Variable responses of benthic communities to anomalously warm sea temperatures on a high-latitude coral reef

High-latitude reefs support unique ecological communities occurring at the
biogeographic boundaries between tropical and temperate marine ecosystems.
Due to their lower ambient temperatures, they are regarded as potential refugia for
tropical species shifting poleward due to rising sea temperatures. However, acute
warming events can cause rapid shifts in the composition of high-latitude reef
communities, including range contractions of temperate macroalgae and bleachinginduced
mortality in corals. While bleaching has been reported on numerous highlatitude
reefs, post-bleaching trajectories of benthic communities are poorly
described. Consequently, the longer-term effects of thermal anomalies on highlatitude
reefs are difficult to predict. Here, we use an autonomous underwater
vehicle to conduct repeated surveys of three 625 m2 plots on a coral-dominated
high-latitude reef in the Houtman Abrolhos Islands, Western Australia, over a fouryear
period spanning a large-magnitude thermal anomaly. Quantification of benthic
communities revealed high coral cover (.70%, comprising three main
morphospecies) prior to the bleaching event. Plating Montipora was most
susceptible to bleaching, but in the plot where it was most abundant, coral cover did
not change significantly because of post-bleaching increases in branching
Acropora. In the other two plots, coral cover decreased while macroalgal cover
increased markedly. Overall, coral cover declined from 73% to 59% over the course
of the study, while macroalgal cover increased from 11% to 24%. The significant
differences in impacts and post-bleaching trajectories among plots underline the
importance of understanding the underlying causes of such variation to improve
predictions of how climate change will affect reefs, especially at high-latitudes.

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