Rubble stabilization alone cannot overcome the deleterious effects of soft coral dominance

Abstract Introduction Without intervention, the material legacies of smashed coral—loose rubble—can persist on reefs for decades and impede coral recruitment. Stabilizing interventions that accelerate rubble binding processes and foster coral growth and survival are commonly tested across narrow methodological, spatial, and temporal scales. Objectives This study determines the efficacy of three approaches to rubble stabilization under identical environmental conditions. Methods We trialed three steel rebar structure designs—flat grid, open pyramid, closed pyramid—that pinned and/or trapped rubble and were either flat or had vertical relief. A total of 36 structures and 33 control plots were installed on the central Great Barrier Reef and monitored over 3 years. Results Structures that pinned rubble (flat grids and closed pyramids) restricted rubble transport and aided stability, compared to control plots. However, this stabilizing effect did not translate to significant increases in rubble binding or scleractinian coral recruitment. Binding increased across the site for all designs and controls, mostly owing to soft coral binds ( Xenia spp.). Soft coral cover reached 87% in the deeper end of the site, and scleractinian recruits here dropped from 3 per 0.25 m 2 to 1 after 3 years. Soft corals also proliferated slightly more on structures than in control plots, suggesting rebar may facilitate their growth. Conclusion Flat grids and closed pyramid designs effectively stabilized rubble, but soft coral overgrowth represented a more important impediment to recovery than rubble transport at this site, particularly in the deeper end.