Understanding differential patterns in coral reef recovery: chronic hydrodynamic disturbance as a limiting mechanism for coral colonization

Abstract

Coral reefs are subject to numerous physical disturbances, and post-disturbance coral recovery potential depends on subsequent re-colonization of impacted habitat. We examined divergent recovery trajectories at 2 proximal reefs disturbed by ship groundings that resulted in clearly delineated areas of altered substrate. Post-disturbance measurements of coral recruitment, survival, and changes in community structure were made approximately annually from 2009−2013 in undisturbed reference areas as well as disturbed pavement and rubble areas. Despite similar initial physical disturbances, there were marked differences between sites, with higher coral recruitment and survival on disturbed pavement than rubble bottom, reference reef, or restoration structures. Subsequent episodic disturbances from rubble mobilization could be a mechanism driving the divergent recovery patterns. To estimate whether local hydrodynamic conditions were sufficient to mobilize rubble, we used a combination of long-term monitoring, hydrodynamic modeling, and rubble transport mechanics to hindcast the potential for substrate mobility. Long-term model simulations of hydrodynamic forcing at the study sites showed multiple events where bottom-orbital velocities exceeded thresholds required to mobilize rubble via sliding or overturning. Our analyses indicate that wave energy mobilizes rubble substrate multiple times annually and suggests a physical limitation on survival of coral recruits relative to those on pavement substrate. Continued mobilizations may establish a positive feedback loop in which continued rubble clast mobilizations cause additional mechanical erosion or breakage and a shift to smaller rubble sizes that would subsequently mobilize at a lower level of hydrodynamic forcing and thus become subject to more frequent and sustained disturbances. The combination of multiple hydrodynamic disturbances and unstable substrate limits coral recovery and thus contributes to prolonged habitat loss.

Cumulative impacts across Australia’s Great Barrier Reef: A mechanistic evaluation

“…Structural collapse occurs immediately after cyclones but is delayed for three years after bleaching and CoTS predation (Sano et al 1987). Coral juveniles do not survive on unconsolidated rubble (Fox et al 2003, Viehman et al 2018), which amounts to reducing their survivorship by the proportion of the reef area covered by rubble. Loose coral rubble tend to stabilize over time with processes of carbonate binding and cementation (Rasser and Riegl 2002).…”

Cumulative impacts across Australia’s Great Barrier Reef: a mechanistic evaluation

“…Structural collapse occurs immediately after cyclones but is delayed for 3 yr after bleaching and CoTS predation (Sano et al 1987). Coral juveniles do not survive on unconsolidated rubble (Fox et al 2003, Viehman et al 2018, which amounts to reducing their survivorship by the proportion of the reef area covered by rubble. Loose coral rubble tends to stabilize over time withs processes of carbonate binding and cementation (Rasser and Riegl 2002).…”

Factors Limiting the Range Extension of Corals into High-Latitude Reef Regions

“…Despite the importance of water motion to coral biology and ecology, few species flourish in areas with high levels of water motion [114]. Even at latitudes conducive to reef accretion, hydrodynamically exposed habitats have lower rates of recruitment [115][116][117], higher rates of colony damage and dislodgement [106,118], reduced growth rates [119], and tend to be dominated by species that can form encrusting or digitate morphologies [114]. At latitudes beyond where reefs accrete, there are fewer structures that attenuate wave energy and so corals are typically found in small areas on the protected sides of islands and headlands [22,120,121], or in bays and inlets [122].…”