Thermal refugia against coral bleaching throughout the northern Red Sea

Abstract

Tropical reefs have been impacted by thermal anomalies caused by global warming that induced coral bleaching and mortality events globally. However, there have only been very few recordings of bleaching within the Red Sea despite covering a latitudinal range of 15° and consequently it has been considered a region that is less sensitive to thermal anomalies. We therefore examined historical patterns of sea surface temperature (SST) and associated anomalies (1982-2012) and compared warming trends with a unique compilation of corresponding coral bleaching records from throughout the region. These data indicated that the northern Red Sea has not experienced mass bleaching despite intensive Degree Heating Weeks (DHW) of >15°C-weeks. Severe bleaching was restricted to the central and southern Red Sea where DHWs have been more frequent, but far less intense (DHWs <4°C-weeks). A similar pattern was observed during the 2015-2016 El Niño event during which time corals in the northern Red Sea did not bleach despite high thermal stress (i.e. DHWs >8°C-weeks), and bleaching was restricted to the central and southern Red Sea despite the lower thermal stress (DHWs < 8°C-weeks). Heat stress assays carried out in the northern (Hurghada) and central (Thuwal) Red Sea on four key reef-building species confirmed different regional thermal susceptibility, and that central Red Sea corals are more sensitive to thermal anomalies as compared to those from the north. Together, our data demonstrate that corals in the northern Red Sea have a much higher heat tolerance than their prevailing temperature regime would suggest. In contrast, corals from the central Red Sea are close to their thermal limits, which closely match the maximum annual water temperatures. The northern Red Sea harbours reef-building corals that live well below their bleaching thresholds and thus we propose that the region represents a thermal refuge of global importance.

Marine heatwaves reveal coral reef zones susceptible to bleaching in the Red Sea

“…Due to the lack of extensive in situ bleaching data in the Red Sea, it is hard to relate a MHW index to bleaching severity. For instance, Yanbu (Figure S5) and Al Khawkhah (Figure S10) experienced medium bleaching (10%–30%) in 1998 (Osman et al, ) yet their MHWs showed duration: 13 versus 11 days, maximum intensity: 2.3 versus 1.7°C, cumulative intensity: 26 versus 14°C‐days, respectively. However, a bleaching severity indicator may be unearthed with further in situ research and analysis into the additional metrics offered by this methodology (Hobday et al, ).…”

“…To do so, we examined a number of different SST percentile thresholds (90th, 95th, 98th and 99th) and different event durations (5, 7 and 10 days). We also investigated different thermal thresholds (30, 30.5, 31, 31.5 and 32°C average SST during events) to set as an additional restriction to MHWs, as corals in the NRS exhibited a high tolerance relative to their maximal summer SST means (Fine et al, ; Osman et al, ). We found that values exceeding the 95th percentile threshold for a minimum duration of 7 days and a minimum average SST of 30°C over the duration of the event was the best combination to meet our objective.…”

Spatial and Intergeneric Variation in Physiological Indicators of Corals in the South China Sea: Insights Into Their Current State and Their Adaptability to Environmental Stress

“…These regions are likely to be in higher latitudes, as several studies have shown that global warming will force a northward and southward migration of corals from the tropics (Beger et al, ; Hoegh‐Guldberg et al, ). Potential coral reef refuges are recognizable through specific features, notably an increase in coral populations through habitat gain or retention, lack of thermal anomalies, higher thermal tolerance, and heightened coral species diversity (Cacciapaglia & van Woesik, ; Osman et al, ; Tkachenko & Soong, ; Yamano et al, ). Based on these characteristics, we can point to several potential coral refuges worldwide.…”

Natural Climate Oscillations may Counteract Red Sea Warming Over the Coming Decades

“…Repetitive exposure to heat stress is considered among the most critical threats to coral reef communities, and could lead to mass coral bleaching events (Hughes et al, 2018). Recently, a number of studies have indicated that the Red Sea coral reefs have been significantly affected by bleaching caused by strong heat waves (Furby et al, 2013;Kotb et al, 2004;Osman et al, 2018). Because of their relatively small volume and the slow rate of their water renewal, marginal semienclosed seas, such as the Red Sea, are more susceptible to global warming.…”