Shading as a mitigation tool for coral bleaching in three common Indo-Pacific species

Coelho, V.R.

Fenner, D.

Caruso, C.

Bayles, B.R.

Huang, Y.

Birkeland, C.

2017

Shading substantially reduced the degree of bleaching in Acropora muricata, Pocillopora damicornis and Porites cylindrica in American Samoa. Experiments were conducted outdoors at two sites on Ofu and Tutuila Islands. An aquarium experiment was set up near some reef-flat pools in the National Park of American Samoa on Ofu Island, using different levels of shading (none, 50% and 75%) early in conditions of cumulative thermal stress corresponding to NOAA's Coral Reef Watch-Bleaching Alert System. We analyzed the effects of cumulative thermal stress regarding coral growth, as well as color changes (evaluated using a standardize reference card) as a proxy for decreases in symbiont density and chlorophyll a content (i.e. bleaching). Thermally stressed corals grew less than controls, but corals without shading experienced a more substantial decrease in growth compared to those under 50% or 75% shade. The analysis of coral color showed that both levels of shading were protective against bleaching in conditions of cumulative thermal stress for all species, but were particularly beneficial for the most sensitive ones: A. muricata and P. cylindrica. Heavier shading (75%) offered better protection than lighter shading (50%) in this experiment, possibly because of the intense light levels corals were subjected to. Although there were limits to the extent shading could mitigate the effects of cumulative heating, it was very effective to at least Degree Heating Week (DHW) 4 and continued to offer some protection until the end of the study (DHW 8). In Tutuila, a shaded/not-shaded platform experiment was carried out in a reef pool in which corals have shown repeated annual summer bleaching for several years. This experiment was designed to investigate if shading could attenuate bleaching in the field and also if there were negative consequences to shading removal. The only factor controlled was light intensity, and our main conclusion was that overall corals on the platform became darker than field colonies in response to shading, but adjusted back to the same color level as field colonies after shade removal. However, the latter results are preliminary and need to be confirmed by future studies under more controlled conditions. As bleaching becomes more frequent and regular due to global warming, we should consider proactively using shading to help mitigate the effects of thermal stress and prolong the survival of at least some coral communities, until solutions to address global climate change become effective.

Coral bleaching

Corals

American Samoa

NMFS (National Marine Fisheries Service)

CoRIS (Coral Reef Information System)

Journal Article

Shading substantially reduced the degree of bleaching in Acropora muricata, Pocillopora damicornis and Porites cylindrica in American Samoa. Experiments were conducted outdoors at two sites on Ofu and Tutuila Islands. An aquarium experiment was set up near some reef-flat pools in the National Park of American Samoa on Ofu Island, using different levels of shading (none, 50% and 75%) early in conditions of cumulative thermal stress corresponding to NOAA's Coral Reef Watch-Bleaching Alert System. We analyzed the effects of cumulative thermal stress regarding coral growth, as well as color changes (evaluated using a standardize reference card) as a proxy for decreases in symbiont density and chlorophyll a content (i.e. bleaching). Thermally stressed corals grew less than controls, but corals without shading experienced a more substantial decrease in growth compared to those under 50% or 75% shade. The analysis of coral color showed that both levels of shading were protective against bleaching in conditions of cumulative thermal stress for all species, but were particularly beneficial for the most sensitive ones: A. muricata and P. cylindrica. Heavier shading (75%) offered better protection than lighter shading (50%) in this experiment, possibly because of the intense light levels corals were subjected to. Although there were limits to the extent shading could mitigate the effects of cumulative heating, it was very effective to at least Degree Heating Week (DHW) 4 and continued to offer some protection until the end of the study (DHW 8). In Tutuila, a shaded/not-shaded platform experiment was carried out in a reef pool in which corals have shown repeated annual summer bleaching for several years. This experiment was designed to investigate if shading could attenuate bleaching in the field and also if there were negative consequences to shading removal. The only factor controlled was light intensity, and our main conclusion was that overall corals on the platform became darker than field colonies in response to shading, but adjusted back to the same color level as field colonies after shade removal. However, the latter results are preliminary and need to be confirmed by future studies under more controlled conditions. As bleaching becomes more frequent and regular due to global warming, we should consider proactively using shading to help mitigate the effects of thermal stress and prolong the survival of at least some coral communities, until solutions to address global climate change become effective.

Grant no. NA09NMF4630105

https://repository.library.noaa.gov/view/noaa/49464

https://doi.org/10.1016/j.jembe.2017.09.016