Three decades of coral reef community dynamics in St. John, USVI: a contrast of scleractinians and octocorals

To better understand phase shifts on Caribbean reefs, we quantified community structure on shallow reefs over 27 yr in St. John, U.S. Virgin Islands, contrasted the community dynamics of scleractinians and octocorals, and evaluated the extent to which community structure was associated with rainfall, temperature, and hurricanes. To gain insight into the likely abundance of octocorals on future reefs with low scleractinian cover, we compared two sites dominated by the major Caribbean reef-building coral Orbicella annularis. Between 1987 and 2013, scleractinian cover declined from 45% to 6% at Yawzi Point, but remained at similar to 30% at Tektite. We compared changes in community structure using four benthic assemblage constructs-scleractinian-focused (cover of scleractinians, macroalgae, and CTB i.e. crustose coralline algae, algal turf, and bare space), octocoral-focused (abundance of octocorals, cover of macroalgae, and CTB), octocoral genera (abundance by genus), and a complete approach (all taxa)-to reveal how a consideration of octocoral abundance influenced the interpretation of coral reef community dynamics. Overall, temporal variation in community structure differed among the four assemblage constructs at both sites and was associated with rainfall and mean seawater temperature. These results suggest that: (1) scleractinian-and octocoral-focused communities in the same location responded differentially to the same environmental conditions, (2) their communities generally were influenced more by the chronic effects of rainfall and temperature than acute effects of storms, and (3) octocoral-focused communities were more resilient to environmental conditions than scleractinian-focused communities. With further declines in cover of scleractinians, octocoral communities are likely to become more common throughout the Caribbean.