Mobilisation thresholds for coral rubble and consequences for windows of reef recovery
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)
- RC1: 'Comment on bg-2023-2', Anonymous Referee #1, 14 Mar 2023 reply
Tania M. Kenyon et al.
Tania M. Kenyon et al.
Viewed (geographical distribution)
This well-written study presents work empirically determining the threshold of mobilisation for individual rubble pieces of varying shapes and sizes, and on different substrate types and slopes, in both controlled and field settings (the Maldives). Rubble movement is relevant because it impacts coral recovery, and many threats to coral (e.g. destructive fishing, storms, bleaching) result in coral breakage and/or death. These pieces become “rubble” of various sizes and shapes. The experiments are clever and thorough, designed to elucidate the probability of rubble ‘rocking’ or various types of ‘transport’ (walk/slide/flip). Unsurprisingly, the authors found similar mobilisation thresholds in the wave flume and in the field, and that the probability of rubble mobilisation increases with higher velocity. Also as common sense and previous work would suggest, it decreases as: (i) rubble size increases; (ii) morphological complexity/’branchiness’ (of both the rubble and of the substrate type) increases; and (iii) as the slope angle decreases (and the contribution of gravity subsequently decreases). Interlocking and ‘settling’ of rubble was a strong inhibitor of mobilisation.
While the authors did find some some nuanced results (e.g. larger rubble is more likely to settle into sand) and differences between the northeastern and western monsoon seasons, overall, their results seem a very sophisticated experimental demonstration of what common sense would predict. While sentences in the Abstract (l 19-21) and Introduction (l 37-40) suggest relevance for managers of reefs that exhibit a significant increase in rubble cover, there is no discussion of what managers can actually do once they have the information presented herein. While there is mention of “rubble stabilisation interventions to enhance coral recruitment and binding,” it’s unclear that the results from this work would actually be needed to predict the likelihood of natural rubble stabilisation and recovery beyond simple first principles. I suggest the discussion at least address potential management relevance, including context for discussions for rubble stabilization, budget needed vs. scale of the problem, etc.
No technical corrections.