40-years of cyclic disturbance / recoveries of coral communities compared with trajectories of coral communities at American Samoa

Cyclones, Acanthaster outbreaks, major heat-stress events, a tsunami and other disturbances have made American Samoa a challenging environment for scleractinian corals. Diseases have been present for decades. Nevertheless, living coral cover is now significantly greater than during the 17 previous surveys dating back to 1982. Although increasing coral cover resulted from colony growth rather than increased abundance, the prevalence and continuity of recruitment was a strong factor in the resilience of American Samoan reefs. Substantial recruitment by most coral taxa indicated that reproductive output was not reduced by physiological stress, and most population densities were high enough to avoid Allee effects. Consistent recruitment of corals is probably favored by the extraordinary prevalence of crustose coralline algae (CCA), as suggested by 94% of successful coral recruits being found on CCA. Community changes such as living coral cover and population densities of coral colonies are usually cyclic (e,g., disturbance/recovery), but there have been long-term trajectories in relative prevalence of particular types of corals. Proportions of massive and encrusting corals have significantly increased while faster growing branching corals, such as Acropora and Pocillopora, have generally decreased. Fagatele Bay National Marine Sanctuary has been an exception where Acropora has increased in coral cover despite repeated disturbances and exposures to heat stress. Elsewhere, Acropora persistence was highest where watershed pollution and other local stressors were low. The reduction of fast-growing branching corals and relative persistence of massive and encrusting corals may be trajectories caused by deteriorating environmental conditions, following patterns previously seen in geologic time.