Coral Reef Science:  Development Highlights

Habib Sekha

U. E. Siebeck, N. J. Marshall, A. Klüter and Ove Hoegh-Guldberg. Monitoring Coral Bleaching Using a Colour Reference Card. Journal Coral Reefs. Volume 25, Number 3/August, 2006.

Communicated by Ecology Editor P.J. Mumby.


Assessment of the extent of coral bleaching has become an important part of studies that aim to understand the condition of coral reefs. In this study a reference card that uses differences in coral colour was developed as an inexpensive, rapid and non-invasive method for the assessment of bleaching. The card uses a 6 point brightness/saturation scale within four colour hues to record changes in bleaching state. Changes on the scale of 2 units or more reflect a change in symbiont density and chlorophyll a content, and therefore the bleaching state of the coral. When used by non-specialist observers in the field (here on an intertidal reef flat), there was an inter-observer error of ± 1 colour score. This technique improves on existing subjective assessment of bleaching state by visual observation and offers the potential for rapid, wide-area assessment of changing coral condition.


The authors are referring to a chart which allows the determination, semi-quantitatively, of the extent of coral bleaching. Other similar charts have been available for several years in some form or another. They are based on the fact that many corals have beautiful pigmentation in their tissue that becomes visible when their zooxanthellae density decreases due to bleaching events. The zooxanthellae are colored different variations of brown, which obscures the host's pigmentation. Hence, once the zooxanthellae density decreases, the coral's pastel-like colors become visible. In extreme bleaching events the coral's tissue becomes white.

Keep in mind that these charts are typically designed for use on corals in natural low-nutrient reefs, thereby showing that pastel coloration is often a result of bleaching caused by factors other than low nutrient levels. Pastel coloration, therefore, might correspond with less than optimal coral health.

Here is an example of one such chart:

More information is available on the following website:

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