ABSTRACT

Disturbance and subsequent recovery have been key factors in the dynamics of coral reef populations, but in the Caribbean, there has been a steady decline in the abundance of hard corals accompanied by increased abundance of algae. This characterization overlooks the observation that at some sites for which there are quantitative data, the abundances of octocorals (sea fans and sea plumes) have increased. Closer inspection of those data reveals that octocorals have also declined at times, but unlike hard corals, they recovered and are now more abundant in at least some locations than 30 years ago. The change in abundances of these organisms would appear to due to the greater resilience of octocorals. The damage inflicted by Hurricanes Irma and Maria on the south shore of St. John, U.S. Virgin Is. provides a window into these processes. Abundances of hard corals on these reefs have been monitored for the past 30 years and the octocorals have been followed for the past 4 years. Colonization of the newly opened space on the reef will be studied to determine if further shifts in abundances can be expected and to understand the basis for the greater resilience of octocoral populations. Characterizing the resilience of octocorals relative to hard corals will have implications for the fate of Caribbean reefs in a world subject of increasing levels of stress. This will be of importance to policy makers and managers, because hard corals and octocorals provide substantially different ecosystem services. In addition to its impact on our fundamental understanding of reef processes, the grant will support the training of graduate students.

Coral reefs have been recognized to be disturbance driven systems for over 50 years and disturbance has been considered to be important in maintaining diversity of reef communities. However, the past 30 years has been characterized by declines in the abundances of scleractinians, often associated with acute events such as hurricanes and bleaching events. A knowledge of the effects of disturbance and subsequent recovery that occurs during these events is critical to understanding the changes in reef structure that have occurred in the last 30 years. At sites for which there are data, changes in the relative abundance of scleractinians and octocorals appear to coincide with disturbance followed by octocoral recovery and a lack of scleractinian recovery. This change must reflect changes in larval supply, settlement, and/or survival, and those differences should be greatest following disturbance. The effects of Hurricanes Irma and Maria on the south shore of St. John provide the opportunity to characterize recruitment following disturbance and identify the processes that may limit successful recruitment on contemporary reefs. Data are already available on the abundance of octocoral and scleractinian species over the past 4 years, and annual censuses have provided data on octocoral recruitment and survival and species level characterizations of octocoral abundances. Recruitment and survival are also being related to spatial complexity of the habitat at the scale of mm and cm and the effects on hurricane driven changes in that spatial complexity will also be assessed. The frequency and intensity of disturbances will at the least continue at historical or greater levels. The comparison of data from before and now after disturbance will provide a powerful assessment of the resilience of octocorals and scleractinians and the role of recruitment and recruit survival in their relative resilience. Thus the effects of Hurricanes Irma and Maria provide a window on the nature of future reefs.

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