Caribbean Corals in Crisis: Record Thermal Stress, Bleaching, and Mortality in 2005
Background: The rising temperature of the world’s oceans has become a major threat to coral reefs globally as the severity and frequency of mass coral bleaching and mortality events increase. In 2005, high ocean temperatures in the tropical Atlantic and Caribbean resulted in the most severe bleaching event ever recorded in the basin.
Methodology/Principal Findings: Satellite-based tools provided warnings for coral reef managers and scientists, guiding both the timing and location of researchers’ field observations as anomalously warm conditions developed and spread across the greater Caribbean region from June to October 2005. Field surveys of bleaching and mortality exceeded prior efforts in detail and extent, and provided a new standard for documenting the effects of bleaching and for testing nowcast and forecast products. Collaborators from 22 countries undertook the most comprehensive documentation of basin-scale bleaching to date and found that over 80% of corals bleached and over 40% died at many sites. The most severe bleaching coincided with waters nearest a western Atlantic warm pool that was centered off the northern end of the Lesser Antilles.
Conclusions/Significance: Thermal stress during the 2005 event exceeded any observed from the Caribbean in the prior 20 years, and regionally-averaged temperatures were the warmest in over 150 years. Comparison of satellite data against field surveys demonstrated a significant predictive relationship between accumulated heat stress (measured using NOAA Coral Reef Watch’s Degree Heating Weeks) and bleaching intensity. This severe, widespread bleaching and mortality will undoubtedly have long-term consequences for reef ecosystems and suggests a troubled future for tropical marine ecosystems under a warming climate.
INTRODUCTION
Coral bleaching has become a major threat to coral reef ecosystems worldwide [1]. Bleaching occurs when stress to the coral-algal symbiosis causes corals to expel their endosymbiotic algae (zooxanthellae) and, if prolonged or particularly severe, may result in partial or complete coral mortality [2]. While many sources of stress have caused corals to bleach, ‘‘mass’’ coral bleaching (at scales of 100 km or more) has only occurred when anomalously warm ocean temperatures, typically coupled with high subsurface light levels, exceeded corals’ physiological tolerances. This was observed during recent major El Nin˜o-Southern Oscillation events (e.g., 1982–83 [3], 1997–98 [4], and 2002 [5]) and verified by laboratory experiments [6,7]. These bleaching events caused coral death at numerous sites around the world, with impacts on reef habitats, structures, and biodiversity that lasted a decade or more [8,9].
From June to October 2005, a warm-water anomaly developed across the tropical Atlantic Ocean and greater Caribbean Sea region. Satellite-based sea surface temperature (SST) observations from the U.S. National Oceanic and Atmospheric Administration (NOAA) [10] detected a large region of warming ocean temperatures that reached a maximum anomaly of +1.2uC vs. the long-term mean when averaged across all Caribbean reef sites. Elevated temperatures persisted for many weeks and helped fuel the most active Atlantic hurricane season on record [11] and the most severe and extensive mass coral bleaching event observed in the Caribbean.
