Foraminiferal Biofacies on a North Coast Fringing Reef (1-75 m), Discovery Bay, Jamaica

Abstract

Recent foraminifera inhabiting Jamaican north coast fringing reefs exhibit variation in distribution patterns that are related to bathymetry and reef morphology. Parallel traverses were run from the back reef (depth 1-4 m) across the reef crest, fore-reef terrace (5-14 m), escarpment (24 m), slope (30-55 m), and upper (75 m) deep fore-reef zones at Discovery Bay, Jamaica. Approximately 150 species belonging to 80 genera were identified. Diversity indices (S, H', J') are lowest on the fore-reef terrace, increase toward the back-reef andfore-reef escarpment and slope, and tend to be highest on the mid-lower slope and deep fore reef. Major operational taxonomic groups of foraminifera (suborders Textulariina, Miliolina, benthic and planktonic Rotaliina) exhibit distinct bathymetric trends, with the Miliolina occurring most abundantly in the back reef, and Textulariina and planktonic Rotaliina being most abundant below 30 m. Benthic rotaliines are prominent across the entire reef environment and may compose >60% of foraminiferal sediment assemblages. Among the largest and most common species, Amphistegina gibbosa (benthic Rotaliina) dominates fore-reef assemblages, whereas Archaias angulatus (Miliolina) is most abundant in back-reef environments. Individual fore-reef zones are poorly delineated in Q-mode cluster analysis of foraminiferal species abundances (2 5% of foraminiferal assemblages at any site) based on standard counting procedures (300 foraminifera from all size-fractions >0.125 mm). Reef zones are, however, clearly defined when the standard countingprocedures are applied to individual size-fractions (1-2, 0.5-1.0, 0.25-0.5, and 0.125-0.25 mm) and species abundances determined from total counts of all four size-fractions combined ('sieve method). Based on the sieve method, Q-mode cluster analysis of common (3% of foraminiferal assemblages at any site) species delineates back reef, fore-reef terrace, and fore-reef slope and upper (75 m) deep fore-reef zones. The alternative counting procedure appears to compensate for operator bias in specimen counts introduced by large and unusually abundant species (e.g., Amphistegina gibbosa) and by small but colorfulforms (e.g., Discorbis rosea), which appear to mask the zonation when using standard counting procedures.