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described in Appendix 3. |
13 |
SECTION 2: |
PARK DESCRIPTION |
Most of Biscayne National Park is within BB, a shallow subtropical estuary |
located along the southeast coast of the Florida Peninsula (Fig 2.1). This coastal lagoon |
extends 56 km from north to south and up to 13 km wide, for a total marine ecosystem |
of about 701 square km, and a watershed spanning 2,434 square kilometer, most of it |
draining urban and agricultural lands. The bay is bordered on the east by barrier islands |
separating it from the Atlantic Ocean, and is rimmed on the west by the City of Miami. |
Most of BB is shallower than three meters (Wang et al. 2003; Harlem et al. 2009) and |
deeper waters are only found in the North Central portion of the bay and in eastern Card |
Sound. |
Biscayne Bay has been traditionally divided for purposes of discussion and |
analysis intro three general areas differing in geographical and hydrodynamic |
characteristics: north (from Dumfoundling Bay south to the Rickenbacker Causeway), |
central (extending from the Rickenbacker Causeway south to Black Point), and south |
BB (from Black Point south to Barnes Sound) (Wang and van de Kreeke, 1994; |
Corcoran et al., 1984; SFWMD 1995; Bellmund et al. 2009). Our modeling work focus |
on Central and South BB and the inland watershed to the west, including urban, |
industrial and agricultural areas as integral part of the analysis. The southern limit of |
North-central BB is marked by the shallow waters on the Featherbed Banks. This |
portion of the bay is significantly affected by urban development in the communities of |
Coral Gables and Coconut Grove and canal inputs on the west and Virginia Key and |
Key Biscayne on the east, and experiences the most active ocean exchange of the |
overall bay through the ―Safety Valve‖ (Fig 2.1). On the west margin of South-central BB |
urban development is less dense and a fringe of wetlands rims the shoreline. While |
direct ocean exchange is limited in South-Central BB, large volumes of freshwater are |
contributed from canals, especially from Black, Princeton, Military and Mowry canals |
14 |
(Fig 2.1). Finally, Card Sound, Barnes Sound and Manatee Bay make South BB. These |
water bodies are somewhat separated from the rest of the bay by extensive shallow |
banks and have restricted oceanic exchange. Their watersheds are mostly located |
within Everglades wetlands with little urban development. |
Figure 2.1: Biscayne National Park and adjacent areas. |
Manatee and Barnes Sound are more like Florida Bay in terms of ecosystem |
structure and biogeochemistry (Boyer and Briceño, 2006; Rudnick et al. 2006), but their |
hydrological connection is with Card Sound. Besides run-off from the wetlands, surface |
freshwater contribution to South BB is mostly from the C-111 canal. |
15 |
GEOLOGY AND SOILS |
The outcropping rock units underlying BB and its watershed are carbonates of |
the Plio-Pleistocene Fort Thompson and Caloosahatchee formations and the |
Quaternary Miami Limestone and the Key Largo Formation (Parkinson, 1989; Wanless |
et. al 1994). The Pleistocene Epoch (1.8 million years to 10,000 years ago) was a |
period of intense shifts in sea level caused by cyclic changes in the earth ice cover. |
Those sea level fluctuations resulted in the accumulation of the Fort Thompson, the Key |
Largo and the Miami formations. The Fort Thompson Fm. is made of marine and fresh |
water shell, sand and lime mud facies typical of shallow coastal environments. The Key |
Largo Fm. is dominated by coralline and reef facies and forms the rock structure of the |
Florida Keys including most of the islands within the Park. The Miami Formation, |
contemporaneous with the Key Largo, forms the low ridge along the Park’s west side |
and the bottom floor of much of BB. Its oolitic sand, carbonate sand and burrowed mud |
facies are interpreted as marine inner-shelf deposits influenced by high tidal |
fluctuations. The karst surface developed on this marine unit has the greatest effect on |
the park’s resources as the principal geomorphological control. |
The Holocene Epoch, which began 10,000 years ago, includes the recent |
modifications done by man. The dominant Holocene sediment in the Park is carbonate |
sand comprised of a gamut of biologically derived components as Halimeda plates, |
mollusk fragments, cohesive pelloids, coral fragments, and invertebrate tests (Wanless |
et al. 1976, 1994). Muddy carbonate sediment is the common matrix component and is |
derived from the breakdown of biological carbonate material. Quartz sand is the third |
most common sediment and is derived from terrestrial sources usually transported to |
the region by longshore drift and submarine currents. |
Figure 2.2 shows the sediment types in BB (Wanless 1976). Holocene sediments |
and soils provide the habitat for most plant species and many important animal species |
in the Park. Much of the bottom of BB is bare rock, especially to the south. Thicker |
intervals of mud-size sediments develop banks in the Safety Valve area and south of |
the Rickenbacker Causeway (Fig 2.1). Sand deposits occur at Featherbed Bank and No |
Name Bank. Seaward of the keys carbonate sedimentation occurs in patch reef |
16 |
complexes, seagrass meadows and the barrier reef. Finally, thick peat deposits |
developed in mangrove forest rim the western shoreline (Harlem et al. 2009). |
Geomorphologically, BB is a shallow depression in bedrock resulting from severe |
differential weathering and erosional events of the Neogene carbonate rock pile and |
from sedimentation processes, both fostered by Pleistocene changes in sea level, under |
significant tectonic stability (Parkinson, 1989; Wanless et. al 1994). During low sea level |
conditions intense chemical weathering, mass movement and river transport and |
sedimentation prevailed. The shoreline and coastal and terrestrial ecosystems migrated |
seaward expanding the land mass. During those periods, BB was a north-south trending |
valley subjected to stream erosion and sedimentation along its main axis. As sea level |
increased, the river valleys were drowned and sediments were remobilized by waves |
and tides. |
Under sea level rising conditions flooding of BB began approximately 3,200 years |
ago (Wanless et. al 1994). This fundamental geomorphologic setting is responsible for |
subsequent development and distribution of habitats and the current ecosystem |
framework. Relevant to the present study are those rock formations supporting the |
landforms and those lying underground which contain the shallow and highly dynamic |
aquifers (Harlem et al. 2009). |
Palaeocological studies of sediment cores in BB (Wingard et al 2004) indicate |
that the bay has been a dynamic environment over the last 500 years, with natural |
salinity changes and marked benthic habitat variability. Some changes may be |
connected to anthropogenic intervention (canal construction, declined freshwater |
supply, etc) while others seem to respond to regional or global changes (sea level rise). |
Salinity has increased while seagrass has declined. Estuarine species assemblages |
were replaced by open-marine species around 1900 AD, and increases in foraminifera |
abundance (Archaias and Articulina) began around 1947; a decline in Thalassia |
appears to have occurred in the 20th century at Featherbed Bank and after 1950 at No |
Name Bank |
. |
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