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mia9

Dataset card Data Studio Files
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6.44k
9
X16
X12 X13 X14
X11 X9 X10
X4
X8
X7
X6
X2 X3
1 – Miami River (boundary)
2 – North Central Inshore
3 – North Central Outer-bay
4 – South Central Inshore
5 – South Central Mid-bay
6 – South Central Outer-bay
7 – Card Sound
8 – Barnes Sound
9 – Manatee Bay
X1
S20
Schematic diagram of the structure of the Biscayne Bay
nutrient box model showing canal discharges, exchanges
(quantified by parameters X1, X2, …), and net discharge
between the boxes. Salinity boundary conditions for the
Atlantic Ocean SB1 and northeast Florida Bay SB2 are
indicated
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transformation of TP exceeded average measured values by between 17 and 44
percent. Of particular interest, the calculated values of TP concentration reflect the
effect of very large loadings of TP caused by hurricanes Katrina, Rita and Wilma in
2005 and Ernesto in 2006. Average values for NOx-N concentrations without
accounting for losses due to net denitrification greatly exceed average measured
values. Calculated values are between about 1.5 and 4 times higher than measured
values. The disparity between calculated and measured values indicates the degree to
which uptake, removal, and wash-out of nitrogen species may exert a significant
influence over NOx-N concentrations in the Bay. When an average net denitrification
rate of 0.3 month-1 was applied Bay-wide in the DIN calculations, calculated values for
DIN concentrations were equal to or substantially less than observed values in all but
one of the sub-basins. In general this means that the net denitrification rate in most
areas of the Bay is capable of being estimated by the default rate or a lower rate.
The exception to the general agreement between calculated and observed
concentrations of DIN occurs in the South Central Inshore (SCI) sub-basin. Six
hypotheses were developed to possibly explain the large discrepancy between
calculated and measured NOx-N and DIN in the SCI sub-basin:
1. Estimated NOx-N and DIN loads are too high,
2. There is an error in the mass-balance calculations,
3. Uptake and removal of NOx-N and DIN occurs at a higher
denitrification rate in the SCI sub-basin than in the rest of the Bay,
4. Biological uptake or other removal processes in addition to
denitrification are at work in the SCI sub-basin and may be at work
in the southernmost sub-basins,
5. Tidal circulation patterns remove some of the NOX-N and DIN to
adjacent boxes,
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6. Measured values of NOx-N and DIN concentration are not
representative of average conditions through the whole SCI subbasin.
Of these hypotheses, the first two hypotheses are less likely to be true, because
the same model calculations and application of monitoring data are used for salinity and
TP without apparent problems. It is unlikely that the third hypothesis is true because the
denitrification rate needed to reduce the calculated DIN concentrations in SCI to the
observed values is much higher than has been documented in the literature that was
reviewed. The sixth hypothesis cannot be tested without a more frequent monitoring
interval in the Bay; these data are not available.
Tidal circulation (hypothesis five) may be an important removal process in the
Bay and acts at a higher temporal resolution than the temporal resolution of the box
model. Examination of tidal circulation plots from a numeric model indicates that tidallygenerated currents may exist that transport waters to both the south and to the north
depending on the point in time in the tidal cycle. This may mean that the high nitrogen
canal discharges may be flushed out of the parts of the Bay that are influenced by the
tide on a sub-daily basis, thereby not affecting the long-term average DIN concentration
that is measured in a monthly grab sample.
Biological uptake of NOx-N and DIN (hypothesis four) may also be an important
factor in the SCI sub-basin. Recent research indicates that the ecosystem in the SCI
sub-basin may be supporting a high rate of uptake and removal of nitrogen from the
water column. A combination of hypotheses four (biological uptake) and five (tidal
circulation) may also be a plausible explanation.
Lastly, the load estimates were used to simulate the effects of large-scale land
use changes from agricultural to urban land uses in the SCI sub-basin. The NCI subbasin is already at urban build-out, and the areas of the NCI and SCI contributing
drainage basins are somewhat similar. At urban build-out, assuming that the SCI
drainage basin will be contributing the same area-based TP loads as the current NCI TP
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loads, the build-out TP loads to the SCI sub-basin will be about 75% of the current SCI
TP loads, about 300 kg/mo. However, the canal TP loads do not appear to be
controlling the water column TP in the NCO, SCM, and SCO sub-basins; instead the
Atlantic Ocean TP load may be the determining factor. In Card Sound and Barnes
Sound the atmospheric TP load and limited oceanic influence will likely determine water
column TP values. In Manatee Bay relatively large TP loads from S-197 discharges will
affect TP (and nitrogen) concentrations.
For NOx-N and DIN, the urban build-out conditions of the NCI drainage basin can
also be used to estimate future loads at urban build-out of south Miami-Dade County. If
this is the case the future NOx-N and DIN loads to the SCI sub-basin will be about 2700
and 4500 kg/mo, respectively. This is about 3% of the current NOx-N SCI loads and
about 5% of the current SCI DIN loads. There will likely be little or no apparent impacts
on nutrient loads to NCI (already at build-out) or to Card Sound, Barnes Sound, and
Manatee Bay from future land use changes in south Miami-Dade County unless S-197
discharges are reduced, which will reduce loads to Manatee Bay and Barnes Sound.
Additionally, because there is little, if any, undisturbed private land in south Miami-Dade
County that remains in a native or otherwise natural state and there are no plans to
purchase developable lands and return them to a protected natural state, little benefit
was seen in evaluating a natural landscape alternative in this metropolitan area. Other
ongoing changes in the diverse mix of land uses present in the watershed as well as the