CANOPY EFFECTS OF OCTOCORAL COMMUNITIES ON SEDIMENTATION: MODERN BAFFLES ON THE SHALLOW-WATER REEFS OF ST. JOHN, USVI

Recent shifts in the presence and abundance of species on shallow Caribbean coral reefs have left octocorals as the dominant species on some reefs due to both their resistance to disturbance and their ability to recover after disturbances. This transition has changed the reef profile, potentially affecting the hydrodynamics of water flow and sedimentation on the reef. To understand this change, this research was designed to determine the effect of dense octocoral canopy as a sediment baffle, and the effects this baffling may have on modern reef systems. Research was conducted over a period of 10 weeks on the west side of Grootpan Bay along the southern shore of St. John, USVI. Fifteen sites with octocoral canopy and fifteen without were initially identified based on the density of octocorals within a square meter. The combined effects of the canopy on water turbulence and unidirectional flow were compared between sites using quantitative clod card measurements, and the effects on sedimentation assessed with sediment traps. The thirty sites were classified further by their characteristic submergence depth, bottom type, grade of slope, and the height and genus of surrounding octocorals. Sediment accumulation and composition were quantified from areas with and without an octocoral canopy using a combination of dry weight, grain size and origin and loss on ignition (LOI). Sediment dry weights indicate greater sedimentation in areas with canopy. Clod card dry weights indicate greater mass loss in regions with canopy. The baffling effect of octocoral canopy produces changes in reef sedimentation which may affect community patterns on a local scale.