Highly variable taxa-specific coral bleaching responses to thermal stresses

Abstract

Complex histories of chronic and acute sea surface temperature (SST) stresses are expected to trigger taxon- and location-specific responses that will ultimately lead to novel coral communities. The 2016 El Niño-Southern Oscillation provided an opportunity to examine large-scale and recent environmental histories on emerging patterns in 226 coral communities distributed across 12 countries from East Africa to Fiji. Six main coral communities were identified that largely varied across a gradient of Acropora to massive Porites dominance. Bleaching intensity was taxon-specific and was associated with complex interactions among the 20 environmental variables that we examined. Coral community structure was better aligned with the historical temperature patterns between 1985 and 2015 than the 2016 extreme temperature event. Additionally, bleaching responses observed during 2016 differed from historical reports during past warm years. Consequently, coral communities present in 2016 are likely to have been reorganized by both long-term community change and acclimation mechanisms. For example, less disturbed sites with cooler baseline temperatures, higher mean historical SST background variability, and infrequent extreme warm temperature stresses were associated with Acropora-dominated communities, while more disturbed sites with lower historical SST background variability and frequent acute warm stress were dominated by stress-resistant massive Porites corals. Overall, the combination of taxon-specific responses, community-level reorganization over time, geographic variation, and multiple environmental stressors suggest complex responses and a diversity of future coral communities that can help contextualize management priorities and activities.

Large geographic variability in the resistance of corals to thermal stress

“…Yet, we show here and elsewhere that sensitivity is highly variable and geographically contextual (McClanahan et al, 2019(McClanahan et al, , 2020.…”

“…Coral sensitivity to thermal stress was based on coordinated field surveys of coral bleaching during the 2016 El Niño‐Southern Oscillation (ENSO). Surveys were conducted within 3 weeks of peak SSTs when bleaching was greatest (McClanahan et al, 2020). We surveyed 226 sites in 12 countries during summer months between March and September 2016.…”

“…Exposure will also vary over time in unique ways for each region (Houk et al, 2020). Climate change and increasing ocean variability in heating patterns mean that exposure and locally adapted sensitivity could change and potentially decouple in the future (Abram et al, 2020;McClanahan et al, 2019McClanahan et al, , 2020. Moreover, there is the question of how good bleaching or the loss of coral colour are at measuring sensitivity to thermal stress (Buddemeier et al, 2004).…”

“…Under the increasing impacts of climate change, coral bleaching, or the rapid decline of coral’s endosymbiotic microalgae, is an early and obvious indicator of thermal stress (Hughes et al, 2018). Nevertheless, bleaching can be patchy in space and time, whereby some corals bleach either more or less than expected given the exposure to thermal stress, and this patchiness can be mediated by historical and current environmental conditions (McClanahan et al, 2020; Sully et al, 2019). Ultimately, the resistance of corals to stress should be influenced by variation in geographic and evolutionary history and associated genotypic diversity, adaptation rates and taxonomic composition (Edmunds & Gates, 2008; Palumbi et al, 2014).…”

The past century of coral bleaching in the Saudi Arabian central Red Sea

“…Porites are generally considered to be relatively resistant to stress ( Darling et al, 2012 ; McClanahan et al, 2020 ) and have shown evidence of acclimatization to repeated marine heatwaves ( DeCarlo et al, 2019 ), meaning that they are not necessarily representative of entire coral communities. It is also possible that some stress bands could arise from disturbances other than high temperatures, such as disease.…”