An Overview of Environmental Engineering Methods for Reducing Coral Bleaching Stress
Daniel P. Harrison
Abstract
Globally, widespread coral bleaching events are increasing in both frequency and severity. Mortality of corals and the resulting reduction in coral cover from these events is recognised as a pressing threat to the ecosystem health of the Great Barrier Reef. An excess of temperature and/or light can contribute to the physiological processes that lead to bleaching. The underlying cause of the contemporary increase in these events has been attributed to rising ocean temperatures caused by anthropogenic warming of the climate. Experiments conducted on corals in aquaria and field observations of bleaching events have indicated that reduction in temperature, light, or both can lower the physiological stress to corals. These observations have led proponents to suggest a range of environmental engineering approaches that aim to mitigate the stress on coral during marine heatwaves to reduce the severity of coral bleaching. Technologies have been suggested that may be applicable across a range of spatial and temporal scales. Methods to directly reduce heat stress include piping and pumping cooler water from depth or enhancing water column mixing to remove stratification. Shading corals from solar radiation may enable them to tolerate higher water temperatures for longer, delay bleaching onset, and reduce both the severity and resulting mortality of bleaching. Actions that could temporarily reduce light intensity include installing physical structures such as shade cloth, ‘whitening’ the upper ocean with floating reflective surface films or microbubbles, and atmospheric options such as creating a fog or mist using seawater or other obscurants. Interventions that reduce incoming solar radiation over large enough areas, for sufficient lengths of time, could reduce both light and temperature stress by regionally altering the radiative transfer of energy into and out of the ocean, thereby cooling the upper water column. Regional scale interventions of this nature include marine haze brightening and marine cloud brightening, which target the direct and indirect aerosol effects of atmospheric aerosols, respectively. In this brief review, these proposed technologies are described with reference to their technical feasibility, potential benefits, limitations, and potential for unintended adverse consequences.