Studies in the last few decades have shown that bleaching of various coral species are mitigated by high water flow. One of the well-supported theories behind is enhancement of mass transfer by water current. High flow causes a thinning of the momentum boundary layer as well as the diffusive boundary layer that facilitates gas flux between the coral and the environment, thus efflux of harmful reactive oxygen species (ROS) from coral tissues increases. However, no study has directly measured ROS in the boundary layer under the water current. I have measured concentrations of hydrogen peroxide, a common ROS, within a few mm above the coral surface under heat stress in a laboratory flume. As expected, under high temperature condition, concentrations of hydrogen peroxide over the corals and bleaching severity in a high-flow condition were lower than those in a low flow. The result supports the theory that the mass transfer of ROS controls degrees of coral bleaching. Also, there was interspecific variability: <em>Acropora tenuis</em> bleached more harshly than <em>Galaxea fascicularis</em> which produced less hydrogen peroxide and may be more heat tolerant. Moreover, it is known that cold temperature can also cause bleaching. Coral bleaching was tested under cold temperature in a similar experimental setup. The outcome is more complex as it did not clearly show ROS productions or expected bleaching responses. Possible reasons will be discussed. Exploring bleaching mechanisms under different temperature/flow regimes will help coral reef management and conservation efforts.

Primary Presenter: Atsushi Fujimura, University of Guam (fujimuraa@triton.uog.edu)

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