Symbiont Diversity on Coral Reefs and Its Relationship to Bleaching Resistance and Resilience

Abstract

Mass coral reef bleaching and mortality as a result of prolonged seawater warming following the 1997–1998 El Niño-Southern Oscillation forced a change in conservation priorities in assessing threats to the health of coral reefs worldwide. By some estimates, approximately one sixth of the world’s coral reefs was destroyed over a single 9-month period during the 1997–1998 bleaching event (Wilkinson 2000). Most of this destruction occurred in the Indian Ocean, where prolonged elevations of sea surface temperature were maintained by prevailing currents that pooled warm water in the western Indo-Pacific. In most cases, coral reef destruction equated to a dramatic reduction in live coral cover on these reefs (e.g., McClanahan 2000; Loch et al. 2002), but it is noteworthy that even the most severely affected reefs maintained significant pockets of live coral scattered throughout their original distributions. Moreover, many coral reef ecosystems that suffered extensive bleaching (e.g., parts of the Caribbean and Great Barrier Reef) did not experience significant eventual mortality (Wilkinson 2002). Consequently, coral reef recovery has in many places been more rapid than initially expected, particularly in the western Pacific. Although 1997–1998 clearly represents an annus horribilis for many coral reefs worldwide, the destruction it witnessed may not be as irreversible or as cumulative as originally thought. How resistant and/or resilient were reef corals (and coral reefs) to this event? How might resistance and resilience change over time in response to rising temperatures and recurrent bleaching episodes? To what extent can we expect the destruction and recovery patterns of 1997–1998 to be common features of reef bleaching and mortality in the years to come?