BENTHIC INVERTEBRATES OF A MODERN CARBONATE RAMP - A PRELIMINARY SURVEY

A preliminary survey of benthic invertebrates off central west Florida provides documentation of modem epifaunal communities on a low-gradie nt carbonate slope. Three large-scale biofacies occur in soft-sediment carbonate environments between 200 and 2,000 m: an Echinoderm biofaci es (200-550 m) dominated by a diverse assemblage of echinoderms, gastr opods, and decapod crustaceans; a Penaeid shrimp-conical mound biofaci es (550-1,200 m) characterized by large bioturbation structures; and a Microbial mat biofacies (1,200-2,000 m) with only rare epifaunal inve rtebrates. A fourth, hard-substrate biofacies reflects the presence of localized Miocene and Pleistocene hardgrounds in water depths of 200- 600 m. This illustrates that hard-substrate biofacies may be laterally correlative with soft-sediment biofacies in a slope setting, thus pro ducing a mosaic of contrasting faunal associations. All four biofacies have low population densities, presumably as a consequence of relativ ely low surface productivity. All four biofacies also show biogeograph ic affinity with other faunas at intermediate depths in the Caribbean region. Depth-related faunal transitions on the west Florida slope cor relate with substrate and current velocity. Decreasing species diversi ty and abundance and a biofacies transition from suspension-feeding to deposit-feeding assemblages correlate with increasing depth, a decrea se in mean grain size, and an increase in organic content of the sedim ent. This biofacies model may provide a modern analogue for faunas of ancient low-gradient slopes such as those of Cretaceous ''shelf-sea'' chalks of northwestern Europe.