Appraisal of coral bleaching thresholds and thermal projections for the northern Red Sea refugia

Abstract

Corals in the northern Red Sea exhibit high thermal tolerance despite the increasing heat stress. It is assumed that corals throughout the Red Sea have similar bleaching thresholds (32°C or higher), and hence greater bleaching tolerance of corals in the northern Red Sea region is likely due to lower ambient water temperatures (25–28°C) that remain well below the corals’ physiological maxima. Whether bleaching patterns across the Red Sea are independent of the local maximum monthly mean of seawater temperature and aligned with an assumed 32°C threshold has yet to be determined. Here, we used remotely sensed surface sea temperature data spanning 1982–2020 to model spatial distributions of Degree Heat Weeks across the Red Sea in relation to assumed coral thermal threshold values of 30, 31, and 32°C. We also used the Coupled Model Intercomparison Project Phase 5 model outputs to predict warming trends in the Red Sea under different greenhouse gas representative concentration pathways (RCPs). We show that applying 32°C thresholds dramatically reduces effective Degree Heat Weeks in the north, but not in central or southern Red Sea regions, a finding that is consistent with historical bleaching observations throughout the Red Sea. Further, model predictions under the most extreme RCP8.5 scenario exhibited ~3°C warming by the end of the 21st century (1998-2020) throughout the Red Sea with less pronounced warming for the northern Red Sea (2–2.5°C) compared to the central and southern regions (2.7–3.1°C). This warming rate will remain below the assumed thermal threshold for the northern Red Sea which should help this region to serve as refugia (i.e., maintaining favorable temperatures) for corals to persist for decades ahead. Together, our results support the notion that corals have similar thresholds throughout the Red Sea; hence, coral bleaching thresholds are independent of the local maximum monthly mean. Consequently, where regional warming projections suggest the northern Red Sea will not reach assumed bleaching thresholds (32°C) before the end of the 21st century, coral reefs in the northern region may be among the last standing against climate change.

Heatwaves and a decrease in turbidity drive coral bleaching in Atlantic marginal equatorial reefs

“…DHW, MHWs, and SSTA are used to detect these temperature changes and are therefore good predictors of bleaching phenomena in this low-latitude region. Some reef regions (e.g., turbid-zone reefs) appear independent of the local thermal threshold (i.e., MMM+1°C) and suggest that other processes may drive the coral bleaching threshold (Eladawy et al, 2022). This may be the case for our study area.…”

Diversification of refugia types needed to secure the future of coral reefs subject to climate change

“…Identifying coral reef refugia to climate change can be improved by recognizing coral sensitivity and recovery, such as adaptive capacity, that reduces sensitivity to exposures (Bairos‐Novak et al., 2021). Adaptive capacity is increasingly being recognized as highly variable among taxa and locations and influenced by connectivity (Asner et al., 2022; Eladawy et al., 2022; McLachlan et al., 2020; McManus et al., 2021). Sensitivity to thermal exposure is variable at many spatial scales and likely driven by interactions between historical exposure and species acclimation and adaptation (Louis et al., 2016; Evensen et al., 2022; McClanahan, Darling, et al., 2020; McClanahan, Maina, et al., 2020).…”

“…Detailed field observations of coral communities over bleaching events that can distinguish the least stressed, most resistant, or fastest recovering corals at smaller scales will be critical to developing metrics and proxies at larger areas relevant to conservation, including national, regional, and global scales. Given that ecoregional differences in coral sensitivity can be shaped by local ecological and evolutionary histories, identifying local characteristics of resistance and refugia may scale up to larger conservation priorities and a global portfolio of refugia (Eladawy et al., 2022; McClanahan, Maina, et al., 2020). By using more environmental metrics and proxies from satellites, one can gain a better understanding of the relationships between the environment and biodiversity at larger spatial scales (Pilowsky et al., 2022).…”

“…Avoidance refugia have stable and cooler water temperatures, resistance refugia have coral assemblages with lower sensitivity to extreme heat and subsequently less bleaching and mortality, and recovery refugia have the ecological capacity to recover after bleaching and mortality. There is abundant evidence that these refugia types vary considerably with geography (Eladawy et al., 2022; Hedouin et al., 2020; McClanahan, Darling, et al., 2020; McClanahan, Maina, et al., 2020; Roff & Mumby, 2012). Ultimately, coral reefs have the best chance to survive and function within these 3 types of refugia when other local pressures are mitigated through good management, such as sustainable fisheries management and pollution reduction.…”

Spatial variation in spawning timing for multi-species Acropora assemblages in the Red Sea

“…The Red Sea spans 18°in latitude and harbors unique and diverse coral reefs that live in contrasting thermal and environmental regimes along the latitudinal gradient (Osman et al, 2018;Berumen et al, 2019) and between west and east coasts (Eladawy et al, 2022). Such diverse environmental conditions drive large variability in spawning timing, based on periodic observations across a few key sites over the past few decades (summarized in Ziegler et al, 2019).…”