Macroalgal feedbacks and substrate properties maintain a coral reef regime shift

Abstract

Coral reefs are among the world's most diverse and productive ecosystems, yet they are also one of the most threatened. The combined effects of local human activities and climate change have led to corals being replaced by macroalgae in various tropical settings, lessening the ecological, social, and economic value of these reefs. Once established, macroalgal regimes are maintained by a range of physical, chemical, and biological feedback mechanisms that suppress the settlement, survival, growth, and hence recovery of coral populations. Our understanding of these feedbacks has come largely from small‐scale experimental studies, but their relative importance in sustaining a regime shift has rarely been examined in situ. We investigated the role of macroalgae in limiting coral recovery on an inshore reef on Australia's Great Barrier Reef that shifted to macroalgal dominance in 2001. Coral recruitment on terracotta tiles in habitats with low cover of macroalgae at the regime‐shifted reef and at comparable habitats at an adjacent coral‐dominated reef was similar, suggesting that neither larval supply nor reef‐wide “avoidance” by coral larvae was contributing to the lack of coral recovery at the regime‐shifted reef. However, within the regime‐shifted reef, recruitment of corals on tiles, and their survival in the first two months post‐settlement, was substantially lower in habitats characterized by dense beds of the brown macroalga Lobophora than in habitats just meters away that were relatively free of macroalgae. Despite the negative effects of Lobophora on recruitment and early recruit survival, there was no effect of Lobophora on the persistence of juvenile corals (1–50 mm diameter). Juvenile coral persistence in beds of Lobophora (50%) was comparable to that in neighboring habitats free of Lobophora (60%) over nine months. Rather, the persistence of juvenile corals was lowest (10%) in unconsolidated rubble habitat, where photographs of fixed quadrats showed that, over nine months, rubble substrate had been redistributed. Our results highlight two bottlenecks to coral recovery; inhibition of coral recruitment and recruit survival by macroalgae, and reduced juvenile coral persistence in patches of loose rubble substrate. Importantly, these processes appear to be habitat‐specific and are unlikely to constrain coral recovery at a reef‐wide scale.

Acute drivers influence recent inshore Great Barrier Reef dynamics

“…A third, related possibility is that historical changes in the environment caused inshore reefs to transition to an alternative stable state that is now reinforced by ecological feedbacks. Such states are difficult to reverse and exhibit little fluctuation [37,94,95], as mooted by Johns et al [96]. This scenario parallels what has been demonstrated in other ecosystems, where recovery dynamics show time lags in responses due to shifting baselines and hysteresis [97].…”

Long‐term dynamics and drivers of coral and macroalgal cover on inshore reefs of the Great Barrier Reef Marine Park

“…We hypothesize that direct and indirect effects of macroalgae on corals, such as shading, abrasion, allelopathy, and inhibition of recruitment, may reach the point where they create previously documented “feedback loops” (Johns et al. ), preventing the return of corals and maintaining macroalgal dominance, once macroalgal cover reaches and/or exceeds 20%. Ultimately, defining thresholds for coral‐macroalgal relationships requires experimental testing, particularly if macroalgal reduction targets are to be applied in manipulative reef restoration programs.…”

Adapting to extreme environments: can coral reefs adapt to climate change?

“…One challenge is the replacement of scleractinian corals by macroalgaea macroalgal regime shiftwhich reduces the ecological, social, and economic value of affected reefs. This is happening globally, not just in the Caribbean [67,68], and leads to two bottlenecks to coral recovery; inhibition of coral recruitment and recruit survival by macroalgae, and reduced juvenile coral persistence in patches of loose rubble [68]. Another problem in the corals themselves is that ocean warming destabilizes the coral symbiosis with the dinoflagellate algae, Symbiodinium¸ producing disparity in benefits and costs to both partners producing symbiont parasitism in the coral [69].…”