Bahamas Slopes

Introduction

The Strait of Florida is bounded to the east by three platforms—Little Bahama Bank, Great Bahama Bank, and Cay Sal Bank—that represent a major component of an extensive carbonate province that includes the submerged Blake Plateau to the north, the Florida peninsula to the west, and the Yucatán peninsula to the southwest.

 

Geology

Drill hole data indicate that the platforms consist chiefly of vast thicknesses (6-14 km) of shallow-water limestones deposited from Jurassic to Recent times. The western margins of these banks are characterized by steep slopes or vertical escarpments that begin as shallow as 60 m and extend to ~160-200 m where they may end more or less abruptly in talus deposits or continue into deeper water as more gently sloping hardgrounds or sediment substrates. Off the southwestern margin of Little Bahama Bank, a narrow bank-margin reef in ~52-69 m gives way to the deep forereef: an irregular, low-relief, sediment-veneered hardbottom slope of usually 40-60°, with locally steeper scarps, rocky outcrops, ledges, loose talus, and rare larger caves. Slope angle generally decreases with increasing depth, and sediment deposition (both Halimeda sand and finer grains) increases correspondingly. Features in deeper water (~300-500 m) include pavements, ledges, benches, and mounds up to 30 m in vertical relief, as well as extensive sediment plains. Upper bank margin sediments are dominated by carbonate grains derived from the shallow bank platforms or bank margin reefs. Sediments below 200 m tend to be chiefly pelagic (foraminiferan and thecosome).

Oceanography

Because the axis of the Florida Current lies closer to the Florida side of the Strait, an average of 25 km offshore Miami to Palm Beach, the Bahama slopes are less directly affected. However, water movement is still generally to the north, and benthic faunas are dominated by suspension-feeding invertebrates that rely on current for food. Submersible observations in ~400 m south of West End, Grand Bahama Island, indicate that bottom flow is usually to the northwest, parallel to the bank margin, at velocities of 5-30 cm sec-1. However, reversals to the southeast of fewer than 12 hours may reach 100 cm s-1. These reversals may be associated with small-scale eddies that spin off the eastern of the Florida Current as it passes north of Great Bahama Bank, or as water flows into the Strait along the northern side of Northwest Providence Channel (between Little and Great Bahama Banks) and out along the southern side.

Because of the geostrophic nature of the Florida Current's northward flow, and the consequent dipping of isotherms from west to east across the Strait, warm water penetrates to much greater depths on the Bahama side, e.g., a mean annual temperature of about 10°C occurs in ~550 m on the Bahama side but only in ~200 m on the Florida side. Bahama slopes are also subject to a salinity maximum (36.6 ppt) in 100-150 m, and a dissolved O2 maximum (4.5 ml l-1) in 250-350 m associated with 18°C water, both associated with water recirculating in the gyre from the North Atlantic.

Biology

Although a substantial literature has documented geological structure and processes of the Bahama margins, relatively little work has focused on biological assemblages except for specifically taxonomic work. On the deep forereef off Grand Bahama Island, octocorals (chiefly Plexauridae and Ellisellidae), antipatharians and sponges are the most abundant attached large organisms below 115 m. The black whip coral, Stichopathes lutkeni may form dense forests in ~130-150 m. The deep forereef ends in 164-190 m, often accompanied by substantial talus. Organisms on sediment-veneered pavements in deeper water include solitary corals, feather stars, sea lilies, brittle and snake stars, primnoid and plexaurid octocorals, antipatharians and sponges. Submersible surveys of the deeper mounds west of Little Bahama Bank reveal assemblages of stony coral fans (Madrepora carolina), octocorals, crinoids and basketstars that become more abundant near the mound tops where currents are stronger. These mounds, like the lithoherms in deeper water, exhibit distinct faunal zonation patterns, for example, with the octocoral Paracalyptrophora duplex on the downslope flank, small antipatharians on the upslope flank, fields of sea lilies (Endoxocrinus parrae and Neo crinus decorus) on the upper surface, and M. carolina and the basketstar Astracme mucronata near the crest. Mounds in deeper water support populations of different sea lilies (Endoxocrinus carolinae and E. prionodes), lithistid sponges and ophiacanthid brittlestars.Sediment plains in >300 m often appear barren, but with the occasional herd of sea biscuit urchins (Palaeopneustidae). Almost no research has been carried out on the biological communities on the deep western margin of Great Bahama Bank and Cay Sal Bank. However, many organisms collected by trawling and dredging along all western Bahama Bank margins have also been collected from similar depths off Cuba. The slope faunas of the western Bahamas and Cuba differ substantially from the faunas at similar depths along the Florida margin of the Strait.

 

Habitats

 

Porifera

DEMOSPONGIAE

Cnidaria

OCTOCORALLIA – Soft Corals and Gorgonians

ANTIPATHARIA – Black Corals

Echinodermata

CRINOIDEA

Fishes