Abstract
Naturally extreme temperature environments can provide important insights into the processes underlying coral thermal tolerance. We determined the bleaching resistance of Acropora aspera and Dipsastraea sp. from both intertidal and subtidal environments of the naturally extreme Kimberley region in northwest Australia. Here tides of up to 10 m can cause aerial exposure of corals and temperatures as high as 37 °C that fluctuate daily by up to 7 °C. Control corals were maintained at ambient nearshore temperatures which varied diurnally by 4-5 °C, while treatment corals were exposed to similar diurnal variations and heat stress corresponding to ~20 degree heating days. All corals hosted Symbiodinium clade C independent of treatment or origin. Detailed physiological measurements showed that these corals were nevertheless highly sensitive to daily average temperatures exceeding their maximum monthly mean of ~31 °C by 1 °C for only a few days. Generally, Acropora was much more susceptible to bleaching than Dipsastraea and experienced up to 75% mortality, whereas all Dipsastraea survived. Furthermore, subtidal corals, which originated from a more thermally stable environment compared to intertidal corals, were more susceptible to bleaching. This demonstrates that while highly fluctuating temperatures enhance coral resilience to thermal stress, they do not provide immunity to extreme heat stress events.
Highlights
Extreme temperature environments can provide important insights into the processes underlying coral thermal tolerance
Coral reefs are in serious decline worldwide[1] and increasingly suffer from episodes of thermally induced stress or coral bleaching, which lead to the breakdown of the vital endosymbiosis with dinoflagellates in the genus Symbiodinium spp.[2,3]
Despite the fact that corals growing in this region experience large daily temperature variability and temperature extremes of up to 37 °C27 (Fig. 1), branching Acropora and massive Dipsastraea corals were highly susceptible to coral bleaching when exposed to heat stress corresponding to ~20 degree heating days
Summary
Extreme temperature environments can provide important insights into the processes underlying coral thermal tolerance. Subtidal corals, which originated from a more thermally stable environment compared to intertidal corals, were more susceptible to bleaching This demonstrates that while highly fluctuating temperatures enhance coral resilience to thermal stress, they do not provide immunity to extreme heat stress events. Thermally variable environments seem to enhance coral thermal tolerance beyond that determined by maximum summer temperatures and can promote an increased resistance to climate change This may partly explain why hindcast predictions of bleaching events based on historical temperature variability rather than climatological maxima showed greater predictive power[17]
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