16 Feb 2023
 | 16 Feb 2023
Status: this preprint is currently under review for the journal BG.

Mobilisation thresholds for coral rubble and consequences for windows of reef recovery

Tania M. Kenyon, Daniel Harris, Tom Baldock, David Callaghan, Christopher Doropoulos, Gregory Webb, Steven P. Newman, and Peter J. Mumby

Abstract. The proportional cover of rubble on reefs is predicted to increase as disturbances increase in intensity and frequency. Unstable rubble can kill coral recruits and impair binding processes that consolidate rubble into a stable substrate for coral recruitment. A clearer understanding of the mechanisms of inhibited coral recovery on rubble requires characterisation of the hydrodynamic conditions that trigger rubble mobilisation. Here, we investigated rubble mobilisation under regular wave conditions in a wave flume and irregular wave conditions in-situ on a coral reef in the Maldives. We examined how changes in near-bed wave orbital velocity influenced the likelihood of rubble motion (e.g., rocking) and transport (by walking, sliding or flipping). Rubble mobilisation was considered as a function of rubble length, branchiness (branched vs. unbranched), and underlying substrate (rubble vs. sand). Rubble was more likely to be transported if ieces were small (4–8 cm) and had no branches, and rubble travelled slightly greater distances (~2 cm) per day on substrates composed of sand than rubble. The effect of near-bed wave orbital velocity on rubble mobilisation was comparable between flume and reef observations. Rubble had a 50 % and 90 % chance of transport when near-bed wave orbital velocities reached 0.30 m/s and 0.43 m/s, respectively, in the wave flume, and 0.34 m/s and 0.55 m/s, respectively, on the reef. Importantly, the probability of rubble transport per day declined over 3-day deployments in the field, suggesting rubble had settled into more hydrodynamically-stable positions or snagged on the first day of deployment. We expect that settled or snagged rubble may have been mobilised more commonly in locations with higher energy and more variable wave environments. Our results show that rubble beds comprised of small rubble pieces and/or pieces with fewer branches are likely to be more unstable. Such rubble beds are likely to have shorter windows of recovery (stability) between mobilisation events, and thus be good candidates for rubble stabilisation interventions to enhance coral recruitment and binding.

Tania M. Kenyon et al.

Status: open (until 07 Apr 2023)

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  • RC1: 'Comment on bg-2023-2', Anonymous Referee #1, 14 Mar 2023 reply

Tania M. Kenyon et al.

Tania M. Kenyon et al.


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Short summary
The movement of rubble on coral reefs can lead to persistent unstable rubble beds that hinder reef recovery. To identify where such rubble beds are, we need to know the minimum flow velocity that will move rubble. We tested this and found that rubble moved more if pieces were small and had no branches. Rubble had a 50% chance of being moved when flow velocities reached ~0.35 m/s. Rubble beds that experience frequent movement would be good candidates for rubble stabilisation interventions.