Physical Disturbance and Community Structure of Exposed and Protected Reefs: A Case Study from St. John, U.S. Virgin Islands

Abstract

Major physical disturbances impacted fringing reefs at 4 to 12 m depth off the south coast of St. John, at intervals of 28, 14 and 12 months over a 6 year period (1985–1991). The most wave exposed habitats were dominated by the hydrocoral Millepora alcicornis and the encrusting gorgonian Erythropodium caribaeorum . Both of these species displayed high resistance stability in response to the 1987 bleaching event and to Hurricanes Gilbert (1988) and Hugo (1989). Although Hurricane Hugo was ranked as the most severe hurricane of the past century, it caused comparatively little damage to the shallow exposed reef, illustrating that the effects of physical disturbance may not always conform to a priori predictions based on wave exposure. This counter-intuitive result is explicable when the chronology of disturbance is considered. For example, the shallow exposed community was more heavily damaged than the protected reef by Hurricane Gilbert occurring one year prior to Hurricane Hugo. Sponges and thick mats of E. caribaeorum had accumulated on the protected reef during a 54 month period of little disturbance and were vulnerable to disturbance by virtue of their elevation above the substratum. These organisms were dislodged by extreme water motion generated by Hurricane Hugo. In contrast, the shallow exposed community was still recovering from Hurricane Gilbert when Hurricane Hugo struck. Monitored populations of two coral species, Agaricia agaricites and Tubastrea coccinea , declined over the six year period, but recruitment in 1990 partially compensated for high mortality caused byHurricane Hugo. There were significant increases in the abundance of algal turf on three out of the four study reefs (4 m exposed and protected, 12 m protected) in response to the creation of patch space by the multiple physical disturbances. This study demonstrates the need to interpret recent disturbances in light of the history of past disturbance on the reef.