Int J Earth Sciences Geol Rundsch) 2001) 90 : 426��437 DOI 10.1007/s005310000125 ORIGINAL PAPER Joan A. Kleypas �� Robert W. Buddemeier Jean-Pierre Gattuso The future of coral reefs in an age of global change Received: 15 October 1999 / Accepted: 10 July 2000 / Published online: 18 October 2000 Springer-Verlag 2000 Abstract Coral reefs are the only ecosystem that is strongly defined by a geological component �� most definitions require that the biological community produces its own build-up of calcium carbonate. In terms of ��reef-building,�� the geological record reveals that coral reefs have flourished over the past few mil- lion years, particularly during interglacial periods. Based on our observations of modern-day reefs, which are limited to the past few centuries, we tend to link ��coral reef health�� to carbonate production however, reef ecosystems face future global-scale environmental changes that may decrease their reef-building capacity. In contrast to past discussions of the factors which determine reef-building potential by a coral reef com- munity, the essential question that arises from this review is: How important is reef building to a coral reef community? Keywords Coral reef �� Global change �� Calcium carbonate �� Saturation state �� Temperature �� CO2 Introduction Coral reefs are threatened directly at local and regional scales by many human activities, including over-harvesting, deforestation, and general decline of coastal environments due to increasing population pressures. Over geological time, reefs have also been affected by global scale changes in seawater chemistry, temperature, and sea-level fluctuations. Of these three, global changes in both seawater chemistry and temperature will probably have the most profound impacts on coral reefs within the next century. In this paper we examine current definitions applied to reefs of the past as well as to those of the present. We then examine the implications of applying such definitions to coral reefs of the future. Past and present views of coral reefs ��C'est une merveille de voir chacun de ces atollons, environn�� d'un grand banc de pierre tout autour, n'y ayant point d'artifice humain.�� ��It is wonderful to see each of the atolls completely surrounded by a great bank of stone, of no human construction.�� Fran��ois Pyrard de Laval 1605) as quoted by Darwin 1842) Early references to reefs reveal fascination with the reef structure. The foregoing quotation by Pyrard de Laval reflects the natural curiosity of early explorers about the origins of the ��great banks of stone��. Such fascination was due partly to the fact that so few Europeans, who provided the first records of coral reefs, had seen coral reefs before. Humans continue to focus on the rock component of reefs for several reasons. One is that coral reefs pose a major threat to navigation. Indeed, the term ��reef�� was originally defined as a navigational hazard, and ��coral reef�� was merely a navigational hazard with corals on it. Another reason is that many of the early reef researchers concentrated on the reef struc- ture. Darwin 1842) was certainly preoccupied with just how reefs formed, as was Dana 1872), and many others of the present century Yonge 1930 Steers 1937 Stoddart et al. 1978). Davis 1928) exemplified the geologist's attitude in the following: J. A. Kleypas )) Oceanography Section, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA E-mail: kleypas@ncar.ucar.edu Phone: +11-303-4971316 Fax: +11-303-4971700 R. W. Buddemeier Kansas Geological Survey, University of Kansas, Lawrence, KS 66047, USA J.-P. Gattuso Observatoire Oc��anologique, ESA 7076 CNRS-UPMC, B.P. 28, 06234 Villefranche-sur-mer Cedex, France

427 The biology and especially the symbiosis of the reefs are unquestionably important subjects in themselves, but the opportunity for the establish- ment and growth of reefs is so largely determined by the physiographic conditions of insular and continental coasts, over which reef-building organ- isms have no control that those physiographic con- ditions necessarily assume the leading role in the problem [i.e., the ��coral reef�� problem] under dis- cussion. Economic geologists enhanced the focus on reefs when it became clear that ancient reef deposits were associated with vast oil reserves Chapman 1977). Geologists naturally focus on reef structures because of their prominence in the geological record, and because they provide us with the materials necessary for studying past environments. To non-geologists, reefs have long been recognized for their role in shoreline protection and for their physical complexity which promotes high biodiversity and gross productivi- ty. Only in the past century has thinking about coral reefs shifted to an ecosystem perspective, similar to the way a rainforest is viewed. Reefs are certainly rec- ognized as complex ecosystems with both biological and geological features, and it is difficult to address reef ecosystems separately from their geological nature. Several papers in the 1970s and 1980s reflected the struggle with defining a coral reef Heckel 1974 Preobrazhensky 1977 Hubbard 1985 Buddemeier and Hopley 1988), and biologists and geologists still appear to favor slightly different definitions: geologists refer to reefs as ��sedimentary systems�� James 1983), whereas biologists focus on the organisms that build and/or reside on reefs. Fagerstrom 1997) and Wood 1998, 1999) provide linkages between the two by emphasizing the importance of framework builders in the definition of fossil reefs: an emphasis which is based on an ecological concept of benthic organism and community function, and which relates to the ��ge- ological�� features of in situ production, topography, and wave resistance Table 1). The bio/geo definition of coral reef is confounded at those ��marginal reef�� areas where reef-building communities occur and appear to function ecologically as reefs, but have not produced a coral reef structure. Buddemeier and Hopley 1988) addressed this dichot- omy by implying that all coral reef communities have the capacity to build reefs, but the surrounding envi- ronment ultimately determines whether these com- munities will ��turn on�� or ��turn off�� in terms of reef building. Drowned reefs, or ��give-up�� reefs a geologi- cal term coined by Neumann and Macintyre 1985) are one example where changing environmental con- ditions e.g., the decrease in light levels due to sea- level rise) turned off carbonate production. Budde- meier and Hopley 1988) also distinguished between ��coral reef,�� a sedimentary structure produced by a living coral reef community ��coral reef community��, which has the potential for reef production and ��coral community�� which does not have that potential. This convention has been adopted in recent coral reef clas- sifications such as in ReefBase McManus and Ver- gara 1998), which attempts to distinguish between coral reefs which have underlying reef structure) and ��non-reef building coral communities.�� Components of a definition of ��coral reef�� Most recent definitions of ��coral reef�� or when ��reef�� or ��organic reef�� implies a coral reef) include both biological and geological components, and occa- sionally include environmental requirements Ta- ble 1). It is obvious that the concept has evolved from its navigational application to a more scientific one, but not to any single consensus definition. Table 1 Key words found in ��coral reef�� definitions Category Key words Referencesa Biological features P Organic, biogenic 2��4, 6, 7, 9, 11, 17, 18, 20 ��coral community��) P Interlocking coral and algal colonies framework) 1, 4, 9, 12, 19 P Mostly ��hermatypic�� corals and algae and other sessile organisms 1, 6, 9, 12, 15, 16, 17 Geological features ��reef��) P Carbonate 2, 3, 6, 7, 16, 17, 18, 20 P In situ buildup 2, 3, 7, 15��17, 19, 20 P Topographic relief 2, 3, 7, 10, 12, 16, 18 P Wave resistant 1, 4, 6, 10, 12, 13, 17 P Cemented/consolidated 8, 9, 12, 13, 19 Environmental requirements P Marine 9, 17 P Warm tropical) 9, 19 P Well lit 9 Other P Navigational hazard 4, 5, 8, 12, 13 a 1 Weyl 1970) 2 Braithwaite 1973) 3 Heckel 1974) 4 Davis 1977) 5 Gross 1977) 6 Ross 1977) 7 Longman 1981) 8 Thurman 1981) 9 Levinton 1982) 10 Schumacher and Zibrowius 1985) 11 Fagerstrom 1987) 12 Stowe 1987) 13 Gross 1987) 14 Scoffin 1987) 15 Buddemeier and Hopley 1988) 16 Riding 1989) 17 Achituv and Dubinsky 1990) 18 Wright and Burchette 1996) 19 Beer 1997) 20 Wood 1999)