The effect of rubble stability on coral settlement and recruitment

The effect of rubble stability on coral settlement and recruitment | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The effect of rubble stability on coral settlement and recruitment Roima Paewai-Huggins, Tania Kenyon, Peter Mumby This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7974089/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Coral reef rubble comprises detached, fragmented, dead coral skeletons that can be unstable and are widely considered to constitute unsuitable substrate for coral recruitment. However, the type, severity, and frequency of a disturbance can generate rubble of diverse sizes, morphologies, and configurations, creating varying typologies of rubble beds. These physical characteristics play a substantial role in determining whether rubble remains stable or is frequently mobilised. We ask how the type of substrate influences the density of corals settling and recruiting onto rubble varying in its level of stability. Within two types of rubble beds, loose and interlocked, and on a control hard carbonate substrate, we measured coral settlement (four months post-spawning) and later-stage recruitment (post-settlement survival within 11 months) on unstable, unfixed rubble and on stabilised, fixed rubble. Both stability and substrate type influenced settlement and recruitment. Settlement and recruitment were higher on fixed than unfixed rubble regardless of the rubble type. This suggests that rubble mobility increases mortality rates. However, effect sizes varied between rubble bed types, likely driven by differing rubble bed characteristics. On the unfixed rubble, settlement and recruitment were higher in the interlocked than in the loose rubble bed. Settlement and recruitment were also higher on the fixed rubble in the interlocked bed than in the loose rubble bed. Thus, environmental effects in the loose rubble appear more severe than the combined environmental and mobility issues associated with the interlocked rubble. While our results show that stability is a key driver of coral recovery, settlement and recruitment on fixed rubble were still lower in rubble beds compared to the hard carbonate reef. These results indicate that each substrate has distinct environmental factors that differentially influence both the settlement and recruitment of corals, even on stabilised rubble surfaces. A greater understanding of coral recruitment dynamics across various rubble bed typologies is important for the management of future reef intervention programs, as the cover of rubble increases on reefs. Marine and Freshwater Ecology Marine and Freshwater Biology rubble coral settlement post-settlement survival substrate habitats Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Scleractinian corals or ‘stony corals’ secrete calcium carbonate skeletons which form coral reefs through accretion, providing a stable and structural habitat (Salinas-de-León et al., 2013 ; Sánchez-Quinto & Falcón, 2019 ). Biological, chemical, and mechanical processes can reduce or diminish the structural complexity of reefs by breaking up and eroding the coral framework into rubble. This is a natural cycle of disturbance and recovery on coral reefs. Wave energy from storm disturbances such as cyclones, hurricanes, and tsunamis has resulted in vast rubble beds (Blanchon et al., 1997 ; Viehman et al., 2018 ). However, impacts from climate change, such as increased intensity of storms and cyclones, coral bleaching, and disease, may alter these cycles and narrow the recovery windows (Ceccarelli et al., 2020 ; Knutson et al., 2020 ). After a disturbance, recovery is heavily reliant upon coral recruitment (Doropoulos et al., 2016 ). The success of coral recruitment is driven by various environmental and ecological factors. A chief prerequisite is thought to be a stable substrate onto which coral larvae can settle and grow (Cameron et al., 2016 ; Ceccarelli et al., 2020 ; Gouezo et al., 2021). Other factors that influence the success of coral recruitment include the abundance and diversity of coral larvae, the cover of recruitment facilitators and inhibitors, hydrodynamic conditions, sedimentation, corallivory and grazing, temperature, eutrophication, grazing pressures, light, connectivity between reefs, and coral larvae substrate preferences (Doropoulos et al., 2018 ; Doropoulos et al., 2016 ; Edmunds, 2000 ; Harrington et al., 2004 ; Harriott & Fisk, 1987 ; Norström et al., 2007 ; Salinas-de-León et al., 2013 ; Ama Wakwella et al., 2020 ). The existence of multiple drivers makes it challenging to determine which scenarios will result in the most successful recruitment outcomes (Doropoulos et al., 2016 ; Edmunds, 2000 ). Studies of the drivers of coral settlement and recruitment generally focus on hard carbonate surfaces, with limited work having been conducted on rubble beds (Cameron et al., 2016 ; Chong-Seng et al., 2014 ; Fox, 2002 ). Rubble can experience greater hydrodynamic energy than a complex reef owing to the reduced drag friction that structural complexity on an intact reef provides (Guihen et al., 2013 ). Greater hydrodynamic energy can make rubble more susceptible to mobilisation (Fox & Caldwell, 2006 ), which can cause the abrasion, dislodgement, and smothering of settled coral larvae (Brown & Dunne, 1988; Chong-Seng et al., 2014 ; Fox, 2002 ). Indeed, this is believed to be the driving force behind the low success of coral recruitment and, consequently, coral recovery on rubble beds (Chong-Seng et al., 2014 ; Fox, 2004 ; Fox & Caldwell, 2006 ; Heyward et al., 2024 ; Page et al., 2024 ). However, rubble can vary in size, morphology, bed thickness, and configuration (Kenyon et al., 2024 ), reflecting the disturbance type, intensity, and pre-disturbance community, i.e., the dominating coral genus and/or coral morphology (Leung & Mumby, 2024 ). Variations of rubble morphometrics can influence rubble mobilisation, biophysical factors (e.g., water flow or sedimentation) and coral recruitment dynamics (Kenyon et al., 2025 ). A rubble bed composed of larger, branched, more structurally complex rubble, with greater void space, may reduce the speed of laminar flow across the bed and increase turbulence, which helps coral larvae to settle (Hata et al., 2017 ). Space between pieces of rubble could also facilitate water movement further into the bed, carrying oxygen and nutrients to settled corals. However, greater flow may also facilitate colonisation of other sessile taxa such as bryozoans, ascidians, and turf algae, which can out-compete corals for space and increase post-settlement mortality (Chong-Seng et al., 2014 ; Doropoulos et al., 2016 ). A flat or less structurally complex rubble bed, with little to no space between rubble pieces and fewer cryptic spaces, may elevate the exposure of settled corals to direct and incidental predation (Lewis & Wainwright, 1985 ; Wilson et al., 2006 ; Woodley et al., 1981 ) and higher deposited sediment (Paewai-Huggins et al., 2025a). Compared to rubble beds with larger void spaces, sediment deposited on flat, tightly packed beds cannot fall deeper into the matrix and might remain on the uppermost layer of rubble. Deposited sediment can inhibit settlement and increase post-settlement mortality due to smothering (Norström et al., 2007 ). Rubble morphometrics and environmental factors will also strongly influence the likelihood of rubble movement. Rubble of shorter length and no branches may be more susceptible to movement than longer, branched rubble (Kenyon et al., 2023 ). Environmental variables of water flow (Fox et al., 2003), the inundation period in response to tidal fluctuation (Thornborough et al., 2012), wind speed and direction (Heyward et al., 1985), and water depth (Bruno et al, 1998; Kenyon et al., 2023 ) all interact with rubble morphometrics to dictate rubble mobilisation. Thus, multiple biophysical factors, other than mobilisation, vary with rubble type and can influence coral recruitment. In a recent study, we asked how the environmental effects associated with two contrasting types of rubble beds influence coral settlement and recruitment (Paewai-Huggins et al., 2025a). We found important environmental effects and associated these with flow and sediment dynamics. To answer that question, we used fixed rubble recruitment tiles to remove any effect of rubble mobility. Here, in the present study, we repeat our previous work but add a new treatment of loose, unfixed rubble. Our motivation here was to gain a holistic understanding of the relative importance of substrate stability and environmental effects. It was necessary to repeat our earlier treatments with fixed rubble in case the drivers of recruitment had changed since the earlier study on environmental effects. Since we focus here on the role of stability and the relative importance of environmental effects, we devote less space to exploring environmental effects and their causation, as this was studied in the earlier manuscript. Methods 3.1. Study sites Two rubble substrates and a hard carbonate substrate, all at a depth of 8–9 m, were identified at Heron Reef, in the Sea Country of the Gooreng Gooreng, Gurang, Bailai and Taribelang Bunda peoples, Southern Great Barrier Reef, Australia (Fig. 1 a). All three substrates were located on the south-west slope of Heron Reef. The two rubble beds are the same habitats that were used in Paewai-Huggins et al. (2025a) rubble study. However, the hard carbonate habitat discussed here is new, located after the first experiments (Paewai-Huggins et al., 2025) concluded. The rubble beds consisted of a loose rubble bed and an interlocked rubble bed. The third substrate was a hard carbonate reef, composed of large ‘bommies’ of solid carbonate rock. Substrates were spaced along and parallel to the reef slope, 200 m apart. Due to their proximity, each substrate was expected to experience similar light exposure, water temperature, wave exposure, and sedimentation. The loose rubble bed (Fig. 1 b) is comprised of small, typically non-branched rubble pieces. The interlocked rubble bed (Fig. 1 c) is composed of larger, branching rubble pieces. For a full description of rubble comprising either bed, see (Paewai-Huggins et al., 2025a). 3.1. 2. Recruitment tiles and loose rubble plot construction and deployment 2. Recruitment tiles and loose rubble plot construction and deployment Two rubble treatments were used: fixed and unfixed. To remove bias from the type of rubble available in each of the two rubble habitats, a collection was made from both loose and interlocked rubble beds, and the resulting material was mixed thoroughly before being treated and deployed back into each habitat and on hard substrate. Treatment involved cleaning rubble with fresh water, bleaching overnight to remove living material, rinsing again, and drying in sunlight. Recruitment tiles comprised an 18 x 18 cm stainless steel mesh onto which rubble pieces were fixed with cable ties in pairs (Fig. 1 d). While this is the same design used in (Paewai-Huggins et al., 2025a), all tiles used here were built specifically for this study; none which were previously used were used here. Each tile consisted of 10 pairs of rubble and was labelled with a unique ID tag. Rubble pieces were placed in pairs as it was easier to attach the rubble to the stainless-steel mesh, and this mimics the fact that rubble is rarely isolated. A total of 60 tiles and 600 fixed rubble pieces were evenly distributed across the three substrate habitats, i.e., 20 tiles per substrate within a 5 x 5 m area. Tiles were connected to the substrate using 1 m basalt reinforcing bar stakes and placed flush with the surrounding rubble or hard carbonate reef. Along with the deployment of fixed rubble tiles, unfixed rubble was deployed within each of the two rubble beds (Fig. 1 e), but not within the hard carbonate habitat, as there is no naturally occurring rubble in this habitat. The unfixed rubble plots consisted of four rebar stakes, spaced 30 cm apart, in a square. Within each permanent plot, 40 unfixed rubble pieces were placed within each plot. Each piece was labelled with a Floy tag with a unique ID, and tags were securely fastened, and tails were snipped so as not to impede rubble movement. A total of 12 plots and 480 unfixed rubble pieces were deployed across the two rubble substrates. Fixed rubble tiles and unfixed rubble were deployed on the 18th of October 2022 to develop a biofilm before the coral spawning event, which took place in November 2022. The new instalment of fixed and unfixed rubble did not overlap in time with rubble recruitment tiles used in (Paewai-Huggins et al., 2025a) experiment. All data presented here is new and distinct from our previous work. 3.1.3. Collection and inspection of rubble recruitment tiles and rubble plots The first collection of rubble occurred on the 2nd, 5th, 8th, 9th and 12th of February 2023, four months after initial deployment (i.e., settlement). During collection, all fixed rubble tiles and unfixed pieces in rubble beds from one substrate were collected at a time, brought back to the lab, and held in tanks with free-flowing seawater. Some of the unfixed rubble had moved during the four months. To find as many of the unfixed pieces as possible, we methodically searched within the rubble plots, lifting untagged in situ rubble in case tagged pieces had fallen between or beneath surrounding pieces. We also swam around the plot, and any tagged rubble not found within a 1 m radius of the plot was considered lost. Tiles were photographed and mapped, and each fixed piece was assigned a unique code to allow replacement to the original location and orientation. Each rubble piece (fixed and unfixed) was photographed on both sides, and the central diameter and length were measured. Surfaces were searched using an Olympus SZ dissection microscope to identify settled coral spat. Each rubble piece was assigned a reference end, and the distance from the reference point end to each coral settler was noted to record its location. The settler’s condition, e.g., alive, dead, or partial mortality, was documented, and rubble was kept in shallow dishes of seawater during the entirety of the inspection. After inspection, tiles and unfixed rubble were redeployed on the 5th, 8th, 9th, 12th, and 15th of February to their original locations. They were then collected for a final time on 22nd, 25th, 28th, and 30th of August 2023, ~ 11 months after initial deployment (i.e., recruitment). The second collection and inspection followed the same protocol as the first. Using the maps of the rubble tiles, the presence or absence of the original coral settlers was documented. 3.1.4. Sedimentation Differences in sedimentation rates were assessed using turfpods and followed the same design as in our previous study (Paewai-Huggins et al., 2025a), which was based on earlier work (Babcock et al., 2002; Latrille et al., 2019 ; Stewart et al., 2006). Five turfpods were deployed in each of the three substrates for seven days at three time points (February, August, and September 2023). Turfpods were deployed flush with surrounding rubble and placed directly onto the hard carbonate. After seven days, the turfpods were carefully collected. To remove any excess sediment, the sides and undersides were gently brushed before the entire pod was placed into a plastic bag, taking caution not to disturb any sediment collected on the turf. In the lab, each plastic bag containing a single turfpod was emptied into a large container, and the turfpod was rinsed thoroughly with fresh water to flush all trapped sediment into the container. This water and sediment mix was left to settle until the seawater could be poured off, and the remaining sample was emptied into a Falcon tube, which had been previously weighed. These samples were topped up with fresh water, left to settle, and excess water was again poured off. This was repeated twice to ensure that all salt had been removed from the sediment sample. Tubes were then oven-dried at 60° C for 24 hours to remove any remaining water and were then reweighed. Data were then standardised to sediment accumulated in grams per m 2 per day (Field et al., 2013). 3.1.5. Flow Using plaster-of-Paris (UniPro), we made 4cm 3 flow cubes to test water flow differences between the three substrates, using the dissolution rates of cubes as a proxy for flow (Fox, 2004 ). A higher dissolution rate indicated greater exposure to water flow. Three deployments of cubes took place in February, August, and September 2023. Here, we only used one cube treatment: ‘surface’, as it is impractical to bury cubes into the hard carbonate structure. Additionally, we previously investigated whether flow can penetrate deeper into the rubble bed, depending on the void spaces between individual rubble pieces, using a ‘buried’ cube treatment in (Paewai-Huggins et al., 2025a). Here, we focus on how the exposure of flow differs between the two rubble beds and the hard carbonate due to differences in the structural composition of each substrate. In February, four cubes were deployed in each habitat, placed directly on the surface of the in-situ surrounding rubble and hard carbonate reef (‘surface’ treatment). In August and September, replication was increased to six surface cubes deployed in each habitat. Before each deployment, cubes were weighed together with their identifying tag. Cubes were deployed for 24 hours before being collected, rinsed in fresh water to remove excess salt, and oven-dried at 60˚C for 24 hours before being weighed (Fox et al., 2003). The dissolution rate was calculated by subtracting the final weight of the flow cube (after deployment) from the cube's weight before deployment and dividing that by the time deployed (i.e., 24 hours) to determine the average dissolution rate per hour. 3.2. Statistical Analysis We first compared the absolute density of coral settlers and recruits per rubble piece across treatments and substrates. We then compared the rates of post-settlement mortality, which illustrates the difference between settlement and recruitment. We also evaluated patterns of sedimentation and flow rates across substrates. For each model, terms were determined to be significant, and backward stepwise model fitting of independent parameters was conducted; non-significant model terms were removed, and the best model was selected using the Akaike Information Criterion (AICc) with the package “MuMIn” (Barton, 2009 ). All statistical analyses were performed using R Core Team (2024). 3.2.1. Coral settlement (Four months post-deployment) To investigate how coral settlement varied across substrates and treatments, a generalised linear mixed-effects model with a negative binomial distribution was used using the package ‘glmmTMB’ (Brooks, 2025). The response variable was coral settler density (i.e., the number of coral settlers per rubble piece). The predictor variables, however, differed depending on whether settlement was being assessed within just the rubble beds (consideration of unfixed and fixed) or across all three substrates (consideration of fixed rubble only). When analysing settlement across the two rubble beds only, the predictor variables were substrate (loose vs. interlocked rubble) and rubble treatment (unfixed vs. fixed rubble) and the interaction between each predictor variable. When analysing settlement across all three substrates, the only predictor variable was substrate (loose vs. interlocked rubble vs. hard carbonate). A random effect of tile ID was included. The rubble surface area of each piece was also included as an offset to account for the fact that there could be more coral settlers on a larger rubble piece. 3.2.2. Coral recruitment (11 months post-deployment) To investigate how coral recruitment varied across substrates and treatments, a generalised linear mixed-effects model with a negative binomial distribution was used. The response variable was coral recruit density (i.e., the number of coral settlers from four months that were still present at 11 months per rubble piece); the predictor variables, again, differed depending on whether recruitment was being assessed within just the rubble beds (predictor variable of substrate and rubble treatment, and the interaction) or across all three substrates (predictor variables of substrate only). 3.2.3. Post-settlement mortality rate (11 months post-deployment) To determine how the probability of mortality occurring from three to 11 months differed between substrates and rubble treatments, generalised linear mixed-effects models were run. Again, predictor variables were substrate and treatment for the analysis considering rubble beds only and substrate only for the analysis considering all three substrates. A binomial error structure was used with a logit link function because the response variable was binary (i.e., 1 for a dead recruit, that had settled at three months and was no longer present at 11 months and 0 for a live recruit, that had settled at three months and was still present at 11 months). A random effect of tile ID was included. 3.2.4. Sedimentation A linear mixed-effect model was run to determine the difference in sedimentation rate (g m 2 per day) between the three substrates. The predictor variable was substrate (loose vs. interlocked vs. hard carbonate) with date included as a random effect to account for temporal variation (i.e. deployment trials from February, August, and September 2023. Although sedimentation rates between rubble beds were previously investigated and reported (Paewai-Huggins et al., 2025a), the data collected from the hard carbonate reef in this study are newly acquired. Therefore, the analysis presented here is novel and reported for the first time. 3.2.5. Flow A linear mixed-effect model was run to determine the difference in the dissolution rate of surface cubes across substrates. The predictor variable was substrate (loose vs. interlocked vs. hard carbonate), and date was included as a random effect to account for any temporal correlation of deployment trials (i.e., February vs. August vs. September 2023). Again, while we have previously investigated and reported on differences in flow between the rubble beds data collected from the hard carbonate reef in this study are newly acquired. Therefore, the analysis presented here is reported for the first time. Results 4. 1. Effects of stability on coral settlement and recruitment At the settlement census after four months, we found and collected 73% of the original unfixed rubble in the loose rubble bed. Of that 73% approximately 30% were found outside the boundary of the unfixed plots, with a few pieces (~ 15) found buried beneath in situ rubble (Paewai-Huggins pers. obvs). At the recruitment census at 11 months, we recovered even less of the original rubble on the loose bed (66%). Losses were slightly higher in the interlocked rubble, with a collection of 66% of unfixed rubble during the first collection and 61% during the second, with many pieces falling deep between the void spaces. Within both rubble beds, coral settlement was greater on the fixed than unfixed rubble (Fig. 2 a). In the interlocked bed, settlement was 3.5 times greater on the fixed rubble ( P < 0.001), and the benefit of stability was even greater at 4.3-fold in the loose rubble bed ( P < 0.001). Yet while the effect of stability was greater in the loose bed, the absolute level of settlement was 4-fold higher in the fixed treatment upon interlocked rubble bed (Fig. 2 a) ( P < 0.001). On unfixed rubble, the settlement was five times greater in the interlocked rubble compared to the loose rubble bed ( P = < 0.001). The patterns of settlement success among treatments mirrored that of recruitment (Fig. 2 c), reflecting consistent patterns of post-settlement mortality (Fig. 2 b). As a result, coral recruitment was greatest on the fixed rubble within the interlocked bed (Fig. 2 c). The likelihood of mortality was high across all treatments and rubble beds (Fig. 2 b), ranging from 85% to 100%. No corals survived for 11 months within the loose rubble bed on the unfixed rubble (Fig. 2 b and c). On average, there were 0.5 coral recruits per fixed piece of rubble within the interlocked rubble bed, nearly 4 times the number of recruits on unfixed rubble in the same substrate ( P < 0.001). 4.2. Comparison of settlement and recruitment rates across all substrates in the absence of mobility constraints (i.e., fixed rubble only) In the absence of mobility constraints, on the fixed rubble tiles, coral settlement increased from the loose rubble bed to the interlocked bed and to the hard carbonate reef (Fig. 3 a). Compared to the loose rubble, settlement was 9.4-fold greater onto tiles in the hard carbonate reef and 3-fold greater onto tiles in the interlocked rubble bed ( P 80%) (Fig. 3 b), recruitment remained highest within the hard carbonate (Fig. 3 c). At 11 months, there were 0.6 recruits per fixed rubble piece within the hard carbonate, which was twice that of the interlocked rubble and six times higher than in the loose rubble bed (Fig. 3 c). 4.3. Sedimentation Sedimentation rates among substrates exhibited minimal variation across the deployment periods (Fig. 4 ). Indicating broadly comparable sediment deposition and retention dynamics across substrates, regardless of temporal variation. On average, deposited sediment collected via turfpods was 5.4 g in the interlocked bed, 6.6 g in the loose bed, and 6.8 g in the hard carbonate reef (Fig. 4 ). 4.4. Flow Flow appeared to be comparable in each substrate, determined by the similarity in dissolution rate of surface cubes deployed (Fig. 5 ). On average, dissolution of surface cubes was 2.3 g in the interlocked bed and 2.4 g in both the loose rubble bed and hard carbonate. Discussion We quantified the effect of stability on coral settlement and recruitment. Rubble mobility clearly increases settlement rates and increases post-settlement mortality, regardless of the rubble substrate. Indeed, recruitment ranged from zero to almost zero (0.01 recruit per rubble) on unstable (unfixed) rubble irrespective of habitat type. While stability had a significant impact on recruitment, habitats were also important, with striking differences between loose, densely packed, low complexity rubble and the structurally complex, interlocked rubble. Settlement and recruitment were both higher in the interlocked habitat, and importantly, recruitment on stabilised rubble was twice as high as that on loose rubble. While stability is clearly highly important, results from the fixed rubble tiles deployed across all three substrates suggest that simply stabilising a rubble bed, whether loose or interlocked, will not result in comparable coral outcomes to those of an undisturbed hard carbonate reef habitat. Settlement and recruitment on the fixed rubble were greatest in the hard carbonate, lower in the interlocked rubble and lowest in the loose rubble bed. These results indicate that substrates have distinct environmental factors that differentially influence the settlement and recruitment of corals, even on stabilised rubble surfaces (Paewai-Huggins et al., 2025a). Sediments, water flow, and competition can all inhibit coral settlement and recruitment in rubble environments. Even modest amounts of sediment can impede the successful settlement or survival of coral larvae. P ocillorpora damicornis , for example, avoids settling on surfaces covered in > 0.9 mg cm − 2 of deposited sediment (Perez III et al., 2014 ). In another study, 0.8 mg cm − 2 d − 1 of deposited sediment resulted in no net recruitment after 12 months (Wakwella et al., 2020 ). We expected that the greater void space between rubble in the interlocked bed would allow sediment to fall into the matrix, away from settled corals. In the loose rubble, we expected sediments to remain on the upper surface due to the tightly packed nature of the rubble. However, the measured deposited sediment was not significantly higher in the loose rubble bed compared to the other substrates. In fact, deposited sediment was comparable across all substrates, making it difficult to conclude how the composition of each substrate influences sedimentation retention as well as the influence sedimentation has on settlement and recruitment rates witnessed here. However, in the past, we have witnessed significant differences in sediment deposition between the two rubble beds (Paewai-Huggins et al., 2025a). This variation in sedimentation processes can change in magnitude over time and is unlikely to be homogeneous. To obtain a clearer understanding of how sedimentation dynamics differ in response to substrate type, higher replication might be useful, as well as reconsidering the method of sediment measurement in rubble beds and hard carbonate habitats. Water flow could also be influenced by the structural complexity of each substrate. Low recruitment on unfixed rubble in the loose bed might have been caused by increased movement driven by the lack of structural complexity and consequent increased laminar flow. Furthermore, flow in the interlocked rubble and hard carbonate might have been broken up by the larger, branched and more complex configuration of rubble in the interlocked bed and the greater coral cover (Paewai-Huggins pers. obvs. ) of the hard carbonate reef. Higher laminar flow and less turbulence are likely to reduce the delivery of nutrients to corals in the loose bed, even if they are stable (Finelli et al., 2006 ). However, due to the comparability in the flow results presented here, few strong conclusions can be determined concerning variations between substrate compositions. Future research should focus on investigating how flow dynamics are influenced by substrates varying in structural complexity to better understand flow effects on coral recruitment. As expected, the effect size of stability on settlement and recruitment was larger in the loose rubble than in the interlocked bed. Thus, stabilisation had more of an impact on settlement and recruitment in the loose bed than it did in the interlocked bed. There is likely higher natural stability within the interlocked rubble due to the size, morphology, and configuration of the rubble (Kenyon et al., 2024 ). The absolute number of settlers and recruits was also higher in the interlocked rubble versus the loose rubble. The larger, branched rubble in the interlocked bed could facilitate coral larval settlement by providing higher structural complexity that breaks up the flow of water and enhances turbulence (Gysbers et al., 2024 ; Hata et al., 2017 ). Furthermore, both the unfixed and fixed rubble in the loose bed might have been subject to burial by the movement of surrounding rubble, whereas rubble in the interlocked bed was better protected by the more stable, interlocked configuration (Kenyon et al., 2024 ). Burial of rubble pieces can cause abrasion-induced damage to coral larvae (Fox, 2004 ) and reduce flow to settled corals. Water flow and turbulence at the benthic boundary layer are crucial components influencing the energy intake, particle capture (Finelli et, al., 2006; Sebens et. al., 1998), and mass transfer of nutrients to corals (Atkinson & Bilger, 1992 ; Dennison & Barnes, 1988 ; Lesser et al., 1994 ; Nakamura & Van Woesik, 2001 ; Patterson, 1991 ; Sebens & Johnson, 1991 ; Stambler et al., 1991 ). Unfortunately, this theory is poorly supported by the flow results presented here, as flow (dissolution) rates appear to be comparable regarding surface cubes between each of the substrate types, regardless of deployment trial. Improvements to our understanding of these substrates, regarding flow, could be made by increasing the sample size and temporal replication of flow measurements. In both rubble beds, stabilisation increased coral settlement and recruitment, indicating that stabilisation can promote positive coral outcomes regardless of substrate. Yet future studies will benefit from exploring the environmental effects that are unrelated to stability. Specifically, studies to improve understanding of how rubble morphometrics and bed configuration influence biophysical factors such as water flow and sedimentation are likely key to determining which types of rubble beds are likely to recover naturally or require restoration. References Atkinson, M., & Bilger, R. (1992). 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Herbivore abundance and grazing intensity on a Caribbean coral reef. Journal of Experimental Marine Biology and Ecology , 87 (3), 215-228. Nakamura, T. v., & Van Woesik, R. (2001). Water-flow rates and passive diffusion partially explain differential survival of corals during the 1998 bleaching event. Marine Ecology Progress Series , 212 , 301-304. Norström, A., Lokrantz, J., Nyström, M., & Yap, H. (2007). Influence of dead coral substrate morphology on patterns of juvenile coral distribution. Marine Biology , 150 , 1145-1152. Page, C. A., Giuliano, C., & Randall, C. J. (2024). Benthic communities influence coral seeding success at fine spatial scales. Restoration Ecology , 32 (7), e14212. Patterson, M. R. (1991). The effects of flow on polyp-level prey capture in an octocoral, Alcyonium siderium. The Biological Bulletin , 180 (1), 93-102. Perez III, K., Rodgers, K. S., Jokiel, P. L., Lager, C. V., & Lager, D. J. (2014). Effects of terrigenous sediment on settlement and survival of the reef coral Pocillopora damicornis. PeerJ , 2 , e387. Roth, F., Saalmann, F., Thomson, T., Coker, D. J., Villalobos, R., Jones, B., Wild, C., & Carvalho, S. (2018). Coral reef degradation affects the potential for reef recovery after disturbance. Marine Environmental Research , 142 , 48-58. Salinas-de-León, P., Dryden, C., Smith, D., & Bell, J. (2013). Temporal and spatial variability in coral recruitment on two Indonesian coral reefs: consistently lower recruitment to a degraded reef. Marine Biology , 160 (1), 97-105. Sánchez-Quinto, A., & Falcón, L. I. (2019). Metagenome of Acropora palmata coral rubble: Potential metabolic pathways and diversity in the reef ecosystem. Plos one , 14 (8), e0220117. Sebens, K., & Johnson, A. (1991). Effects of water movement on prey capture and distribution of reef corals. Hydrobiologia , 226 , 91-101. Smith, L., & Hughes, T. (1999). An experimental assessment of survival, re-attachment and fecundity of coral fragments. Journal of Experimental Marine Biology and Ecology , 235 (1), 147-164. Stambler, N., Popper, N., Dubinsky, Z., & Stimson, J. (1991). Effects of nutrient enrichment and water motion on the coral Pocillopora damicornis. Tunnicliffe, V. J. (1980). Biological and physical processes affecting the survival of a stony coral, Acropora cervicornis . Yale University. Viehman, S., Hench, J. L., Griffin, S. P., Malhotra, A., Egan, K., & Halpin, P. N. (2018). Understanding differential patterns in coral reef recovery: chronic hydrodynamic disturbance as a limiting mechanism for coral colonization. Marine Ecology Progress Series , 605 . https://doi.org/10.3354/meps12714 Wakwella, A., Mumby, P. J., & Roff, G. (2020). Sedimentation and overfishing drive changes in early succession and coral recruitment. Proceedings of the Royal Society B-Biological Sciences , 287 (1941). https://doi.org/ARTN 20202575 10.1098/rspb.2020.2575 Wakwella, A., Mumby, P. J., & Roff, G. (2020). Sedimentation and overfishing drive changes in early succession and coral recruitment. Proceedings of the Royal Society B , 287 (1941), 20202575. Wilson, S. K., Graham, N. A., Pratchett, M. S., Jones, G. P., & Polunin, N. V. (2006). Multiple disturbances and the global degradation of coral reefs: are reef fishes at risk or resilient? Global change biology , 12 (11), 2220-2234. Wolfe, K., Kenyon, T. M., & Mumby, P. J. (2021). The biology and ecology of coral rubble and implications for the future of coral reefs. Coral reefs , 40 (6), 1769-1806. Woodley, J., Chornesky, E., Clifford, P., Jackson, J., Kaufman, L., Knowlton, N., Lang, J., Pearson, M., Porter, J., & Rooney, M. (1981). Hurricane Allen's impact on Jamaican coral reefs. Science , 214 (4522), 749-755. Wulff, J. L. (1984). Sponge-Mediated Coral-Reef Growth and Rejuvenation. Coral reefs , 3 (3), 157-163. https://doi.org/Doi 10.1007/Bf00301960 Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7974089","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":536540374,"identity":"60d52456-86d3-457c-b846-a28fe21787bd","order_by":0,"name":"Roima 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12:06:32","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":112751,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/53bdf491dc4caf7c8fa1ccde.html"},{"id":94760973,"identity":"471363ef-3110-4de6-93bb-b8da01fa3732","added_by":"auto","created_at":"2025-10-30 12:06:32","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":323911,"visible":true,"origin":"","legend":"\u003cp\u003eA panel of the study sites and rubble treatments, (a) Heron Island Reef and Wistari Reef (Google Earth, 2021); (b) close-up of the loose rubble bed; (c) close-up of the interlocked rubble bed; (d) fixed rubble recruitment tiles; (e) unfixed rubble plots.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/7ab3247726552d0b5f3c9522.png"},{"id":94823665,"identity":"2456ff14-047f-4e0f-94d6-5e87ae307c5e","added_by":"auto","created_at":"2025-10-31 06:47:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":61681,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of stability and substrate type on (a) the abundance (mean ± SE) of coral settlers four months post-deployment, (b) the likelihood of post-settlement mortality between 4 and 11 months, and (c) the abundance (mean ± SE) of recruits at 11 months post-deployment for fixed and unfixed rubble in loose and interlocked rubble beds. Differing letters signify a significantly different mean. Significance is only investigated between the two rubble treatments within a single substrate (i.e. unfixed rubble vs. fixed rubble in the loose rubble bed) or between the same rubble treatment across the two substrates (i.e., unfixed rubble in the loose bed vs. unfixed rubble in the interlocked bed). We do not investigate the significance between the different treatments across the two substrates (i.e., unfixed rubble in the loose bed vs. fixed rubble in the interlocked bed).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/f70636c1443163b57cd24283.png"},{"id":94760972,"identity":"1e9b67fb-085e-4006-a944-92ed55b386e6","added_by":"auto","created_at":"2025-10-30 12:06:32","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":47721,"visible":true,"origin":"","legend":"\u003cp\u003eRates of (a) the abundance (mean ± SE) of coral settlers four months post-deployment, (b) the likelihood of post-settlement mortality between 4 and 11 months, and (c) the abundance (mean ± SE) of recruits at 11 months post-deployment on fixed rubble across loose rubble, interlocked rubble and hard carbonate of Heron Reef. Differing letters signify a significantly different mean.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/9af311a4d4d5497cd2496fc3.png"},{"id":94760974,"identity":"1194cb78-c091-4744-9337-6b21890b65b4","added_by":"auto","created_at":"2025-10-30 12:06:32","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":20046,"visible":true,"origin":"","legend":"\u003cp\u003eThe average (mean ± SE) deposited sediment collected on the turf pods in 2023 in each substrate.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/3b6fe98e043d8ba20e5f39fc.png"},{"id":94824786,"identity":"b6cdeb35-a4f5-4e41-8d1c-a745490ba932","added_by":"auto","created_at":"2025-10-31 06:49:20","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":20464,"visible":true,"origin":"","legend":"\u003cp\u003eAverage (mean ± SE) dissolution (g) of plaster-of-Paris flow cubes between substrates in 2023.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/7764a3d683d653def41a138d.png"},{"id":95221054,"identity":"09dabcae-b2f2-42a6-8b2c-596bb9be6fe4","added_by":"auto","created_at":"2025-11-05 16:18:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1113175,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7974089/v1/1224c95d-425c-4ab1-8a30-c6fb65bdfbce.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eThe effect of rubble stability on coral settlement and recruitment\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eScleractinian corals or ‘stony corals’ secrete calcium carbonate skeletons which form coral reefs through accretion, providing a stable and structural habitat (Salinas-de-León et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Sánchez-Quinto \u0026amp; Falcón, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Biological, chemical, and mechanical processes can reduce or diminish the structural complexity of reefs by breaking up and eroding the coral framework into rubble. This is a natural cycle of disturbance and recovery on coral reefs. Wave energy from storm disturbances such as cyclones, hurricanes, and tsunamis has resulted in vast rubble beds (Blanchon et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Viehman et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). However, impacts from climate change, such as increased intensity of storms and cyclones, coral bleaching, and disease, may alter these cycles and narrow the recovery windows (Ceccarelli et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Knutson et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAfter a disturbance, recovery is heavily reliant upon coral recruitment (Doropoulos et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The success of coral recruitment is driven by various environmental and ecological factors. A chief prerequisite is thought to be a stable substrate onto which coral larvae can settle and grow (Cameron et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Ceccarelli et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Gouezo et al., 2021). Other factors that influence the success of coral recruitment include the abundance and diversity of coral larvae, the cover of recruitment facilitators and inhibitors, hydrodynamic conditions, sedimentation, corallivory and grazing, temperature, eutrophication, grazing pressures, light, connectivity between reefs, and coral larvae substrate preferences (Doropoulos et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Doropoulos et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Edmunds, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Harrington et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Harriott \u0026amp; Fisk, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Norström et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Salinas-de-León et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Ama Wakwella et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The existence of multiple drivers makes it challenging to determine which scenarios will result in the most successful recruitment outcomes (Doropoulos et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Edmunds, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2000\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eStudies of the drivers of coral settlement and recruitment generally focus on hard carbonate surfaces, with limited work having been conducted on rubble beds (Cameron et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Chong-Seng et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Fox, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Rubble can experience greater hydrodynamic energy than a complex reef owing to the reduced drag friction that structural complexity on an intact reef provides (Guihen et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Greater hydrodynamic energy can make rubble more susceptible to mobilisation (Fox \u0026amp; Caldwell, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), which can cause the abrasion, dislodgement, and smothering of settled coral larvae (Brown \u0026amp; Dunne, 1988; Chong-Seng et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Fox, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Indeed, this is believed to be the driving force behind the low success of coral recruitment and, consequently, coral recovery on rubble beds (Chong-Seng et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Fox, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fox \u0026amp; Caldwell, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Heyward et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Page et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). However, rubble can vary in size, morphology, bed thickness, and configuration (Kenyon et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), reflecting the disturbance type, intensity, and pre-disturbance community, i.e., the dominating coral genus and/or coral morphology (Leung \u0026amp; Mumby, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Variations of rubble morphometrics can influence rubble mobilisation, biophysical factors (e.g., water flow or sedimentation) and coral recruitment dynamics (Kenyon et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eA rubble bed composed of larger, branched, more structurally complex rubble, with greater void space, may reduce the speed of laminar flow across the bed and increase turbulence, which helps coral larvae to settle (Hata et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Space between pieces of rubble could also facilitate water movement further into the bed, carrying oxygen and nutrients to settled corals. However, greater flow may also facilitate colonisation of other sessile taxa such as bryozoans, ascidians, and turf algae, which can out-compete corals for space and increase post-settlement mortality (Chong-Seng et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Doropoulos et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). A flat or less structurally complex rubble bed, with little to no space between rubble pieces and fewer cryptic spaces, may elevate the exposure of settled corals to direct and incidental predation (Lewis \u0026amp; Wainwright, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Wilson et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Woodley et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1981\u003c/span\u003e) and higher deposited sediment (Paewai-Huggins et al., 2025a). Compared to rubble beds with larger void spaces, sediment deposited on flat, tightly packed beds cannot fall deeper into the matrix and might remain on the uppermost layer of rubble. Deposited sediment can inhibit settlement and increase post-settlement mortality due to smothering (Norström et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Rubble morphometrics and environmental factors will also strongly influence the likelihood of rubble movement. Rubble of shorter length and no branches may be more susceptible to movement than longer, branched rubble (Kenyon et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Environmental variables of water flow (Fox et al., 2003), the inundation period in response to tidal fluctuation (Thornborough et al., 2012), wind speed and direction (Heyward et al., 1985), and water depth (Bruno et al, 1998; Kenyon et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) all interact with rubble morphometrics to dictate rubble mobilisation. Thus, multiple biophysical factors, other than mobilisation, vary with rubble type and can influence coral recruitment.\u003c/p\u003e\u003cp\u003eIn a recent study, we asked how the environmental effects associated with two contrasting types of rubble beds influence coral settlement and recruitment (Paewai-Huggins et al., 2025a). We found important environmental effects and associated these with flow and sediment dynamics. To answer that question, we used fixed rubble recruitment tiles to remove any effect of rubble mobility. Here, in the present study, we repeat our previous work but add a new treatment of loose, unfixed rubble. Our motivation here was to gain a holistic understanding of the relative importance of substrate stability and environmental effects. It was necessary to repeat our earlier treatments with fixed rubble in case the drivers of recruitment had changed since the earlier study on environmental effects. Since we focus here on the role of stability and the relative importance of environmental effects, we devote less space to exploring environmental effects and their causation, as this was studied in the earlier manuscript.\u003c/p\u003e\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Methods","content":"\u003ch2\u003e3.1. Study sites\u003c/h2\u003e\u003cp\u003eTwo rubble substrates and a hard carbonate substrate, all at a depth of 8–9 m, were identified at Heron Reef, in the Sea Country of the Gooreng Gooreng, Gurang, Bailai and Taribelang Bunda peoples, Southern Great Barrier Reef, Australia (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). All three substrates were located on the south-west slope of Heron Reef. The two rubble beds are the same habitats that were used in Paewai-Huggins et al. (2025a) rubble study. However, the hard carbonate habitat discussed here is new, located after the first experiments (Paewai-Huggins et al., 2025) concluded.\u003c/p\u003e\u003cp\u003eThe rubble beds consisted of a loose rubble bed and an interlocked rubble bed. The third substrate was a hard carbonate reef, composed of large ‘bommies’ of solid carbonate rock. Substrates were spaced along and parallel to the reef slope, 200 m apart. Due to their proximity, each substrate was expected to experience similar light exposure, water temperature, wave exposure, and sedimentation.\u003c/p\u003e\u003cp\u003eThe loose rubble bed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) is comprised of small, typically non-branched rubble pieces. The interlocked rubble bed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) is composed of larger, branching rubble pieces. For a full description of rubble comprising either bed, see (Paewai-Huggins et al., 2025a).\u003c/p\u003e\u003ch2\u003e3.1.\u003cem\u003e2. Recruitment tiles and loose rubble plot construction and deployment\u003c/em\u003e\u003c/h2\u003e\u003cem\u003e2. Recruitment tiles and loose rubble plot construction and deployment\u003c/em\u003e\u003cp\u003eTwo rubble treatments were used: fixed and unfixed. To remove bias from the type of rubble available in each of the two rubble habitats, a collection was made from both loose and interlocked rubble beds, and the resulting material was mixed thoroughly before being treated and deployed back into each habitat and on hard substrate. Treatment involved cleaning rubble with fresh water, bleaching overnight to remove living material, rinsing again, and drying in sunlight. Recruitment tiles comprised an 18 x 18 cm stainless steel mesh onto which rubble pieces were fixed with cable ties in pairs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). While this is the same design used in (Paewai-Huggins et al., 2025a), all tiles used here were built specifically for this study; none which were previously used were used here. Each tile consisted of 10 pairs of rubble and was labelled with a unique ID tag. Rubble pieces were placed in pairs as it was easier to attach the rubble to the stainless-steel mesh, and this mimics the fact that rubble is rarely isolated. A total of 60 tiles and 600 fixed rubble pieces were evenly distributed across the three substrate habitats, i.e., 20 tiles per substrate within a 5 x 5 m area. Tiles were connected to the substrate using 1 m basalt reinforcing bar stakes and placed flush with the surrounding rubble or hard carbonate reef.\u003c/p\u003e\u003cp\u003eAlong with the deployment of fixed rubble tiles, unfixed rubble was deployed within each of the two rubble beds (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee), but not within the hard carbonate habitat, as there is no naturally occurring rubble in this habitat. The unfixed rubble plots consisted of four rebar stakes, spaced 30 cm apart, in a square. Within each permanent plot, 40 unfixed rubble pieces were placed within each plot. Each piece was labelled with a Floy tag with a unique ID, and tags were securely fastened, and tails were snipped so as not to impede rubble movement. A total of 12 plots and 480 unfixed rubble pieces were deployed across the two rubble substrates. Fixed rubble tiles and unfixed rubble were deployed on the 18th of October 2022 to develop a biofilm before the coral spawning event, which took place in November 2022. The new instalment of fixed and unfixed rubble did not overlap in time with rubble recruitment tiles used in (Paewai-Huggins et al., 2025a) experiment. All data presented here is new and distinct from our previous work.\u003c/p\u003e\u003ch2\u003e3.1.3. Collection and inspection of rubble recruitment tiles and rubble plots\u003c/h2\u003e\u003cp\u003eThe first collection of rubble occurred on the 2nd, 5th, 8th, 9th and 12th of February 2023, four months after initial deployment (i.e., settlement). During collection, all fixed rubble tiles and unfixed pieces in rubble beds from one substrate were collected at a time, brought back to the lab, and held in tanks with free-flowing seawater. Some of the unfixed rubble had moved during the four months. To find as many of the unfixed pieces as possible, we methodically searched within the rubble plots, lifting untagged in situ rubble in case tagged pieces had fallen between or beneath surrounding pieces. We also swam around the plot, and any tagged rubble not found within a 1 m radius of the plot was considered lost.\u003c/p\u003e\u003cp\u003eTiles were photographed and mapped, and each fixed piece was assigned a unique code to allow replacement to the original location and orientation. Each rubble piece (fixed and unfixed) was photographed on both sides, and the central diameter and length were measured. Surfaces were searched using an Olympus SZ dissection microscope to identify settled coral spat. Each rubble piece was assigned a reference end, and the distance from the reference point end to each coral settler was noted to record its location. The settler’s condition, e.g., alive, dead, or partial mortality, was documented, and rubble was kept in shallow dishes of seawater during the entirety of the inspection.\u003c/p\u003e\u003cp\u003eAfter inspection, tiles and unfixed rubble were redeployed on the 5th, 8th, 9th, 12th, and 15th of February to their original locations. They were then collected for a final time on 22nd, 25th, 28th, and 30th of August 2023, ~ 11 months after initial deployment (i.e., recruitment). The second collection and inspection followed the same protocol as the first. Using the maps of the rubble tiles, the presence or absence of the original coral settlers was documented.\u003c/p\u003e\u003ch2\u003e3.1.4. Sedimentation\u003c/h2\u003e\u003cp\u003eDifferences in sedimentation rates were assessed using turfpods and followed the same design as in our previous study (Paewai-Huggins et al., 2025a), which was based on earlier work (Babcock et al., 2002; Latrille et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Stewart et al., 2006). Five turfpods were deployed in each of the three substrates for seven days at three time points (February, August, and September 2023). Turfpods were deployed flush with surrounding rubble and placed directly onto the hard carbonate. After seven days, the turfpods were carefully collected. To remove any excess sediment, the sides and undersides were gently brushed before the entire pod was placed into a plastic bag, taking caution not to disturb any sediment collected on the turf. In the lab, each plastic bag containing a single turfpod was emptied into a large container, and the turfpod was rinsed thoroughly with fresh water to flush all trapped sediment into the container. This water and sediment mix was left to settle until the seawater could be poured off, and the remaining sample was emptied into a Falcon tube, which had been previously weighed. These samples were topped up with fresh water, left to settle, and excess water was again poured off. This was repeated twice to ensure that all salt had been removed from the sediment sample. Tubes were then oven-dried at 60° C for 24 hours to remove any remaining water and were then reweighed. Data were then standardised to sediment accumulated in grams per m\u003csup\u003e2\u003c/sup\u003e per day (Field et al., 2013).\u003c/p\u003e\u003ch2\u003e3.1.5. Flow\u003c/h2\u003e\u003cp\u003eUsing plaster-of-Paris (UniPro), we made 4cm\u003csup\u003e3\u003c/sup\u003e flow cubes to test water flow differences between the three substrates, using the dissolution rates of cubes as a proxy for flow (Fox, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). A higher dissolution rate indicated greater exposure to water flow.\u003c/p\u003e\u003cp\u003eThree deployments of cubes took place in February, August, and September 2023. Here, we only used one cube treatment: ‘surface’, as it is impractical to bury cubes into the hard carbonate structure. Additionally, we previously investigated whether flow can penetrate deeper into the rubble bed, depending on the void spaces between individual rubble pieces, using a ‘buried’ cube treatment in (Paewai-Huggins et al., 2025a). Here, we focus on how the exposure of flow differs between the two rubble beds and the hard carbonate due to differences in the structural composition of each substrate.\u003c/p\u003e\u003cp\u003eIn February, four cubes were deployed in each habitat, placed directly on the surface of the in-situ surrounding rubble and hard carbonate reef (‘surface’ treatment). In August and September, replication was increased to six surface cubes deployed in each habitat.\u003c/p\u003e\u003cp\u003eBefore each deployment, cubes were weighed together with their identifying tag. Cubes were deployed for 24 hours before being collected, rinsed in fresh water to remove excess salt, and oven-dried at 60˚C for 24 hours before being weighed (Fox et al., 2003). The dissolution rate was calculated by subtracting the final weight of the flow cube (after deployment) from the cube's weight before deployment and dividing that by the time deployed (i.e., 24 hours) to determine the average dissolution rate per hour.\u003c/p\u003e\u003ch2\u003e3.2. Statistical Analysis\u003c/h2\u003e\u003cp\u003eWe first compared the absolute density of coral settlers and recruits per rubble piece across treatments and substrates. We then compared the rates of post-settlement mortality, which illustrates the difference between settlement and recruitment. We also evaluated patterns of sedimentation and flow rates across substrates. For each model, terms were determined to be significant, and backward stepwise model fitting of independent parameters was conducted; non-significant model terms were removed, and the best model was selected using the Akaike Information Criterion (AICc) with the package “MuMIn” (Barton, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). All statistical analyses were performed using R Core Team (2024).\u003c/p\u003e\u003ch2\u003e3.2.1. Coral settlement (Four months post-deployment)\u003c/h2\u003e\u003cp\u003eTo investigate how coral settlement varied across substrates and treatments, a generalised linear mixed-effects model with a negative binomial distribution was used using the package ‘glmmTMB’ (Brooks, 2025). The response variable was coral settler density (i.e., the number of coral settlers per rubble piece). The predictor variables, however, differed depending on whether settlement was being assessed within just the rubble beds (consideration of unfixed and fixed) or across all three substrates (consideration of fixed rubble only).\u003c/p\u003e\u003cp\u003eWhen analysing settlement across the two rubble beds only, the predictor variables were substrate (loose vs. interlocked rubble) and rubble treatment (unfixed vs. fixed rubble) and the interaction between each predictor variable. When analysing settlement across all three substrates, the only predictor variable was substrate (loose vs. interlocked rubble vs. hard carbonate). A random effect of tile ID was included. The rubble surface area of each piece was also included as an offset to account for the fact that there could be more coral settlers on a larger rubble piece.\u003c/p\u003e\u003ch2\u003e3.2.2. Coral recruitment (11 months post-deployment)\u003c/h2\u003e\u003cp\u003eTo investigate how coral recruitment varied across substrates and treatments, a generalised linear mixed-effects model with a negative binomial distribution was used. The response variable was coral recruit density (i.e., the number of coral settlers from four months that were still present at 11 months per rubble piece); the predictor variables, again, differed depending on whether recruitment was being assessed within just the rubble beds (predictor variable of substrate and rubble treatment, and the interaction) or across all three substrates (predictor variables of substrate only).\u003c/p\u003e\u003ch2\u003e3.2.3. Post-settlement mortality rate (11 months post-deployment)\u003c/h2\u003e\u003cp\u003eTo determine how the probability of mortality occurring from three to 11 months differed between substrates and rubble treatments, generalised linear mixed-effects models were run. Again, predictor variables were substrate and treatment for the analysis considering rubble beds only and substrate only for the analysis considering all three substrates. A binomial error structure was used with a logit link function because the response variable was binary (i.e., 1 for a dead recruit, that had settled at three months and was no longer present at 11 months and 0 for a live recruit, that had settled at three months and was still present at 11 months). A random effect of tile ID was included.\u003c/p\u003e\u003ch2\u003e3.2.4. Sedimentation\u003c/h2\u003e\u003cp\u003eA linear mixed-effect model was run to determine the difference in sedimentation rate (g m\u003csup\u003e2\u003c/sup\u003e per day) between the three substrates. The predictor variable was substrate (loose vs. interlocked vs. hard carbonate) with date included as a random effect to account for temporal variation (i.e. deployment trials from February, August, and September 2023. Although sedimentation rates between rubble beds were previously investigated and reported (Paewai-Huggins et al., 2025a), the data collected from the hard carbonate reef in this study are newly acquired. Therefore, the analysis presented here is novel and reported for the first time.\u003c/p\u003e\u003ch2\u003e3.2.5. Flow\u003c/h2\u003e\u003cp\u003eA linear mixed-effect model was run to determine the difference in the dissolution rate of surface cubes across substrates. The predictor variable was substrate (loose vs. interlocked vs. hard carbonate), and date was included as a random effect to account for any temporal correlation of deployment trials (i.e., February vs. August vs. September 2023). Again, while we have previously investigated and reported on differences in flow between the rubble beds data collected from the hard carbonate reef in this study are newly acquired. Therefore, the analysis presented here is reported for the first time.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e4.\u003cem\u003e1. Effects of stability on coral settlement and recruitment\u003c/em\u003e\u003c/h2\u003e\u003cp\u003eAt the settlement census after four months, we found and collected 73% of the original unfixed rubble in the loose rubble bed. Of that 73% approximately 30% were found outside the boundary of the unfixed plots, with a few pieces (~\u0026thinsp;15) found buried beneath in situ rubble (Paewai-Huggins \u003cem\u003epers. obvs).\u003c/em\u003e At the recruitment census at 11 months, we recovered even less of the original rubble on the loose bed (66%). Losses were slightly higher in the interlocked rubble, with a collection of 66% of unfixed rubble during the first collection and 61% during the second, with many pieces falling deep between the void spaces.\u003c/p\u003e\u003cp\u003eWithin both rubble beds, coral settlement was greater on the fixed than unfixed rubble (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). In the interlocked bed, settlement was 3.5 times greater on the fixed rubble (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), and the benefit of stability was even greater at 4.3-fold in the loose rubble bed (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Yet while the effect of stability was greater in the loose bed, the absolute level of settlement was 4-fold higher in the fixed treatment upon interlocked rubble bed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). On unfixed rubble, the settlement was five times greater in the interlocked rubble compared to the loose rubble bed (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u0026lt;\u0026thinsp;0.001). The patterns of settlement success among treatments mirrored that of recruitment (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), reflecting consistent patterns of post-settlement mortality (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). As a result, coral recruitment was greatest on the fixed rubble within the interlocked bed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe likelihood of mortality was high across all treatments and rubble beds (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), ranging from 85% to 100%. No corals survived for 11 months within the loose rubble bed on the unfixed rubble (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb and c). On average, there were 0.5 coral recruits per fixed piece of rubble within the interlocked rubble bed, nearly 4 times the number of recruits on unfixed rubble in the same substrate (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e\u003cp\u003e\u003cem\u003e4.2. Comparison of settlement and recruitment rates across all substrates in the absence of mobility constraints (i.e., fixed rubble only)\u003c/em\u003e\u003c/p\u003e\u003cp\u003eIn the absence of mobility constraints, on the fixed rubble tiles, coral settlement increased from the loose rubble bed to the interlocked bed and to the hard carbonate reef (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Compared to the loose rubble, settlement was 9.4-fold greater onto tiles in the hard carbonate reef and 3-fold greater onto tiles in the interlocked rubble bed (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAlthough the likelihood of mortality on fixed rubble was high across all three substrates (\u0026gt;\u0026thinsp;80%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), recruitment remained highest within the hard carbonate (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). At 11 months, there were 0.6 recruits per fixed rubble piece within the hard carbonate, which was twice that of the interlocked rubble and six times higher than in the loose rubble bed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.3. Sedimentation\u003c/h2\u003e\u003cp\u003eSedimentation rates among substrates exhibited minimal variation across the deployment periods (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Indicating broadly comparable sediment deposition and retention dynamics across substrates, regardless of temporal variation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eOn average, deposited sediment collected via turfpods was 5.4 g in the interlocked bed, 6.6 g in the loose bed, and 6.8 g in the hard carbonate reef (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.4. Flow\u003c/h2\u003e\u003cp\u003eFlow appeared to be comparable in each substrate, determined by the similarity in dissolution rate of surface cubes deployed (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eOn average, dissolution of surface cubes was 2.3 g in the interlocked bed and 2.4 g in both the loose rubble bed and hard carbonate.\u003c/p\u003e\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eWe quantified the effect of stability on coral settlement and recruitment. Rubble mobility clearly increases settlement rates and increases post-settlement mortality, regardless of the rubble substrate. Indeed, recruitment ranged from zero to almost zero (0.01 recruit per rubble) on unstable (unfixed) rubble irrespective of habitat type. While stability had a significant impact on recruitment, habitats were also important, with striking differences between loose, densely packed, low complexity rubble and the structurally complex, interlocked rubble. Settlement and recruitment were both higher in the interlocked habitat, and importantly, recruitment on stabilised rubble was twice as high as that on loose rubble.\u003c/p\u003e\u003cp\u003eWhile stability is clearly highly important, results from the fixed rubble tiles deployed across all three substrates suggest that simply stabilising a rubble bed, whether loose or interlocked, will not result in comparable coral outcomes to those of an undisturbed hard carbonate reef habitat. Settlement and recruitment on the fixed rubble were greatest in the hard carbonate, lower in the interlocked rubble and lowest in the loose rubble bed. These results indicate that substrates have distinct environmental factors that differentially influence the settlement and recruitment of corals, even on stabilised rubble surfaces (Paewai-Huggins et al., 2025a).\u003c/p\u003e\u003cp\u003eSediments, water flow, and competition can all inhibit coral settlement and recruitment in rubble environments. Even modest amounts of sediment can impede the successful settlement or survival of coral larvae. P\u003cem\u003eocillorpora damicornis\u003c/em\u003e, for example, avoids settling on surfaces covered in \u0026gt;\u0026thinsp;0.9 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e of deposited sediment (Perez III et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In another study, 0.8 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of deposited sediment resulted in no net recruitment after 12 months (Wakwella et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). We expected that the greater void space between rubble in the interlocked bed would allow sediment to fall into the matrix, away from settled corals. In the loose rubble, we expected sediments to remain on the upper surface due to the tightly packed nature of the rubble. However, the measured deposited sediment was not significantly higher in the loose rubble bed compared to the other substrates. In fact, deposited sediment was comparable across all substrates, making it difficult to conclude how the composition of each substrate influences sedimentation retention as well as the influence sedimentation has on settlement and recruitment rates witnessed here. However, in the past, we have witnessed significant differences in sediment deposition between the two rubble beds (Paewai-Huggins et al., 2025a). This variation in sedimentation processes can change in magnitude over time and is unlikely to be homogeneous. To obtain a clearer understanding of how sedimentation dynamics differ in response to substrate type, higher replication might be useful, as well as reconsidering the method of sediment measurement in rubble beds and hard carbonate habitats.\u003c/p\u003e\u003cp\u003eWater flow could also be influenced by the structural complexity of each substrate. Low recruitment on unfixed rubble in the loose bed might have been caused by increased movement driven by the lack of structural complexity and consequent increased laminar flow. Furthermore, flow in the interlocked rubble and hard carbonate might have been broken up by the larger, branched and more complex configuration of rubble in the interlocked bed and the greater coral cover (Paewai-Huggins \u003cem\u003epers. obvs.\u003c/em\u003e) of the hard carbonate reef. Higher laminar flow and less turbulence are likely to reduce the delivery of nutrients to corals in the loose bed, even if they are stable (Finelli et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). However, due to the comparability in the flow results presented here, few strong conclusions can be determined concerning variations between substrate compositions. Future research should focus on investigating how flow dynamics are influenced by substrates varying in structural complexity to better understand flow effects on coral recruitment.\u003c/p\u003e\u003cp\u003eAs expected, the effect size of stability on settlement and recruitment was larger in the loose rubble than in the interlocked bed. Thus, stabilisation had more of an impact on settlement and recruitment in the loose bed than it did in the interlocked bed. There is likely higher natural stability within the interlocked rubble due to the size, morphology, and configuration of the rubble (Kenyon et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The absolute number of settlers and recruits was also higher in the interlocked rubble versus the loose rubble. The larger, branched rubble in the interlocked bed could facilitate coral larval settlement by providing higher structural complexity that breaks up the flow of water and enhances turbulence (Gysbers et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Hata et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Furthermore, both the unfixed and fixed rubble in the loose bed might have been subject to burial by the movement of surrounding rubble, whereas rubble in the interlocked bed was better protected by the more stable, interlocked configuration (Kenyon et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Burial of rubble pieces can cause abrasion-induced damage to coral larvae (Fox, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) and reduce flow to settled corals. Water flow and turbulence at the benthic boundary layer are crucial components influencing the energy intake, particle capture (Finelli et, al., 2006; Sebens et. al., 1998), and mass transfer of nutrients to corals (Atkinson \u0026amp; Bilger, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Dennison \u0026amp; Barnes, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1988\u003c/span\u003e; Lesser et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Nakamura \u0026amp; Van Woesik, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Patterson, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Sebens \u0026amp; Johnson, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Stambler et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). Unfortunately, this theory is poorly supported by the flow results presented here, as flow (dissolution) rates appear to be comparable regarding surface cubes between each of the substrate types, regardless of deployment trial. Improvements to our understanding of these substrates, regarding flow, could be made by increasing the sample size and temporal replication of flow measurements.\u003c/p\u003e\u003cp\u003eIn both rubble beds, stabilisation increased coral settlement and recruitment, indicating that stabilisation can promote positive coral outcomes regardless of substrate. Yet future studies will benefit from exploring the environmental effects that are unrelated to stability. Specifically, studies to improve understanding of how rubble morphometrics and bed configuration influence biophysical factors such as water flow and sedimentation are likely key to determining which types of rubble beds are likely to recover naturally or require restoration.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAtkinson, M., \u0026amp; Bilger, R. (1992). Effects of water velocity on phosphate uptake in coral reef‐hat communities. \u003cem\u003eLimnology and Oceanography\u003c/em\u003e,\u003cem\u003e\u0026nbsp;37\u003c/em\u003e(2), 273-279.\u003c/li\u003e\n \u003cli\u003eAuthority, G. B. R. M. P. (1997). The Great Barrier Reef: science use and management, a national conference. Proceedings volumes 1 and 2.\u003c/li\u003e\n \u003cli\u003eBarton, K. (2009). 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A., Pratchett, M. S., Jones, G. P., \u0026amp; Polunin, N. V. (2006). Multiple disturbances and the global degradation of coral reefs: are reef fishes at risk or resilient? \u003cem\u003eGlobal change biology\u003c/em\u003e,\u003cem\u003e\u0026nbsp;12\u003c/em\u003e(11), 2220-2234.\u003c/li\u003e\n \u003cli\u003eWolfe, K., Kenyon, T. M., \u0026amp; Mumby, P. J. (2021). The biology and ecology of coral rubble and implications for the future of coral reefs. \u003cem\u003eCoral reefs\u003c/em\u003e,\u003cem\u003e\u0026nbsp;40\u003c/em\u003e(6), 1769-1806.\u003c/li\u003e\n \u003cli\u003eWoodley, J., Chornesky, E., Clifford, P., Jackson, J., Kaufman, L., Knowlton, N., Lang, J., Pearson, M., Porter, J., \u0026amp; Rooney, M. (1981). Hurricane Allen\u0026apos;s impact on Jamaican coral reefs. \u003cem\u003eScience\u003c/em\u003e,\u003cem\u003e\u0026nbsp;214\u003c/em\u003e(4522), 749-755.\u003c/li\u003e\n \u003cli\u003eWulff, J. L. (1984). Sponge-Mediated Coral-Reef Growth and Rejuvenation. \u003cem\u003eCoral reefs\u003c/em\u003e,\u003cem\u003e\u0026nbsp;3\u003c/em\u003e(3), 157-163. https://doi.org/Doi 10.1007/Bf00301960\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"University of Queensland","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"rubble, coral settlement, post-settlement survival, substrate habitats","lastPublishedDoi":"10.21203/rs.3.rs-7974089/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7974089/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCoral reef rubble comprises detached, fragmented, dead coral skeletons that can be unstable and are widely considered to constitute unsuitable substrate for coral recruitment. However, the type, severity, and frequency of a disturbance can generate rubble of diverse sizes, morphologies, and configurations, creating varying typologies of rubble beds. These physical characteristics play a substantial role in determining whether rubble remains stable or is frequently mobilised. We ask how the type of substrate influences the density of corals settling and recruiting onto rubble varying in its level of stability. Within two types of rubble beds, loose and interlocked, and on a control hard carbonate substrate, we measured coral settlement (four months post-spawning) and later-stage recruitment (post-settlement survival within 11 months) on unstable, unfixed rubble and on stabilised, fixed rubble. Both stability and substrate type influenced settlement and recruitment. Settlement and recruitment were higher on fixed than unfixed rubble regardless of the rubble type. This suggests that rubble mobility increases mortality rates. However, effect sizes varied between rubble bed types, likely driven by differing rubble bed characteristics. On the unfixed rubble, settlement and recruitment were higher in the interlocked than in the loose rubble bed. Settlement and recruitment were also higher on the fixed rubble in the interlocked bed than in the loose rubble bed. Thus, environmental effects in the loose rubble appear more severe than the combined environmental and mobility issues associated with the interlocked rubble. While our results show that stability is a key driver of coral recovery, settlement and recruitment on fixed rubble were still lower in rubble beds compared to the hard carbonate reef. These results indicate that each substrate has distinct environmental factors that differentially influence both the settlement and recruitment of corals, even on stabilised rubble surfaces. A greater understanding of coral recruitment dynamics across various rubble bed typologies is important for the management of future reef intervention programs, as the cover of rubble increases on reefs.\u003c/p\u003e","manuscriptTitle":"The effect of rubble stability on coral settlement and recruitment","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-30 12:06:27","doi":"10.21203/rs.3.rs-7974089/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"029309ea-dd45-44ee-92fb-3a6372a97ed2","owner":[],"postedDate":"October 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":57050093,"name":"Marine and Freshwater Ecology"},{"id":57050094,"name":"Marine and Freshwater Biology"}],"tags":[],"updatedAt":"2025-10-30T12:06:27+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-30 12:06:27","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7974089","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7974089","identity":"rs-7974089","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}