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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 |
. |
17 |
Figure 2.2: Sediment type map for Biscayne Bay bottom (from Harlem et al. 2009) |
Little is known about the distribution of soil types within BNP and there seems to |
be three soil types inside the park. First, silty marl (carbonate mud) is the dominant soil |
type along the west side of the park landward of the mangrove fringes (Gaiser, et al., |
2006b, Ross, et al. 2003). The second major soil type in the Park is peat derived from |
mangrove detritus. This is the common soil along the fringe of the coastline, along |
former freshwater streams and seeps, and on most of the islands. The third major soil |
type in Biscayne National Park are histosols developed under hardwood hammock |
vegetation on the barrier islands. These organic soils develop in well-drained upland |
18 |
settings that combine relatively high above ground production with some level of |
recalcitrance to decomposition (Coultas, 1977; Ross et al. 2003; Harlem et al. 2009). |
WATER RESOURCES |
Natural habitats in BB have been severely affected by rapid urbanization and |
coastal development linked with the growth of the Greater Miami area, and by disruption |
of its natural drainage system. Southern BB has been additionally impacted by |
agricultural activities. Overall, most of the surface water flow into BB is now controlled |
by a system of canals, levees and structures contributing to the bay's transition from an |
estuary to a marine lagoon over the last 100 years (Harlem, 1979; 2009). Biscayne Bay |
estuarine waters have experienced the impact of anthropogenic interventions since the |
early 1900’s, including major disruptions of its hydrology (RECOVER 2009) and |
sustained urban and agricultural development. Hence, despite their oligotrophic nature, |
BB aquatic ecosystems bear the heritage and signals of such a long and sustained |
influence. |
Prior to construction of the South Dade Conveyance System coastal wetlands |
and nearshore areas in BB maintained estuarine condition most of the year (Smith |
1896; Meeder et al. 1999, Meeder et al. 2001) and supported a diverse assemblage of |
estuarine biota, including oyster reefs and American crocodile (Kushlan and Mazzotti |
1989; SFWMD 1995; SFNRC 2006). The southern bays and sounds have experienced |
significant reductions in historical freshwater flows, leading to hyper saline conditions |
during extended periods of low rainfall (Wingard et al. 2003, 2004). Nevertheless, BB |
remains very productive and its biodiversity is still noteworthy. |
These outstanding features have driven the establishment of protected areas |
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