Quantifying damage to coral colonies by waterborne debris during hydrodynamic disturbances

Physical bombardment by waterborne debris is a common disturbance in shallow coral reef systems. During hydrodynamic disturbances, such as tropical storms, increases in water velocity elevate drag forces acting on objects—ranging in size from sand to boulders and coral colonies—dislodging and propelling them into nearby coral colonies. Impact by debris can cause a number of injuries, ranging from intra-colony damage (e.g., branch breakage or tissue death) to whole-colony dislodgment. However, the bombardment process is poorly understood, given that it is difficult to observe in situ as hydrodynamic disturbances occur. Using 3D coral scans representing five characteristically different growth forms and finite element analysis, I simulated bombardment scenarios by applying increasing point forces to colony meshes. I measured the force required to cause breakage and where that breakage occurred, and found high rates of intra-colony breakage (a mean of 23% across growth forms). There was a significant interaction between colony surface area to volume ratios (SA:V) and the damage outcome (branch breakage or whole-colony dislodgement), but generally the impact force necessary to result in damage decreased as SA:V increased. Traditional models of coral damage during storms only consider hydrodynamic force, however, the results presented here show that bombardment may be the dominant process damaging and killing reef corals during hydrodynamic disturbances.