Unravelling the links between heat stress, bleaching and disease: fate of tabular corals following a combined disease and bleaching event.

While links between heat stress and coral bleaching are clear and predictive tools for bleaching risk are well advanced, links between heat stress and outbreaks of coral diseases are less well understood. In this study, the effects of accumulated heat stress on tagged colonies of tabular Acropora were monitored over the 2017 austral summer at Beaver Reef, which is located in the central region of the Great Barrier Reef. The initial surveys in midsummer (21 February) coincided with an accumulated heat stress metric of 4.5 °C-weeks, and documented high coral cover (74.0 ± 6.5%), extensive bleaching (71% of all corals displayed bleaching signs) and an outbreak of white syndromes (WSs) (31% of tabular acroporid corals displayed white syndrome signs). Repeat assessments of the impacts of bleaching and disease on these corals provided real-time information to reef managers by tracking the unfolding reef health incident on 100 colonies of Acropora hyacinthus (Dana, 1846), tagged in mid-March and surveyed intermittently until late October 2017. Heat stress increased rapidly on Beaver Reef, peaking at 8.3 °C-weeks on 31 March, which coincided with the highest prevalence of WS recorded in the study. Of the 85 tagged colonies surviving on 31 March, 41 (~ 48%) displayed WS signs, indicating a link between heat stress and WS. When re-surveyed at eight months (24 October), 68 of 100 tagged colonies had suffered whole-colony mortality and only four colonies had not displayed signs of bleaching or disease (WS) in any of our surveys. Overall, coral cover on Beaver Reef was reduced by more than half to 31.0 ± 11.2%. Significant tissue loss due to severe bleaching was observed with up to 20 times greater tissue loss on severely bleached colonies (i.e. categorised as > 50% bleached) compared to mildly/moderately bleached colonies (< 50% bleached) at the heat stress peak (31 March). This suggests that for Acropora hyacinthus, a threshold of 50% colony bleaching is a good indicator that substantial mortality at both the colony and population level is likely to follow a heat stress event. Across all levels of bleaching, colonies displaying WS signs exhibited up to seven times greater tissue loss than bleached-only colonies. WS caused a threefold increase in accumulated tissue loss (69.6 ± 10.5% tissue lost) in the mildly bleached category, suggesting that disease exacerbated mortality in bleached corals and contributed significantly to the substantial loss of corals on the GBR in 2017. [ABSTRACT FROM AUTHOR]