The effects of rubble mobilisation on coral fragment survival, partial mortality and growth

The sliding and overturning of unconsolidated rubble by hydrodynamic forcing is expected to cause physical damage to settled coral recruits and asexual fragments by scouring and smothering. Yet, few empirical studies have tested the relationship between rubble mobilisation frequency and the survival and growth of these corals. Here, we tested the response of small coral fragments to varying levels of experimental scouring and smothering, proxies for rubble mobilisation impacts, on two coral species with distinct functional traits; Porites rus (robust, encrusting) and Pocillopora verrucosa (sensitive, branching). This study investigated: 1) how coral fragment survival was affected by inversion on rubble or sand substratum over ~2 w (i.e. smothering); and 2) how the survival, growth and tissue properties of fragments were affected by rubble being overturned (i.e. experiencing scouring or abrasion) at different frequencies over ~10 w. Smothering substratum had a profound influence on the probability of fragment survival. For both species, survival was <2% after 5 d when inverted on sand, whereas nearly all survived when inverted on rubble. Following scouring, most P. rus fragments survived over the ~10-w period irrespective of scour frequency. However, P. verrucosa showed high mortality (45% loss) across all treatments, likely influenced by initial handling and fragmentation. For P. rus, increasing scour frequency caused sublethal effects of significantly reduced tissue growth and pigmentation, and increased partial mortality. These findings indicate that small fragments – and potentially recruits – are unlikely to successfully recruit in frequently-mobilised rubble beds regardless of their functional morphotype, but may persist in rubble beds in lower-energy environments. This provides valuable insight into the limitations on coral recovery in rubble beds, which could inform reef recovery programs considering substratum stabilisation. Combined with an understanding of a reef's hydrodynamic environment, this research can assist managers in prioritising damaged reefs for active interventions.