Environmental Drivers of Variation in Bleaching Severity of Acropora Species during an Extreme Thermal Anomaly

Abstract

High sea surface temperatures caused global coral bleaching during [2015][2016]. During this thermal stress event, we quantified within-and among-species variability in bleaching severity for critical habitat-forming Acropora corals. The objective of this study was to understand the drivers of spatial and species-specific variation in the bleaching susceptibility of these corals, and to evaluate whether bleaching susceptibility under extreme thermal stress was consistent with that observed during less severe bleaching events. We surveyed and mapped Acropora corals at 10 sites (N = 596) around the Lizard Island group on the northern Great Barrier Reef. For each colony, bleaching severity was quantified using a new image analysis technique, and we assessed whether small-scale environmental variables (depth, microhabitat, competition intensity) and species traits (colony morphology, colony size, known symbiont clade association) explained variation in bleaching. Results showed that during severe thermal stress, bleaching of branching corals was linked to microhabitat features, and was more severe at reef edge compared with lagoonal sites. Bleaching severity worsened over a very short time-frame (∼1 week), but did not differ systematically with water depth, competition intensity, or colony size. At our study location, within-and among-species variation in bleaching severity was relatively low compared to the level of variation reported in the literature. More broadly, our results indicate that variability in bleaching susceptibility during extreme thermal stress is not consistent with that observed during previous bleaching events that have ranged in severity among globally dispersed sites, with fewer species escaping bleaching during severe thermal stress. In addition, shaded microhabitats can provide a refuge from bleaching which provides further evidence of the importance of topographic complexity for maintaining the biodiversity and ecosystem functioning of coral reefs.

Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event

“…Corals assimilate nutrients from seabirds nesting on adjacent islands (Lorrain et al, 2017), and the ratio of nitrogen to phosphorous in seawater adjacent to seabird colonies is within the range considered optimal for increasing coral growth, photosynthetic efficiency and resistance to heat stress (N:P ratio of 13-32 in seawater compared to optimal ratio of 11-29; Allgeier et al, 2014;Savage, 2019;Wiedenmann et al, 2013). Other field studies have similarly demonstrated reduced susceptibility to bleaching for corals in sheltered areas with low water flow (Hoogenboom et al, 2017;McClanahan, Ateweberhan, Muhando, Maina, & Mohammed, 2007;Pineda et al, 2013;Sheppard, 1999). The lagoon that maintained most of its live coral is the smallest and most enclosed and therefore experiences the least water flow, warmest temperatures and highest temperature fluctuations under normal conditions (Pugh & Rayner, 1981;Sheppard et al, 2017).…”

“…The lagoon that maintained most of its live coral is the smallest and most enclosed and therefore experiences the least water flow, warmest temperatures and highest temperature fluctuations under normal conditions (Pugh & Rayner, 1981;Sheppard et al, 2017). Other field studies have similarly demonstrated reduced susceptibility to bleaching for corals in sheltered areas with low water flow (Hoogenboom et al, 2017;McClanahan, Ateweberhan, Muhando, Maina, & Mohammed, 2007;Pineda et al, 2013;Sheppard, 1999). One likely explanation for this pattern is that corals from small lagoons are adapted to warmer water and/or greater temperature variability and thus are better able to withstand temperature stress (Donner, 2011;Guest et al, 2012;Middlebrook, Hoegh-Guldberg, & Leggat, 2008;Oliver & Palumbi, 2011;Safaie et al, 2018;Schoepf, Stat, Falter, & McCulloch, 2015).…”

Coral microbiome diversity reflects mass coral bleaching susceptibility during the 2016 El Niño heat wave

“…However, the mass bleaching observed at both our relatively turbid and clear sites (of similar extent, Figure ) would thus suggest small‐scale environmental variability afforded through complex reef habitats (e.g., shading from overhangs, Cacciapaglia & Woesik, ) are more important in providing refuge from heat stress. Bleaching of P. lutea and loss of C15 type cells at both sites similarly suggests localized small‐scale amplification of heat stress, for example, by high light (Hoogenboom et al, ). Second, alternate symbiont types of unbleached typically heat‐sensitive A. muricata (D1 type, Praslin) and bleached typically heat‐tolerant C. aspera (D1 type) reflects some capacity for Symbiodiniaceae community re‐organization needed for thermal acclimatization.…”

“…Microbiomes were generally conserved among bleached versus unbleached colonies for the two species of Acropora sp. suggesting small‐scale environmental variability (e.g., shading from surrounding substrates, sensu Hoogenboom et al, ) are likely critical in ensuring persistence of heat stress‐sensitive corals in oceanically isolated reef systems such as Seychelles.…”

“…C3z/ C3 associations, lack of bleaching for A. gemmifera at Praslin may indicate small-scale environmental variability, as similarly observed for in-shore Acropora sp. colonies on the Great Barrier Reef also in 2016(Hoogenboom et al, 2017). Turbidity has been suggested to be critical in providing refuge against bleaching intensity(LaJeunesse et al, 2010;Oliver & Palumbi, 2009;Ulstrup & Van Oppen 2003).…”