Trajectories and agents of binding in stabilized and unstabilized coral rubble across environmental gradients

Natural ecosystems are routinely impacted by acute disturbances that generate space for early colonizers. Following disturbances, the interaction strengths of top-down and bottom-up factors across environmental gradients influence community succession. On coral reefs, rubble beds commonly form following major disturbances and can persist for decades. Yet, there is little understanding of the successional pathways that lead to rubble binding-where rubble is bound and consolidated to form stable substrate suitable for coral recruitment-and subsequent coral recovery. This study used observational and experimental methods to determine: (1) binding likelihood in unstabilized in situ rubble beds 2.5 years following a coral bleaching event in 2016 in the Maldives, and how it varied according to rubble characteristics across environmental gradients; and (2) how the number of binds and binder community composition on experimentally stabilized rubble varied temporally over 1.5 years across environmental gradients. Surveys of rubble beds found that binding was lowest on the reef flat (8% of rubble was bound) and highest at exposed deeper sites (38%), where flow appears low enough to maintain rubble stability but high enough to support binder growth. When experimentally stabilized, similar to 100% of rubble was bound by at least one bind within 6 months. Yet, while the number of binds per rubble piece in experimental units continued to increase over time on the reef slope, it remained low on the reef flat, and binder community composition was distinct between reef flat and slope-likely due to higher sediment transport on the reef flat. Community composition also was distinct between exposed and cryptic rubble microhabitats. On reefs where rubble is mobilized more frequently than every 6 months, rubble beds will likely have low binding potential and delayed coral recruitment. Where sediment flux and deposition is high, recovery is unlikely even if rubble is actively stabilized. In contrast, infrequently mobilized areas with lower sediment flux are more likely to facilitate natural binding and coral recovery, and thus may not require intervention. Our findings can help to effectively guide managers toward the best strategies that facilitate the recovery of rubble-dominated coral reefs, while optimizing limited intervention resources through careful prioritization.