Marine heatwave causes unprecedented regional mass bleaching of thermally resistant corals in northwestern Australia

Morane Le Nohaïc,Ryan Lowe,Christopher E Cornwall,Verena Schoepf,Malcolm T Mcculloch,Steeve Comeau,Claire L Ross

doi:10.1038/s41598-017-14794-y

Abstract

In 2015/16, a marine heatwave associated with a record El Niño led to the third global mass bleaching event documented to date. This event impacted coral reefs around the world, including in Western Australia (WA), although WA reefs had largely escaped bleaching during previous strong El Niño years. Coral health surveys were conducted during the austral summer of 2016 in four bioregions along the WA coast (~17 degrees of latitude), ranging from tropical to temperate locations. Here we report the first El Niño-related regional-scale mass bleaching event in WA. The heatwave primarily affected the macrotidal Kimberley region in northwest WA (~16°S), where 4.5–9.3 degree heating weeks (DHW) resulted in 56.6–80.6% bleaching, demonstrating that even heat-tolerant corals from naturally extreme, thermally variable reef environments are threatened by heatwaves. Some heat stress (2.4 DHW) and bleaching (<30%) also occurred at Rottnest Island (32°01’S), whereas coral communities at Ningaloo Reef (23°9’S) and Bremer Bay (34°25’S) were not impacted. The only other major mass bleaching in WA occurred during a strong La Niña event in 2010/11 and primarily affected reefs along the central-to-southern coast. This suggests that WA reefs are now at risk of severe bleaching during both El Niño and La Niña years.

Highlights

Coral reefs are in serious decline worldwide due to a combination of increasing local and global anthropogenic pressures[1,2]
We show here that marine heatwaves associated with extreme climatic events such as the record-strength 2015/16 El Niño have the potential to cause unprecedented regional-scale mass bleaching, even in coral reef regions that harbour naturally heat-resistant corals that have escaped mass bleaching in previous El Niño years
This occurred in the macrotidal Kimberley region in northwestern Australia during the austral summer of 2016, where highly diverse and naturally stress-resistant coral reef communities have been observed to thrive under conditions that corals from more typical reef environments would usually not survive

Summary

Introduction

Coral reefs are in serious decline worldwide due to a combination of increasing local and global anthropogenic pressures[1,2]. In 2015/16, unusually high ocean temperatures associated with one of the strongest El Niño events on record triggered an unprecedented global coral reef crisis, initiating what would become the third documented global mass bleaching event[17] This event was estimated to have impacted 38% of the world’s coral reefs and became the longest and most severe mass bleaching event on record[5,17]. The macrotidal Kimberley region is one of the most extreme natural coral reef environments in the world, with tidal ranges up to 12 m, turbid waters associated with strong tidal currents and terrestrial sediment discharge, and sea surface temperatures (SST) exceeding 30 °C for five months per year[22,23,24,25]. Since El Niño events will likely increase in frequency and intensity due to climate change[36], understanding how these extreme climatic events impacts coral reefs in WA is critical to predict their persistence under continued ocean warming and climate change

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