A remote coral reef shows macroalgal succession following a mass bleaching event

Abstract

Coral reefs worldwide are exposed to increased levels of thermal stress due to global warming. A coral reef at the remote island of Kapou (Lisianski) in Papahānaumokuākea experienced an unprecedented level of heat stress in 2014, which resulted in coral bleaching and subsequent mass mortality that resulted in nearly 100 % loss of live coral cover. Here, we describe successional changes in benthic communities occurring on the reef from 2014 to 2021 based on our surveys utilizing underwater photogrammetric techniques. Despite having > 85 % Montipora live coral cover before the bleaching event, the newly available substrata created by the loss of coral were quickly colonized by the green macroalga Halimeda, along with an ephemeral bloom of the green macroalga Boodlea in 2016. While Halimeda continued to increase in benthic cover, other algae (Neomeris, Asparagopsis and unidentified filamentous red and green algae) also started colonizing the reef between 2017 and 2021. Erosion of the reef substrata was evident in both in-situ and three-dimensional surveys in 2015 following the bleaching event and has continued to progress through time. The high abundance of Halimeda may indicate a slow process of coral recovery, but the overall benthic diversity increased in 2021 due to the presence of other algae and an increase in hard substrata and turf algal cover. New colonies of Montipora coral were also observed during annual surveys following the bleaching-induced mortality. Future monitoring efforts should continue to track coral and algal communities and survey herbivorous fish and reef binders that play important ecological roles in algal control, reef erosion and sediment binding. Such efforts should reveal interactions among these different ecological processes that enable reef succession following mass coral mortality.