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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
approach is simple to implement compared to the more costly alternative of developing
a coupled hydrodynamic/water quality model, and the simpler approach lends itself to
the analysis of uncertainty and alternatives by sensitivity analysis. However, the down-
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side of the simpler approach is that the mass-balance calculations are not able to
predict the short-term response of nutrient concentrations in the Bay to changes in
loads. As a result the model calculations may not account for the effect of higher
frequency (relatively) processes that may drive nutrient fluxes to and from the water
column within the Bay itself. To a large extent, these limitations are imposed by the data
that are available to calibrate and verify water quality calculations no matter what model
is used. Even so the model was capable of being used in a sensitivity analysis mode to
provide a rough-order-of-magnitude estimate of future nutrient loads to Biscayne Bay
and the resulting water quality in the Bay.
Feedback received at a workshop convened to review the results of this project
recommended that further work be undertaken to understand better and refine the
mass-balance calculations. Seasonal variation in nutrient concentrations and the
effects of singular events, such as hurricanes, have not been examined in detail, even
though these are evident in the monitoring data and in the model calculations. More
work is needed to understand the fate of the NOx-N that is apparently ―missing‖ from
the South Central Inshore region, including the effects of tidal circulation and biological
uptake. Once these questions are satisfactorily addressed, the results of calculations
can be refined to include the effect of water management policies and practices on the
seasonal and inter-annual variations in freshwater inflows and nutrient loads.
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SECTION 5:
SUMMARY OF THREATS TO BISCAYNE
BAY AND BISCAYNE NATIONAL PARK
WATER RESOURCES
The following chapter is a summary of a recent assessment of natural resource
conditions in Biscayne Bay (Fig 5.1), based on the evaluation of a review/compilation of
existing information on Park’s natural resources (Harlem et al. 2009). This review of
existing data was used to evaluate threats and stressors, and is intended to improve
understanding of BNP resources in order to help guide Park management to properly
address the identified threats. Threats to the resources of Biscayne National Park are
multiple as are gaps in our understanding of the functioning of the Biscayne Bay
ecosystem. Harlem et al. (2009) focused on several broad resource components,
namely terrestrial resources and aquatic systems including wetlands, canals, bay
waters, marine/reef areas and ground waters. Both, biotic and abiotic resource
components were considered in the study, and the main objectives of the assessment
were:
1. Provide a review/compilation of existing information on BNP natural resources.
2. Provide a list and description of a suite of threats/stressors to these resources.
3. Generate a semi-quantitative estimation and ranking of such threats to defined
resource components, and the richness of existing information
4. Identify research needs based on information gaps and degree of threat to
said resources.
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Figure 5.1: Biscayne National Park. Also shown, canals, ditches and tidal creeks
dissecting the mainland west of Biscayne Bay.
HABITAT LOSS AND IMPAIRMENT
Coastal Development
Biscayne Bay’s watershed has experienced an enormous expansion in urban
development in the last 100 years, from 12,000 people in 1910 to 2,460,000 people in
2010 for Miami-Dade alone (http://www.census.gov/popest/data/index.html). Former
wetlands and more recently farm fields have been transformed into residential areas
(Figure 5.2). More people near the Park increases impact on resources by augmenting
usage of the parklands for recreation, increasing pollution, and further requiring
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measures to reduce flooding which, in turn, impacts surface and groundwater flows to
the Park. Projections of considerable further development to south Miami-Dade County
are alarming in the scope and magnitude of sprawl development.
Figure 5.2: Changes in land cover/land use in Miami’s Greater Metropolitan area since
1972. Modified after Migliaccio et al (2009).
Channelization/Sheet Flow Barriers
Canals in South Florida were designed to collect surface water and remove it
from the landscape thus virtually eliminating sheet flow which once dominated the