Abstract
With on-going climate change, coral susceptibility to thermal stress constitutes a central concern in reefconservation. In the Persian Gulf, coral reefs are confronted with a high seasonal variability in water temperature, and both hot and cold extremes have been associated with episodes of coral bleaching and mortality. Using physiological performance as a measure of coral health, we investigated the thermal susceptibility of the common acroporid, Acropora downingi, near Hengam Island where the temperature oscillates seasonally in the range 20.2–34.2 °C. In a series of two short-term experiments comparing coral response in summer versus winter conditions, we exposed corals during each season (1) to the corresponding seasonal average and extreme temperature levels in a static thermal environment, and (2) to a progressive temperature deviation from the annual mean toward the corresponding extreme seasonal value and beyond in a dynamic thermal environment. We monitored four indictors of coral physiological performance: net photosynthesis (Pn), dark respiration (R), autotrophic capability (Pn/R), and survival. Corals exposed to warming during summer showed a decrease in net photosynthesis and ultimately died, while corals exposed to cooling during winter were not affected in their photosynthetic performance and survival. Coral autotrophic capability Pn/R was lower at the warmer thermal level within eachseason, and during summer compared to winter. Corals exposed to the maximum temperature of summer displayed Pn/R < 1, inferring that photosynthetic performance could not support basal metabolic needs under this environment. Our results suggest that the autotrophic performance of the Persian Gulf A. downingi is sensitive to the extreme temperatures endured in summer, and therefore its populations may be impacted by future increases in water temperature.
Highlights
Increasing evidence of climate change toward a warmer environment with more frequent thermal anomalies points to the importance of assessing thermal susceptibility of corals in reef conservation (Berkelmans, 2002; Hoegh-Guldberg et al, 2014)
Experiment one: exposure to average and peak seasonal temperatures Coral net photosynthesis was relatively consistent between the seasonal extreme and mean thermal levels of winter (0.12 ± 0.03 versus 0.13 ± 0.03 SE mgO2 respectively, p = 0.946), while it was considerably lower at the seasonal extreme compared to the mean temperature in summer (0.06 ± 0.02 versus 0.12 ± 0.02 SE mgO2 respectively, p < 0.0001)
The net photosynthesis to dark respiration ratio photosynthesis to respiration ratio (Pn/R) showed a significant decrease with increasing temperature both within and between seasons (Fig. 2C)
Summary
Increasing evidence of climate change toward a warmer environment with more frequent thermal anomalies points to the importance of assessing thermal susceptibility of corals in reef conservation (Berkelmans, 2002; Hoegh-Guldberg et al, 2014). Estimating the thermal sensitivity of Persian Gulf corals is important for more effective reef conservation in the region. It can benefit our general understanding of the ability of corals to adapt or acclimate to temperature variations by providing quantitative information on the limits of coral performance in populations exposed to recurring thermal stress (Feary et al, 2013). The thermal threshold of bleaching, does not give much information about the effects of the non-bleaching range of temperature on physiological performance of corals, and cannot be generalized to reef areas where records of water temperature and bleaching events are scarce
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