What grew on a remote reef after nearly all its live coral was lost?

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Coral reefs worldwide face increasing thermal stress due to global warming. In 2014, a reef at the remote island of Kapou (Lisianski) in Papahānaumokuākea experienced unprecedented heat stress, causing coral bleaching and subsequent mass mortality that led to nearly 100% loss of live coral cover.

Researchers used underwater photogrammetric techniques to survey the reef and describe succession in its benthic communities from 2014 to 2021. Before bleaching, live Montipora coral covered more than 85% of the reef; after the coral died, green macroalga Halimeda quickly colonized the newly available substrata, accompanied by a short-lived bloom of green macroalga Boodlea in 2016. As Halimeda continued to increase in benthic cover, other algae—Neomeris, Asparagopsis, and unidentified filamentous red and green algae—also began colonizing the reef between 2017 and 2021.

Erosion of the reef substrata was evident in both in-situ and three-dimensional surveys in 2015 after bleaching and continued to progress over time. The high abundance of Halimeda may indicate slow coral recovery, but overall benthic diversity increased in 2021 due to the presence of other algae and increased cover of hard substrata and turf algae. Annual surveys after the bleaching-induced mortality also observed new Montipora coral colonies.

The researchers recommend continuing to monitor coral and algal communities and surveying herbivorous fish and reef binders, which play important ecological roles in algal control, reef erosion and sediment binding. They say these efforts should reveal interactions among the ecological processes that enable reef succession after mass coral mortality.