Material Legacies on Coral Reefs: Rubble Length and Bed Thickness Are Key Drivers of Rubble Bed Recovery

Disturbances on coral reefs—which are increasing in intensity and frequency—generate material legacies. These are commonly in the form of rubble beds, which depend on rubble stability and/or binding to facilitate coral recruitment and recovery. Yet, our understanding of rubble stability and binding dynamics across environmental gradients is limited. Characterising and categorising rubble material legacies in context of their likely recovery trajectory is imperative to the effective deployment of active intervention strategies used to restore degraded reefs, such as rubble stabilisation, coral outplanting and larval seeding techniques. We quantified rubble characteristics across environmental gradients on the Great Barrier Reef. The likelihood of rubble stability and binding increased with rubble length and rubble bed thickness, and rubble length was a good predictor of bed thickness and rubble branchiness. Thin rubble bed profiles (< ~10 cm depth), those with small, unbranched rubble pieces (< ~10 cm length), and beds at the base of sloped rubble screes, had lower stability and binding likelihoods. These kinds of beds are expected to persist with low recovery prospects, and could be good candidates for rubble stabilisation interventions. Thicker rubble beds with larger, branched rubble pieces tended to exhibit higher stability and binding likelihoods. However, these beds had nuanced effects on coral cover, and interventions may still be necessary where competition is high, for example from macroalgae. A rapid assessment of rubble length—while also considering shelf location, geomorphic zone, slope angle and underlying substrate—can indicate the potential direction of a rubble bed's recovery trajectory. Our findings have been summarised into a rapid rubble bed assessment tool available in the Supporting Information, that can be incorporated into current reef monitoring to optimize prioritisation of intervention strategies at disturbed sites.