REPORT William K. Fitt á Barbara E. Brown Mark E. Warner á Richard P. Dunne Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals Received:28June2000/Accepted:29November2000/Publishedonline:19May2001 Ó Springer-Verlag 2001 Abstract `Itshouldbeclearthattheuppertemperature limit for life cannot be accurately de®ned' Schmidt- Nielsen1996).Thethermalphysiologyofzooxanthellate reef corals is reviewed in this paper in the context of organismalandbiochemicalresponsesoccurringduring coralbleaching,withemphasisonmethodsofdetection andinterpretationofanimalandalgalsymbiontstress. Coralbleaching,aspresentlyde®nedintheliterature,is a highly subjective term used to describe a variety of conditions pertaining to low symbiont densities in the coral±algal complex, including response to thermal stress. Three general types of high-temperature bleach- ing are de®ned: physiological bleaching, which may or may not include higher-than-normal temperature re- sponses;algal-stressbleaching,involvingdysfunctionof symbioticalgaeathighlightand/orhightemperatures; andanimal-stressbleaching,wherecoralcellscontaining symbioticalgaeareshedfromthegastrodermallayerof cells. Since none of these methods of bleaching is mu- tually exclusive, a combination of intrusive and non- intrusive techniques is necessary to determine which mechanisms of symbiont loss are occurring. While quanti®cationofsymbiontdensities,algalpigments,and coral tissue biomass provide unambiguous evidence of bleaching severity, measurements of physiological and biochemicaldegradationoeradditionalcorrelativeev- idence of temperature stress. Pulse-amplitude-modulat- ed PAM) ¯uorometry has emerged as an easy and relatively inexpensive non-invasive technique for moni- toring symbiotic algal function both in situ and in the laboratory, when proper assumptions and interpreta- tions are made. The roles of global warming, water quality, acclimation/adaptation processes, and relation to coral disease and reef heterogeneity are also dis- cussed. A thorough understanding of the organismal responses occurring during bleaching will help explain changes in coral populations and in the coral reef community, and perhaps assist in predicting the future ofreefcoralsandcoralreefsduringthenextcenturyof globalclimatechange. Keywords Coral á Bleaching á Zooxanthellae á Chlorophyll¯uorescence á PAM¯uorometry á Temperaturethreshold Introduction and terminology Bleaching,de®nedasthelossofalgalsymbiontsand/or theirpigments,isaclassicresponseoftropicalsymbiotic coralsandrelatedcnidariansandmolluskstoavariety ofenvironmentalstresses.Thesestressesinclude,butare notlimitedto,decreasedsalinityreviewedinColesand Jokiel 1992); increased temperature reviewed in Jokiel andColes1990);lowtemperatureSteenandMuscatine 1987;Gatesetal1992);exposureatlowtideVaughan 1914; Yonge and Nicholls 1931a); sedimentation Bak 1978; Dollar and Grigg 1981); darkness Yonge and Nicholls1931a);solarradiationBrownetal.1994);ora combinationofthesefactors. Overthelast17years,recurrentlocal,regional,and global bleaching events have occurred, many of which have resulted in signi®cant coral mortality Jokiel and Coles 1990; Glynn 1993; Brown 1997a). The most widespreadandseverebleachingoccurredduring1997± 1998whenreefsinover42countrieswereaected,with massive coral mortality being noted in southern Japan Loya, personal communication) and Sri Lanka, the Maldives, India, Kenya, Tanzania, the Seychelles, and CoralReefs2001)20:51±65 DOI10.1007/s003380100146 W.K.Fitt&) á M.E.Warner InstituteofEcology,UniversityofGeorgia, Athens,Georgia30602,USA E-mail: ®tt@sparrow.ecology.uga.edu Tel.:+1-706-5423328 Fax:+1-706-5423344 M.E.Warner DepartmentofBotany,UniversityofGeorgia, Athens,Georgia30602,USA B.E.Brown á R.P.Dunne DepartmentofMarineSciencesandCoastalManagement, UniversityofNewcastleuponTyne,NE17RU,UK