Did corals on a turbid reef bleach less during the 2020 heat stress event?

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The impacts of climate change have become more evident in recent years. Projections suggest that heat stress events will likely become more frequent and severe over the next century, threatening the Coral Triangle’s high diversity. Several studies have found that shallow turbid reefs have an inherent resilience to heat stress events, suggesting they may dampen some of these effects. To test this hypothesis, researchers assessed coral responses to the 2020 heat stress event.

They conducted bleaching surveys in two contrasting habitats in Darvel Bay, Sabah: the turbid Sakar reef and the clear-water Blue Lagoon reef. In 2019, both reefs had relatively high coral cover of 40–43%. Underwater data loggers monitored temperature and light. The researchers scored coral colonies from 100 m video transects at depths of 5 m and 10 m at both locations, plus an additional transect at 15 m in Blue Lagoon. In total, 1,326 coral colonies were assessed for the presence and severity of bleaching using a six-point scoring method.

Bleaching severity differed significantly between the reefs. On the highly turbid Sakar reef, bleaching impacts were low, with an average of 9.6% of colonies bleached. On the clear-water Blue Lagoon reef, an average of 37.1% of colonies were affected by bleaching. Severity also differed significantly by depth: in Blue Lagoon, deeper corals bleached less.

Foliose coral forms were most affected in Blue Lagoon, whereas massive forms suffered most at Sakar. Bleaching responses also differed significantly among coral genera. Ctenactis and Herpolitha were consistently resistant at both reefs, while Leptoseris, Fungia and Goniopora were most affected at 5 m in Blue Lagoon. Pachyseris was notably more affected at 10 m on Sakar reef than in Blue Lagoon.

Overall, bleaching indices at these two reefs were lower than at most reefs assessed around the globe during heat stress events in 2020. The findings support the hypothesis that turbid reefs host resilient coral communities in the face of climate change.