Mechanical limitations of reef corals during hydrodynamic disturbances

Abstract

[Extract] During hydrodynamic disturbances such as cyclones and tsunamis, coral colonies are partially damaged whereby colony branches or segments are removed or dislodged from the substrate altogether (Woodley et al. 1981; Connell et al. 1997). Afterwards, those coral fragments that are unable to reattach to the substrate (Smith and Hughes 1999) are tumbled about by ambient water motion, resulting in extensive mortality regimes (Knowlton et al. 1981; Woodley et al. 1981). Despite the well-known role of hydrodynamic disturbance in structuring coral reef communities (Connell 1978), the fate of different colonial morphologies is unknown because researchers are unable to observe the reef during such destructive disturbances. To circumvent this obstacle, the present study uses a classical engineering approach to demonstrate the likely consequences for coral colonies when pushed to their mechanical limits by water motion. Contrary to common belief (Chamberlain 1978; Schuhmacher and Plewka 1981; Vosburgh 1982), this study finds that the overall mechanical integrity of coral colonies is highly unlikely to be determined by the strength of their carbonate skeletons, but rather by the highly variable and significantly weaker strength of the reef substrate to which they are attached. Furthermore, as the hydrodynamic force increases, the engineering model predicts that the pruning of colony branches is unlikely to occur before whole-colony dislodgement. It is suggested therefore that the superior strength of coral skeleton, which is found to be increasingly stronger in more mechanically vulnerable morphologies, may be an evolutionary solution for reducing the likelihood of branch removal by localized impacts of waterborne debris during hydrodynamic disturbances