Big Muddy River TMDL-Appendix P
Final Report 7 October 2004
For most stations in the lake and for the lake as a whole (considering data from all four
stations), there is no correlation (r2,0.15) between total phosphorus and pH, chlorophyll-a
and pH or total phosphorus and chlorophyll-a. Moderate correlations exist between total
phosphorus and chlorophyll-a at station RNC-3 (r2=0.46) and for the entire lake
(r2=0.37), and between chlorophyll-a and pH at station RNC-1 (r2=0.37). For station
RNC-4, which has the highest total phosphorus concentrations, but no exceedences of the
pH standard, there is no correlation between any of the variables (r2 </=0.10).
Response: The relationships presented in Section 7 were intended to confirm that
there is a relationship between pH and chlorophyll-a in addition to chlorophyll-a
and phosphorus. These general relationships have been established in literature,
which suggest that reducing algae (chlorophyll-a) maintains pH and reducing
phosphorus concentrations reduce algae (chlorophyll-a). Additional data collection
for Kinkaid Lake may establish the same relationships found in literature. All the
r2 values are low, and the figures were intended to show general trends and provide
examples of what has been established through literature. The relationships
established in literature were used as the basis for the focus of this TMDL – control
of phosphorus and thereby control of algae (chlorophyll-a) and pH.
22. We also find the discussion of loading capacity confusing. The section states that
load reductions were modeled for RNC-4 because it violates the phosphorus standard (but
not the pH standard), but then says that the effects of the modeled reductions are only
shown for stations RNC-1 and RNC-3 because they are the only stations that violated the
pH standard (RNC-1 never violates the phosphorus standard and at RNC-3 only one
sample since 1994 has exceeded the standard. At both stations, the mean total
phosphorus concentration is well below the standard).
Response: Phosphorus loadings in sub-basins 2, 3 and 4 were less than sub-basin 1.
Therefore sub-basin 1 and its contributions to station RNC-4 were targeted for
reductions. Phosphorus flows through the lake as well as settles and can cause algae
growth in various parts of the lake. Since the largest loads were generated from
sub-basin 1, it was the focus of the reductions to maintain pH throughout the lake.
23. The relationships provided in Figures 7-1 through 7-4 are weak, at best. At RNC-1,
there is no relationship between total phosphorus and chlorophyll-a and only a moderate
correlation between chlorophyll-a and pH. There is also no direct correlation between
total phosphorus and pH (r2=0.04). At RNC-3, there is only a moderate correlation
between total phosphorus and chlorophyll-a and no correlation between chlorophyll-a and
pH. There is also no direct correlation between total phosphorus and pH (r2=0.0007).
We also note that the predicted mean total phosphorus values after a 40 percent reduction
in phosphorus loadings shown in Table 8-3 exceed the mean of the observed values at
RNC-1 in 1997 and 2000 and at RNC-3 in 2000.
Response: See response to question 21.
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