Cryptic regime shift in benthic community structure on shallow reefs in St. John, US Virgin Islands

We evaluated whether long-term changes on fringing reefs (7-9 m depth) around St. John, US Virgin Islands, conformed to the 'coral reef crisis' involving ongoing collapse of community structure. Annual photoquadrats at 6 sites were used to measure octocoral abundance (by genus) and the cover of scleractinians (by species), macroalgae, crustose coralline algae, algal turf, and bare space. Community structure changed between 1992 and 2014, but did not correspond to large changes in response to pulse disturbances; octocoral abundance declined from 1992 to 2002, but increased to 2014, scleractinians remained uncommon (similar to 4.5% cover), and macroalgae became more abundant. Richness and diversity of octocorals and scleractinians increased following 2000, and together defined 2 communities characterized by distinct taxonomic assemblages and temporally segregated to 1992-2001 versus 2002-2014. Smoothed temporal variation in community structure assessed at generic (Octocorallia) and species (Scleractinia) level was associated most strongly with rainfall, and to a lesser extent with hurricane intensity, thereby underscoring the roles of chronic disturbances in driving gradual changes in community structure. Together, these changes did not conform to the typical coral reef crisis construct, and instead summed to cryptic regime change that has escaped attention by researchers focused on scleractinians and macroalgae.