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

Abstract

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 bleaching-induced mortality in corals. While bleaching has been reported on numerous high-latitude reefs, post-bleaching trajectories of benthic communities are poorly described. Consequently, the longer-term effects of thermal anomalies on high-latitude reefs are difficult to predict. Here, we use an autonomous underwater vehicle to conduct repeated surveys of three 625 m(2) plots on a coral-dominated high-latitude reef in the Houtman Abrolhos Islands, Western Australia, over a four-year 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.

Figure 1. Location of the Houtman Abrolhos Islands, Western Australia (a); location of the study site Geebank between the Easter and Southern (Pelseart) island groups (b).

Black squares indicate the location of replicate plots (c).

Figure 2. Autonomous Underwater Vehicle (AUV) images showing examples of shifts in community composition from 2010–2013; (a) high abundance of branching Acropora, plating Acropora and Montipora in 2010; (b) bleached Montipora adjacent to unbleached Acropora in 2011; (c) red macroalga Asparagopsis colonising substrate exposed by coral decline in 2013.

Figure 3. Principal coordinates plot of shifts in composition of dominant benthic taxa from 2010–2013.

Shapes indicate the different plots (circles = plot 1, diamonds = plot 2 and crosses = plot 3, while colours indicate different years (blue = 2010, red = 2011, orange = 2012 and green 2013). A general trend of declines in plating corals and increased macroalgae were observed in all plots, although coral decline was most pronounced in plot 3. Declines in plating corals were offset by increases in branching Acropora in plots 1 and 2 between 2012 and 2013.

Figure 4. Changes in percent cover of hard coral (a) and macroalgae (b) in each plot from 2010 to 2013.

Figure 5. Changes in the abundance of the dominant coral and macroalgal taxa across the three plots from 2010–2013.