Coral Growth under Thermal Stress and Different Levels of Light Intensity in Acropora aspera.

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Klein, Devan Y; Barana, Mary W; Obilor, Leeanne A
Ecology and Evolutionary Biology; Marine Biology; Terrestrial and Aquatic Ecology
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In this study we examined the growth rates of coral species Acropora aspera in conditions of high thermal stress which are expected to become more common because of climate change. During higher than normal temperature periods, corals can experience bleaching which is the loss of algal symbionts from their tissue. Since corals depend on these photosynthetic organisms for most of their nutrition, this loss can lead to mass mortality. However, if the period of thermal stress happens during cloudy weather, bleaching events are less severe, and thus it is possible that shading reefs could help mitigate the effects of warmer waters. In our experiment a total of 250 corals, about 5-10 cm tall, originally from American Samoa, were separated into eight tanks with approximately 30 to 32 specimens in each. Two of the aquaria were controls at 26 degrees Celsius and the remaining six tanks were maintained at 31 degrees Celsius. Among the latter ones, light intensity was reduced by 50 percent in two of aquaria, and by 75 percent in another two, after degree heating week (DHW) 01 was reached (as described in the methodology designed by the Coral Reef Watch Satellite Bleaching Alert System of the National Oceanic and Atmospheric Administration). Coral buoyant weight was taken at the beginning and at the end of the experiment when DHW 08 was reached (in approximately 34 days). Daily coral growth was estimated by subtracting the final weight from the initial one, and dividing that value by the total number of days of the experiment. Water quality was monitored daily, and temperature was recorded every 10 min using loggers. The mean monthly growth for control corals was 0.27 g (SE± 0.054, n=62, buoyant weight), for corals in high temperature without shading was 0.047 g (SE ± 0.058, n=64), with 50 percent shading was 0.29 g (SE ± 0.084, n=61), and with 75 percent shading was 0.26 g (SE± 0.060, n=63). The statistical analysis showed that there was a significant difference in growth between corals in high temperature without shading in comparison to all other corals. However there was no statistical difference between controls and corals under thermal stress that received shading, nor was there a significant difference between growth under different shading treatments. We concluded that by reducing irradiance, by either 50 or 75 percent, these corals were able to better cope with conditions of cumulative thermal stress, and therefore this strategy could be effective in managing wild populations.