discharge from S-160 and estimates of ungaged flow below S-160 resulted in
statistically significant (at α =0.05) relationships for 34 taxa, although flow alone
accounts for only a small fraction of the abundance (median r 2 =0.24).
Three taxa were selected because of their positive response to freshwater and their
significance to the regional ecology (bay anchovy and mysid shrimp) or their
commercial value (pink shrimp). The abundance of each taxon was estimated for the
median baseline flow condition (Flow at S-160 that would have occurred between 1983
and 2003 if no diversions or withdrawals had occurred for water supply reasons) and for
a series of hypothetical reduced flow scenarios. The daily median of the 20-year flow
was evaluated in order to allow for the natural variation in flow, recognizing that there
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Executive Summary
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