REPORT C. D. Storlazzi Æ E. K. Brown Æ M. E. Field K. Rodgers Æ P. L. Jokiel A model for wave control on coral breakage and species distribution in the Hawaiian Islands Received: 3 May 2004 / Accepted: 5 June 2004 / Published online: 5 November 2004 Ó Springer-Verlag 2004 Abstract The fringing reef off southern Molokai, Hawaii, is currently being studied as part of a multi-disciplinary project led by the US Geological Survey. As part of this study, modeling and field observations were utilized to help understand the physical controls on reef morphol- ogy and the distribution of different coral species. A model was developed that calculates wave-induced hydrodynamic forces on corals of a specific form and mechanical strength. From these calculations, the wave conditions under which specific species of corals would either be stable or would break due to the imposed wave- induced forces were determined. By combining this hydrodynamic force-balance model with various wave model output for different oceanographic conditions experienced in the study area, we were able to map the locations where specific coral species should be stable (not subject to frequent breakage) in the study area. The combined model output was then compared with data on coral species distribution and coral cover at 12 sites along Molokai’s south shore. Observations and model- ing suggest that the transition from one coral species to another may occur when the ratio of the coral colony’s mechanical strengths to the applied (wave-induced) forces may be as great as 5:1, and not less than 1:1 when corals would break. This implies that coral colony’s mechanical strength and wave-induced forces may be important in defining gross coral community structure over large (orders of 10’s of meters) spatial scales. Keywords Hawaiian Islands Æ Montipora Æ Pocillopora Æ Porites Æ Wave forces Introduction Computer models are being increasingly used to simu- late aspects of coral reefs, including carbonate produc- tion (Aigner et al. 1989; Bosence and Waltham 1989) and the geologic development of reef structures over the course of sea-level fluctuations (Bosscher and Schlager 1992). Such simulations help in understanding the vari- ous controls on reef structures; however, separating the influence of individual controls remains difficult. With- out quantification of the individual controls, computer simulations on reef growth will depend on the use of empirical data on biologic, geologic, and environmental variables (Graus et al. 1984; Scaturo et al. 1989; Bosscher and Schlager 1992). Typically, most simula- tions combine data from different geological and geo- graphical settings, causing significant problems when trying to understand how a specific biologic, geologic, or oceanographic system interacts to form a specific reef structure. It has been known for some time that there are strong qualitative correlations between wave energy and coral distribution (Rosen 1975; Geister 1977; Vosburgh 1977b; Dollar 1982; Done 1983; Massel and Done 1993; Rogers 1993; Blanchon and Jones 1997). Grigg (1998) has most recently discussed the interplay between wave energy and reef properties in the Hawaiian Islands, but as in most past studies, wave energy is classified in terms of the loosely divided categories of ‘‘low’’, ‘‘medium’’, or ‘‘high’’ wave energy regimes. Jokiel et al. (2004) show that maximum wave height in Hawaii is negatively correlated with coral cover, diversity, and species richness. There has yet to be, however, a large-scale quantitative investigation of these relationships that compares the motions exerted by waves upon the reef to the distribution of different stony coral species. Our goal Communicated by Geological Editor P.K. Swart C. D. Storlazzi (&) Æ M. E. Field Coastal and Marine Geology Program, US Geological Survey, Pacific Science Center, 1156 High Street, Santa Cruz, CA 95064, USA E-mail: cstorlazzi@usgs.gov Tel.: +1-831-4274721 Fax: +1-831-4274748 E. K. Brown Æ K. Rodgers Æ P. L. Jokiel Hawaii Institute of Marine Biology, University of Hawaii at Manoa, P.O. Box 1346, Kaneohe, Hawaii 96744, USA Coral Reefs (2005) 24: 43–55 DOI 10.1007/s00338-004-0430-x