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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
political and regulatory activities also affect the ability to infer current and future nutrient
loads simply from land use.
The mass-balance calculations provide an incomplete description of the
processes contributing to the variation of nutrient concentrations in Biscay Bay, and one
must keep in mind the particular capabilities and limitations of this modeling approach in
interpreting the results. First, the mass-balance model represents the long-term
balance between nutrient loads from external sources and nutrient removal by
advection and exchange of surface water. Therefore, the nutrient concentrations
calculated by the model are comparable only to long-term average concentrations in the
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water column and they are not predictive of short-term variations around the mean
concentrations. Second, with the exception of a net rate of denitrification in the
calculation of dissolved inorganic nitrogen (DIN) concentrations, internal fluxes due to
the uptake of nutrients by biota, excretion by organisms, and exchange with benthic
sediments are not included in the model calculations. These internal fluxes of nutrients
may affect the variation of nutrients in the water column in response to short-term
events, such as runoff from storm events, and seasonally. By excluding internal
processes of nutrient uptake and transformation, it is assumed that they make a
negligible contribution to balancing nutrient inputs from external sources over the longterm. Third, mass-balance calculations for nitrogen do not account for all nitrogen
species known to be present in the water column. Calculations are applied only to NOxN (NO2 plus NO3) and DIN (the sum of NOx-N and NHx-N) concentrations.
Even with these limitations, this study showed that the mass-balance nutrient
calculations work well for an evaluation of estimated nutrient loads from the watershed
by comparing the long-term average concentrations calculated based on these loads to
the long-term average of concentrations measured in the Bay. The average
concentrations calculated for total phosphorous and dissolved inorganic nitrogen in the
Bay are generally within 25 percent of the average of measured concentrations for the
recent period July 1997 through June 2007. This lends confidence that the nutrient
loads developed in this study are reasonable estimates of the actual long-term average
nutrient loads to Biscayne Bay under current conditions, and that the box model may be
a useful tool for investigating the fate and transport of nutrients in the Bay.
In conclusion, the mass-balance approach to linking nutrient loads to Biscayne
Bay with the resulting water column concentrations has proven to be useful as a check
on the magnitude of the nutrient loads estimated from available data as well as to
provide information on the fate and transport of nutrient loads. The mass-balance