Long-term spatial variations in turbidity and temperature provide new insights into coral-algal states on extreme/marginal reefs

Globally, coral reefs are under threat, with many exhibiting degradation or a shift towards algal-dominated regimes following marine heat waves, and other disturbance events. Marginal coral reefs existing under naturally extreme conditions, such as turbid water reefs, may be more resilient than their clear water counterparts as well as offer some insight into how reefs could look in the future under climate change. Here, we surveyed 27 benthic habitats across an environmental stress gradient in the Exmouth Gulf region of north Western Australia immediately following a marine heatwave event. We used multidecadal remotely sensed turbidity (from an in-situ validated dataset) and temperature, to assess how these environmental drivers influence variability in benthic communities and coral morphology. Long-term turbidity and temperature variability were associated with macroalgal colonisation when exceeding a combined threshold. Coral cover was strongly negatively associated with temperature variability, and positively associated with depth, and wave power, while coral morphology diversity was positively associated with turbidity. While moderate turbidity (long-term average similar to 2 mg/L suspended matter) appeared to raise the threshold for coral bleaching and macroalgal dominance, regions with higher temperature variability (> 3.5 degrees C) appeared to have already reached this threshold. The region with the least turbidity and temperature variability had the highest amount of coral bleaching from a recent heatwave event and moderate levels of both these variables may confer resilience to coral reefs.

Automated summary

Globally, coral reefs are facing threats, such as degradation and algal dominance, due to marine heatwaves and other disturbance events. Marginal coral reefs in extreme environments, like turbid water reefs, may be more resilient and provide insights into future reefs under climate change. This study examined benthic habitats in the Exmouth Gulf region of north Western Australia to understand the influence of environmental drivers, such as turbidity and temperature, on benthic communities and coral morphology. The results showed that long-term turbidity and temperature variability were connected to macroalgal colonization, while coral cover was negatively associated with temperature variability and positively associated with depth and wave power. Coral morphology diversity was positively associated with turbidity. Moderate turbidity appeared to raise the threshold for coral bleaching and macroalgal dominance, while regions with higher temperature variability had already reached this threshold. The least turbid and temperature variable region experienced severe coral bleaching from a recent heatwave, suggesting that moderate levels of these variables may confer resilience to coral reefs.