Hillary A. Smith, Dylan A. Brown, Chaitanya V. Arjunwadkar, Stella E. Fulton, Taylor Whitman, Bambang Hermanto, Elissa Mastroianni, Neil Mattocks, Adam K. Smith, Peter L. Harrison, Lisa Boström-Einarsson, Ian M. McLeod, David G. Bourne
Abstract. Declining coral cover on tropical coral reefs often results in a concomitant increase in macroalgae. When proliferation of macroalgae persists outside of regular seasonal growth, it can shift the ecosystem dominance away from corals into a permanently altered system. Such an altered system is unlikely to recover naturally, despite ample supply of coral larvae, as coral settlement and survival is reduced by the presence of macroalgae. Physical removal of macroalgae has been proposed to overcome this biotic barrier to recovery, though empirical evidence demonstrating the effects of removal on phase-shifted reefs is lacking. Here, we manually removed macroalgae from twelve 25 m2experimental plots (88.5 ± 6.2 kg wet weight per plot; 90% benthic cover decrease) on a degraded reef prior to coral mass spawning across two years and recorded the number of coral recruits to settlement tiles and natural substrata. Four months after each spawning event, we found a three-fold increase in coral recruits to tiles in plots where macroalgae had been removed (n = 12 plots; February 2019: mean 45.9 ± 12.7 recruits per tile; February 2020: mean 53.9 ± 5.9 recruits per tile) compared to control plots where macroalgae remained (n = 12 plots; February 2019 mean: 13.6 ± 2.8 recruits per tile; February 2020 mean: 17.5 ± 3.5 recruits per tile). These results suggest that, at small scales, macroalgae removal may be a useful intervention to boost recruitment on degraded reefs. Longer-term monitoring is needed to document if coral survivorship, growth, and subsequent reef recovery occurs.