Oliver et al (2004) Monitoring bleaching

Introduction
Coral bleaching occurs when corals lose the single celled algae (zooxanthellae) which live within their
tissues. These golden brown coloured algae (Figure 1) occur in varying densities in reef corals (and
other reef invertebrates), and give them a light tan to deep chocolate brown colour. Where additional
pigments exist within the animal cells, this brown colour can be overlain by different additional hues
such as blue, green, purple or yellow (Figure 2). When corals lose their zooxanthellae, the white
skeleton can be seen through the transparent animal tissue, making the corals looked bleached white. In
cases where the corals possess additional animal pigments, bleached corals take on vivid fluorescent
hues, with no trace of the normal brown background colour (Figure 3).
Corals which have bleached are not dead. On close inspection it is possible to see the transparent
polyps and tentacles of bleached colonies. However corals cannot remain bleached indefinitely. If the
stress is not too severe or prolonged, stressed colonies can slowly regain their zooxanthellae and
survive. But in severe bleaching events many corals subsequently die, casing major changes to the
structure and function of the reef ecosystem, and possible cascading impacts on other organisms.
Although bleaching in individual corals was first observed early in this century, massive events
affecting reefs spanning tens to hundreds of kilometres have only been documented in the last two
decades . In the laboratory, or on a small scale in the field, bleaching has been attributed to a number of
factors which cause stress, including extremes (both high and low) of temperature and light, low
salinity, low oxygen, high concentration of toxic chemicals which affect respiration and
photosynthesis. In the case of “mass coral bleaching” involving a range of coral species over large
areas, elevated temperatures are the primary stress factor (Coles & Brown, 2003; Hoegh-Guldberg
1999). Elevated sea temperatures may be driven by regional phenomena (such as El Niño events), but
local conditions, such as periods of calm weather and clear skies, also play a major role in determining
sea temperatures in reefal regions. Decreased salinity from excessive rainfall and flood plumes can
increase the amount of bleaching, and may be a primary cause in some situations (mostly over limited
spatial scales). During the worst global bleaching event in 1998, more than 680 records of coral
bleaching were reported from over 55 different reef regions. Many of the affected reefs suffered
massive mortality from which they are still recovering (Wilkinson 2002). This event was also the most
intensively studied, and satellite data showing thermal anomalies or “hot spots” clearly showed that
large patterns of abnormally and consistently high temperature were very well correlated with
bleaching patterns, and in some cases appeared to be able to predict the events (Toscano et al. 2000,
Wilkinson et al 1999).
Coral bleaching is now considered to be one of the most significant and widespread threats to coral
reefs Hughes et al., 2003). When predicted temperature increases due to global warming over the next
100 years have been compared to the known temperature bleaching limits of corals, the depressing
conclusion is that by 2020 coral reefs in many parts of the world may suffer bleaching and mortality
every year Hoegh-Guldberg, 1999). Unless corals are able to adapt to raising temperatures, reefs may
suffer progressive deterioration and species loss, resulting in major ecological impacts and consequent
social and economic impacts on the human communities in many countries that depend on reefs for
their livelihoods. Global warming and greenhouse gas emission thus represent a relatively new and
severe threat to the sustainability and productivity of coral reefs and the services that that provide to
humans (Hughes et al., 2003; Wellington et al., 2001; Cesar et al., 2003).
There is an urgent need to obtain better information on bleaching events around the world. This
information is crucial to scientific understanding of the fate of coral reefs and to the feasibility and
practicality of developing management strategies to increase the resistance and resilience of reefs to
bleaching and associated mortality events. This protocol aims to provide a simple yet consistent set of
procedures to:
1. Document the spatial dis tribution, timing and severity of large scale bleaching events and how
they compare with maps of global temperature anomalies and other meteorological and
oceanographic factors. This will help in the developing and testing of predictive models of
bleaching which can focus attention on susceptible reefs which might benefit from better
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