An Investigation of Flow Regimes Around Coral Colonies and Implications for Coral Bleaching

Abstract

Coral reefs are fundamental to the prosperity of many countries throughout the world. In these countries, coral reefs support food production, tourism and emerging biotechnology development, as well as providing coastal protection from natural disasters. However, these reefs are incredibly fragile, as dramatically highlighted in 1997-1998 when 16% of the world‘s reef was effectively destroyed by a coral disease known as bleaching. Coral bleaching, which is the loss of symbolic algae and their pigments, has been associated with a variety of environmental triggers including sea-surface temperature, increased solar irradiation, sedimentation, and pollution. While coral bleaching is only one of several diseases experienced by reefs, the frequency and worldwide distribution of bleaching events places a priority on the understanding of its causes and consequences. Thermal stress is viewed as the primary cause, but unfortunately, due to the variability of coral bleaching, its etiology is mostly unknown. Little is understood of how hydrodynamic processes can influence coral resistance during a bleaching event. This study presents the first quantitative investigation of flow velocity around massive and branching coral colonies under oscillatory wave and unidirectional current conditions. Flume-based experiments are used to accurately manipulate the magnitude of wave and current, a factor that has been lacking in previous field studies. Both massive and branching coral forms were used in this study to ensure an accurate representation of the majority of corals. A total of 124 experimental runs were conducted in the wave flume with three different sizes of massive and branching coral colonies that were subjected to eight different flow regimes. Five clear conclusions were derived: (1) the presence of corals has predictable effects on hydrodynamic process around coral colonies; (2) different flow regimes yield different flow patterns around coral colonies; (3) for a given flow regime, velocity pattern is consistent regardless of coral growth form; (4) there are spatial variations in velocity around coral colonies; and (5) magnitude of change in velocity scales with colony size. Spatial variation in velocity around the massive and branching growth form at the colony scale resulted in localized regions with high and low water flow around coral colonies. As a result, there are localized regions around the coral colonies in which mass transfer may be enhanced or limited, resulting in the patchy nature of coral bleaching. The findings add to the understanding of coral reef hydrodynamics on a colony scale, and identify some factors that may impact the corals resistance and resilience in bleaching events.