An enigmatic decoupling between heat stress and coral bleaching on the Great Barrier Reef.

Thomas M Decarlo,Hugo B Harrison

doi:10.7717/peerj.7473

Abstract

Ocean warming threatens the functioning of coral reef ecosystems by inducing mass coral bleaching and mortality events. The link between temperature and coral bleaching is now well-established based on observations that mass bleaching events usually occur when seawater temperatures are anomalously high. However, times of high heat stress but without coral bleaching are equally important because they can inform an understanding of factors that regulate temperature-induced bleaching. Here, we investigate the absence of mass coral bleaching on the Great Barrier Reef (GBR) during austral summer 2004. Using four gridded sea surface temperature data products, validated with in situ temperature loggers, we demonstrate that the summer of 2004 was among the warmest summers of the satellite era (1982–2017) on the GBR. At least half of the GBR experienced temperatures that were high enough to initiate bleaching in other years, yet mass bleaching was not reported during 2004. The absence of bleaching is not fully explained by wind speed or cloud cover. Rather, 2004 is clearly differentiated from bleaching years by the slow speed of the East Australian Current (EAC) offshore of the GBR. An anomalously slow EAC during summer 2004 may have dampened the upwelling of nutrient-rich waters onto the GBR shelf, potentially mitigating bleaching due to the lower susceptibility of corals to heat stress in low-nutrient conditions. Although other factors such as irradiance or acclimatization may have played a role in the absence of mass bleaching, 2004 remains a key case study for demonstrating the dynamic nature of coral responses to marine heatwaves.

Highlights

Mass coral bleaching events have increased in frequency since the 1980s in concert with rising sea surface temperature (SST) (Donner et al 2017; Hughes et al 2018a)
In 1998, Coral Reef Watch (CRW) implies that degree heating weeks (DHW) across nearly the whole Great Barrier Reef (GBR) did not exceed 2-3 °C-weeks, whereas OI-SSTv1 implies that large swaths of the southern and far northern GBR were exposed to 7-8 °C-weeks
DHWs derived from CRW during both 2002 and 2004 were high in the Coral Sea but nearly the entire GBR remained less than 3 °C-weeks

Summary

Introduction

Mass coral bleaching events have increased in frequency since the 1980s in concert with rising sea surface temperature (SST) (Donner et al 2017; Hughes et al 2018a). The correspondence of mass bleaching events with anomalously high SST is often considered indicative of a direct relationship between heat stress and coral bleaching The severity of mass bleaching events varies in space and time, and a range of environmental factors besides temperature can affect the bleaching sensitivity of corals. Four mass bleaching events have been recorded on Australia’s iconic Great Barrier Reef (GBR) and all have been associated with anomalously high SST. The GBR experienced another large-scale bleaching event just four years later in 2002, which affected ~54% of reefs (Berkelmans et al 2004). In both cases, severe bleaching was predominantly observed on coastal reefs (Berkelmans et al.2004). The most severe and widespread bleaching on the GBR occurred in 2016, when comparative surveys suggest approximately 91% of reefs showed signs of bleaching, with severe bleaching leading to widespread mortality in the northern third of the GBR (Hughes et al 2017)

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Discussion

Conclusion