Alafia River Minimum Flows and Levels - Southwest Florida Water ...

1) The DISTRICT acknowledges the many problems of impounded estuaries. The
predation noted is the result of physical constraints imposed by a confined channel,
devoid of protective habitat that terminates in a flood control structure. Peebles ( 2004)
reports that a flow of >100 cfs displaces the medusa downstream and away from the
control structure. Such flows are essentially the median baseline discharge (97 cfs) from
S-160 and are well beyond what might be considered as low flow. Peebles and others
have also questioned whether the attractiveness of a freshwater source truncated by a
physical barrier is beneficial or detrimental to estuarine-dependent species. The
management conundrum for impounded flood control projects is whether there is
justifiable benefit to the ecological resources associated with a minimum flow that may
increase predation.
2) Residence times were calculated from the hydrodynamic model and summarized
(see page 3-40) for a range of flows. Additional details can be found in Luther and
Meyers (2005) available from the District. Chlorophyll (as surrogate for phytoplankton)
was not explicitly evaluated because of a prior TBC task force determination that salinity
and dissolved oxygen were the primary water quality parameters critical to setting an
MFL. The relationship between dissolved oxygen and flow was evaluated and reported
(Section 3.8). Surface DO (expressed as percent saturation) was poorly related to flow
and lagged flow terms, suggesting that residence time is not an important factor. For
some segments, flow was not a significant predictor of percent saturation (at p=0.05).
The evaluations were conducted on segments and any relationship between residence
time and chlorophyll would be expected to show up in the DO to flow evaluations.
3) The DISTRICT readily acknowledges that biological data is highly variable and that
organism abundance is the result of many environmental and ecological forcing
functions. The DISTRICT also understands that low correlations are typical for many
types of biological data. However, the MFL statutes pertain to management of flows,
which in the case of the TBC account for approximately 15 percent of significant fish
and invertebrate abundances. Thus, 85 percent of the resource response is unrelated
to the establishment of an MFL. The DISTRICT does not feel it is prudent to establish
an MFL at this time based on those conditions.
TBEP comment on the correlation coefficient issue references a flow of 66 cfs. It should
be noted the evaluation was not based on a single flow, but included a range of flows
and lag flows associated with the fish and invertebrate sampling effort. Over the course
of the 2000-2003 sampling, gaged and ungaged flows ranged from 6 to 4,600 cfs.
(Actual sample date flows ranged from 6 to 870 cfs). The confusion appears to be
rooted in a 15 percent reduction in abundance when the 20-year median flows are
reduced from 92 cfs to 66 cfs.
4) The suggested re-evaluation of abundance based on segmentation cannot be related
to inflows at the same time because the location of the organisms changes with inflow.
Thus, at inflow ‘X’ the organisms might be found in a downstream segment. Increasing
Appendix D ix