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 functional group on some reefs, creating an ‘animal forest’ with an associated canopy. This transition changes the reef profile potentially affecting flow and sedimentation. We examined the effects of an octocoral forest on the depositional environment on a shallow-water fringing reef system on the south shore of St. John, USVI. The depositional environment was characterized as canopy or non-canopy based on octocoral density. The effect of the octocoral canopy on flow and sedimentation was assessed using clod cards and sediment traps at 15 paired locations. The octocoral canopy altered flow resulting in greater levels of turbulence within the canopy. Sediment traps in areas of dense octocoral canopy accumulated greater amounts of sediment, with coarser, more rounded grains. Organic content of sediments collected in the traps was greater within the canopy than outside of it. The increase in turbulence within the canopy was likely due to wave-driven oscillatory flow interacting with the octocoral colonies. Sediment traps in the canopy likely had greater sediment accumulation due to both resuspension of sediments from within the canopy and deposition of imported sediments as flow decreased within the canopy. The presence of octocoral canopies and the reworking of sediment within them may affect the success of settling larvae and the evolution of reef structure.