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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
western BNP shoreline. Road construction produced elevated structures with adjacent
canal-like borrow ditches and levee structures which cross the western coast in many
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places. All these structures are barriers to flow, some affect groundwater, and all
fragment the coastal environments into small disconnected parcels (Fig 5.1). This has a
negative effect on the wetlands themselves and downstream consequences for those
marine/bay ecosystems which depend on both quantity and quality of freshwater
entering the estuary, groundwater flow and dynamics of seawater intrusion.
Assessments of perceived impacts of canal and groundwater discharges to southern
Biscayne Bay include those of Szmant (1987), Byrne and Meeder (1999), Lietz (1999),
Langevin (2000) and Graves et al (2004, 2005). Findings suggest nitrogen-enriched
groundwater enters Biscayne Bay through canals and underwater springs. Szmant
(1987) reported a change in seagrass community composition near the mouth of C-102
and C-103 and found lower salinities and higher water column nutrient concentrations in
the affected areas
Habitat Fragmentation
Habitat fragmentation has both physical and biotic impact. Most fragmentation
occurs on the western coastal zone affecting the coastal wetlands and mangrove fringe.
This compartmentalization of formerly connected wetlands includes urban areas,
canals, roads, and other structures which impede water flow and isolate biota.
Therefore habitat fragmentation is a current problem with fair documentation for the
terrestrial environments, although its potential impacts on the Bay are only inferred.
Dredging and filling for marinas and residential boat access as well as channels dug
through shallow water and bridges/causeways are currently a minimal problem in
Biscayne National Park, but affects the Bay to the north of the Park.
Power Plants
Electrical generating power plants, all run by Florida Power and Light, are located
adjacent to Biscayne National Park. The principal plants are: (1) the Turkey Point
Nuclear facility located just west of the Park’s SW corner and includes a fossil fuel
peaker plant on site; (2) the Cutler Power Plant, located on the shore of Biscayne Bay
northwest of the Park; and (3) one fossil fuel peaker plant which has been permitted
south of the Princeton canal just west of the Park perimeter. Since 2007, the NPS has
expressed concerns about the planned expansion, indicating those plans affect
mangrove wetlands within protected areas, and the recharge area which maintains the