Coral reef bleaching: facts, hypotheses and implications

Abstract

Coral reef bleaching, the temporary or permanent loss of photosynthetic microalgae (zooxanthellae) and/or their pigments by a variety of reef taxa, is a stress response usually associated with anthropogenic and natural disturbances. Degrees of bleaching, within and among coral colonies and across reef communities, are highly variable and difficult to quantify, thus complicating comparisons of different bleaching events. Small‐scale bleaching events can often be correlated with specific disturbances (e.g. extreme low/high temperatures, low/high solar irradiance, subaerial exposure, sedimentation, freshwater dilution, contaminants, and diseases), whereas large scale (mass) bleaching occurs over 100s to 1000s of km2, which is more difficult to explain. Debilitating effects of bleaching include reduced/no skeletal growth and reproductive activity, and a lowered capacity to shed sediments, resist invasion of competing species and diseases. Severe and prolonged bleaching can cause partial to total colony death, resulting in diminished reef growth, the transformation of reef‐building communities to alternate, non‐reef building community types, bioerosion and ultimately the disappearance of reef structures. Present evidence suggests that the leading factors responsible for large‐scale coral reef bleaching are elevated sea temperatures and high solar irradiance (especially ultraviolet wavelengths), which may frequently act jointly.

High-precision 238U–234U–230Th disequilibrium dating of the recent past: a review

“…4). This is consistent with the observed increase in the frequency of mass coral bleaching events since the late 1970s (Glynn, 1996). However, the frequency of coral mortality on Meiji Reef seems to have remained unchanged over the past two centuries.…”

Climate change impacts on U.S. Coastal and Marine Ecosystems

“…Reef ecosystems will be susceptible to changes in the frequency and/or magnitude of temperature extremes because many coral species live near their upper limits of thermal tolerance. Bleaching can occur when these thermal tolerances are exceeded and the corals' symbiotic algae (zooxanthellae) are subsequently expelled, slowing or halting growth, skeletal accretion, and sexual reproduction, and increasing the susceptibility to pathogens (Glynn 1996). Satellite data show an increase in the extent of tropical ocean hotspots, areas 2°C to 3°C above average water temperatures (Strong et al 2000), and these hotspots have been useful predictors of bleaching events.…”

Corals from the Persian/Arabian Gulf as models for thermotolerant reef-builders: Prevalence of clade C3 Symbiodinium, host fluorescence and ex situ temperature tolerance

“…Bleaching due to thermal stress is considered to be induced by the algal production of reactive oxygen species (ROS), particularly H 2 O 2 (Lesser, 2006;Smith et al, 2005;Suggett et al, 2008;Tchernov et al, 2011). Although the animal host might survive and recover, mass mortality is frequently observed among bleached corals Glynn, 1996;Wilkinson, 1998). Over the past decades, mass bleaching events associated with elevated seawater temperatures have become more frequent and have contributed to the rapid degradation of coral reefs Burt et al, 2011;Hughes et al, 2003).…”

“…In general, temperatures as little as 1°C above the average annual maximum can trigger bleaching, but the actual bleaching threshold depends also on the duration of temperature stress and the stress history of the corals Glynn, 1996). Furthermore, the threshold for heat-induced bleaching can be lowered by unfavourable concentrations of inorganic nutrients in the water column .…”