Abstract
Climate change is one of the greatest threats to the long-term maintenance of coral-dominated tropical ecosystems, and has received considerable attention over the past two decades. Coral bleaching and associated mortality events, which are predicted to become more frequent and intense, can alter the balance of different elements that are responsible for coral reef growth and maintenance. The geomorphic impacts of coral mass mortality have received relatively little attention, particularly questions concerning temporal recovery of reef carbonate production and the factors that promote resilience of reef growth potential. Here, we track the biological carbonate budgets of inner Seychelles reefs from 1994 to 2014, spanning the 1998 global bleaching event when these reefs lost more than 90% of coral cover. All 21 reefs had positive budgets in 1994, but in 2005 budgets were predominantly negative. By 2014, carbonate budgets on seven reefs were comparable with 1994, but on all reefs where an ecological regime shift to macroalgal dominance occurred, budgets remained negative through 2014. Reefs with higher massive coral cover, lower macroalgae cover and lower excavating parrotfish biomass in 1994 were more likely to have positive budgets post-bleaching. If mortality of corals from the 2016 bleaching event is as severe as that of 1998, our predictions based on past trends would suggest that six of eight reefs with positive budgets in 2014 would still have positive budgets by 2030. Our results highlight that reef accretion and framework maintenance cannot be assumed from the ecological state alone, and that managers should focus on conserving aspects of coral reefs that support resilient carbonate budgets.
Highlights
Ocean warming and climate change are considered the greatest threats to the long-term maintenance of coral-dominated tropical ecosystems
The fundamental process of coral and reef growth that provides the structural complexity of reefs and underpins their biodiversity, fisheries and coastal protection functions may be substantial reduced [10], and has implications that may not be apparent from measures of total coral cover alone
We found that nine of twelve pre-bleaching predictor variables had more than 10% relative influence on carbonate budgets
Summary
Ocean warming and climate change are considered the greatest threats to the long-term maintenance of coral-dominated tropical ecosystems. Recent studies indicate that carbonate budgets can be dynamic across relatively short time scales and can respond to site-specific emergence of different dominant coral taxa, which may have very different calcification rates [8,11,20,21] Such short-term dynamics are known to be driven by rapid ecological change trajectories as demonstrated by empirical pre- and postdisturbance event (e.g. bleaching) studies at individual locations, including substantial declines in reef accretion potential [11,20]. We selected variables that were likely to be influential across these time scales, such as abundance of bioeroders [39] and ecological predictors important in determining regime shifts (biomass of browsing, excavating and scraping parrotfish, cover of macroalgae, branching, massive and encrusting coral, depth, wave exposure, structural complexity and reserve status—see the electronic supplementary material, table S5 for details). To identify a threshold of probability above which reefs were more likely than not to be in a net positive carbonate budget state, we used the optimal.threshold model with the PredPrev 1⁄4 Obs methods in the PresenceAbsence package [41]
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