Abstract
Coral bleaching is the breakdown of symbiosis between coral animal hosts and their dinoflagellate algae symbionts in response to environmental stress. On large spatial scales, heat stress is the most common factor causing bleaching, which is predicted to increase in frequency and severity as the climate warms. There is evidence that the temperature threshold at which bleaching occurs varies with local environmental conditions and background climate conditions. We investigated the influence of past temperature variability on coral susceptibility to bleaching, using the natural gradient in peak temperature variability in the Gilbert Islands, Republic of Kiribati. The spatial pattern in skeletal growth rates and partial mortality scars found in massive Porites sp. across the central and northern islands suggests that corals subject to larger year-to-year fluctuations in maximum ocean temperature were more resistant to a 2004 warm-water event. In addition, a subsequent 2009 warm event had a disproportionately larger impact on those corals from the island with lower historical heat stress, as indicated by lower concentrations of triacylglycerol, a lipid utilized for energy, as well as thinner tissue in those corals. This study indicates that coral reefs in locations with more frequent warm events may be more resilient to future warming, and protection measures may be more effective in these regions.
Highlights
Coral bleaching is a stress response in which corals lose their symbiotic dinoflagellate algae [1]
We examined changes in coral skeletal growth rates and partial mortality scars [11] to investigate the impact of the bleaching event in 2004 [16] on corals from different temperature variability regimes
Corals from Butaritari were more severely affected by bleaching in 2004 (Figure 2); on average corals from Butaritari had a 45% reduction in skeletal extension rates in 2004 compared with a 22% reduction at Abaiang and North Tarawa (p = 0.055, permutation test between sites, Figure 4)
Summary
Coral bleaching is a stress response in which corals lose their symbiotic dinoflagellate algae [1]. The accumulation of temperature stress in excess of coral bleaching thresholds, often expressed as the accumulation of degree-heating-months (DHM) or degree-heating-weeks (DHW), is commonly used to predict mass bleaching events [5,6]. Observations indicating that mass bleaching events have recently become more common [7,8,9,10,11,12], combined with projected increases in heat stress, have prompted dire predictions for the future of coral reefs under unabated greenhouse gas emissions scenarios [13,14]. Corals might acquire more thermally-resistant symbionts [26,27], or might increase their own physiological mechanisms to reduce bleaching susceptibility by producing oxidative enzymes [28] or photoprotective compounds [29]. On a reef-wide scale, more resistant taxa may increase in dominance after bleaching [30,31]
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