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The forereef slope is a well-defined zone of the reef biotope. In Jamaica, its maximum limits lie between about -20 m and -120 m, roughly from wave base to the depth of compensation for photosynthesis of green algae and zooxanthellae. In practice, the upper and lower limits of the forereef slope habitat appear to be defined by the 15 and 0.3% isophotes respectively.
Morphologically, the Jamaican forereef slope is divided horizontally by rimmed terraces at about the -18-m, -35-m, -65-m, and -120-m levels. Each terrace is set back from the one below by a steep coral-covered drop-off which may be vertical or even overhanging. These levels are related to eustatic sea level changes during the late Pleistocene and Holocene marine transgressions.
Biologically, the forereef slope has a rich and diverse benthos whose biomass may in some places exceed that of all other reef zones combined. From -20 m to -60 m, hermatypic corals are dominant, below that Porifera prevail. The algal flora is dominated by immense populations of various species of Halimeda, the chief sand builders of this zone. Frame cementation is mainly by encrusting coralline red algae and the colonial foraminifer, Gypsina. A common attribute of many groups in this habitat is gigantism, notably among the sponges, corals, Gorgonia, Antipatharia, and Halimeda. There is a high degree of endemism; i.e., the forereef slope harbors many species not found elsewhere in the reef. Among these are the recently discovered sclerosponges which are important frame cementers of th twilight zone in caves, crevices, and subreef tunnels. Many of the endemic forms are precise habitat indicators, and thus may be of considerable paleoecologic significance.
Sedimentologically, the forereef slope is a region of accelerated deposition and erosion. There, transient sediment of shallow-water origin mingles with locally produced skeletal detritus, resulting in a poorly sorted mixture with a high proportion of fines. The rimmed terraces usually dam large pools of talus through which project nunataklike isolated pinnacles and outcrops of the substrate. The drop-offs in front of the terraces are steep, and dissected by gullies through which drainage of sediment into deep water takes place. Few organisms grow in these chutes, the richest organic communities are on the precipitous rocky promontories between the chutes. Mass transport of sediment downslope is by creep, turbidity currents, and slides. Sporadic fallout of corals results in an imbrica ed scree piled against the source area. Avalanches spread large amounts of coral detritus in disordered heaps far downslope. Slumping displaces very large blocks of reef framework into deep water. Extensive submarine organic and inorganic lithification tends to stabilize quickly the masses of detritus, even on very steep slopes and in spite of the structural weakening due to boring sponges.
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