Optimizing coral reef recovery with context-specific management actions at prioritized reefs

Going with the flow: Leveraging reef-scale hydrodynamics for upscaling larval-based restoration

Anthropogenic pressures are impacting coastal marine ecosystems, necessitating large-scale interventions to accelerate recovery. Propagule-based restoration holds the potential for restoring shallow coastal systems at hectare scales by harnessing natural dispersal. However, predicting propagule dispersal remains challenging due to the complex hydrodynamic nature of coastal marine ecosystems and the complex behaviors of marine propagules. To improve predictions of fine-scale larval dispersal patterns, we developed a 3D reef-scale (~30-m resolution) dispersal model for Lizard Island, Australia, with the aim to predict the effect of island-scale hydrodynamics on the distribution of coral spawn slicks and larvae. Using in situ field observations and dispersal simulations, we assessed the model's capability to (1) forecast hydrodynamic conditions, (2) predict coral spawn slick convergence zones for collection efforts, and (3) identify optimal locations and timeframes where high particle residence time may enhance local settlement following larval delivery to damaged reefs. Predictions of convergence zones in the upper water column aligned well with field observations of coral spawn slicks. At the reef benthos, the model captured variability in current speed and direction at ~58% of studied locations. At other locations, the model did not resolve hydrodynamic conditions due to sheltering effects and associated hydrodynamic processes occurring at a scale below 50 m. At locations where the model performed well, propagules could remain within a 1-ha area around the delivery site for 5-15 h depending on locations and the timing of larval release. These high retention conditions were infrequent but occurred at least once at 15 of the 25 studied sites. Observations of local currents a posteriori confirmed model predictions, showing periods of little water movement lasting from 6.5 to 15 h. Overall, our study highlights fine-scale dispersal modeling as a key tool for scaling up larval-based reef restoration, while also acknowledging the need for better predictions of local conditions in complex, shallow environments. Applications of fine-scale modeling, coupled with local knowledge of reproductive timing and larval behavioral ecology, assist with the mass collection of propagules upon release and in identifying areas and times of optimal larval deployment to achieve the greatest impact.

Using Community Composition and Successional Theory to Guide Site-Specific Coral Reef Management

High spatial or temporal variability in community composition makes it challenging for natural resource managers to predict ecosystem trajectories at scales relevant to management. This is commonly the case in nearshore marine environments, where the frequency and intensity of disturbance events vary at the sub-kilometer to meter scale, creating a patchwork of successional stages within a single ecosystem. The successional stage of a community impacts its stability, recovery potential, and trajectory over time in predictable ways. Here we demonstrate the value of successional theory for interpreting fine-scale community heterogeneity using Hawaiian coral reefs as a case study. We tracked benthic community dynamics on 36 forereefs over a 6-year period (2017-2023) that captures impacts from high surf events, a marine heatwave, and unprecedented shifts in human behavior due to the COVID-19 pandemic. We document high spatial variation in benthic community composition that was only partially explained by island and environmental regime. Through hierarchical clustering, we identify three distinct community types that appear to represent different successional stages of reef development. Reefs belonging to the same community type exhibited similar rates of change in coral cover and structural complexity over time, more so than reefs located on the same island. Importantly, communities that were indicative of early succession (low coral cover reefs dominated by stress-tolerant corals) were most likely to experience an increase in coral cover over time, while later-stage successional communities were more likely to experience coral decline. Our findings highlight the influence of life history and successional stage on community trajectories. Accounting for these factors, not simply overall coral cover, is essential for designing effective management interventions. Site-specific management that accounts for a community's unique composition and history of disturbance is needed to effectively conserve these important ecosystems.

Material Legacies on Coral Reefs: Rubble Length and Bed Thickness Are Key Drivers of Rubble Bed Recovery

Disturbances on coral reefs-which are increasing in intensity and frequency-generate material legacies. These are commonly in the form of rubble beds, which depend on rubble stability and/or binding to facilitate coral recruitment and recovery. Yet, our understanding of rubble stability and binding dynamics across environmental gradients is limited. Characterising and categorising rubble material legacies in context of their likely recovery trajectory is imperative to the effective deployment of active intervention strategies used to restore degraded reefs, such as rubble stabilisation, coral outplanting and larval seeding techniques. We quantified rubble characteristics across environmental gradients on the Great Barrier Reef. The likelihood of rubble stability and binding increased with rubble length and rubble bed thickness, and rubble length was a good predictor of bed thickness and rubble branchiness. Thin rubble bed profiles (< ~10 cm depth), those with small, unbranched rubble pieces (< ~10 cm length), and beds at the base of sloped rubble screes, had lower stability and binding likelihoods. These kinds of beds are expected to persist with low recovery prospects, and could be good candidates for rubble stabilisation interventions. Thicker rubble beds with larger, branched rubble pieces tended to exhibit higher stability and binding likelihoods. However, these beds had nuanced effects on coral cover, and interventions may still be necessary where competition is high, for example from macroalgae. A rapid assessment of rubble length-while also considering shelf location, geomorphic zone, slope angle and underlying substrate-can indicate the potential direction of a rubble bed's recovery trajectory. Our findings have been summarised into a rapid rubble bed assessment tool available in the Supporting Information, that can be incorporated into current reef monitoring to optimize prioritisation of intervention strategies at disturbed sites.

Macroalgal cover on coral reefs: Spatial and environmental predictors, and decadal trends in the Great Barrier Reef

Macroalgae are an important component of coral reef ecosystems. We identified spatial patterns, environmental drivers and long-term trends of total cover of upright fleshy and calcareous coral reef inhabiting macroalgae in the Great Barrier Reef. The spatial study comprised of one-off surveys of 1257 sites (latitude 11-24°S, coastal to offshore, 0-18 m depth), while the temporal trends analysis was based on 26 years of long-term monitoring data from 93 reefs. Environmental predictors were obtained from in situ data and from the coupled hydrodynamic-biochemical model eReefs. Macroalgae dominated the benthos (≥50% cover) on at least one site of 40.4% of surveyed inshore reefs. Spatially, macroalgal cover increased steeply towards the coast, with latitude away from the equator, and towards shallow (≤3 m) depth. Environmental conditions associated with macroalgal dominance were: high tidal range, wave exposure and irradiance, and low aragonite saturation state, Secchi depth, total alkalinity and temperature. Evidence of space competition between macroalgal cover and hard coral cover was restricted to shallow inshore sites. Temporally, macroalgal cover on inshore and mid-shelf reefs showed some fluctuations, but unlike hard corals they showed no systematic trends. Our extensive empirical data may serve to parameterize ecosystem models, and to refine reef condition indices based on macroalgal data for Pacific coral reefs.

Limitations to coral recovery along an environmental stress gradient

Positive feedbacks driving habitat-forming species recovery and population growth are often lost as ecosystems degrade. For such systems, identifying mechanisms that limit the re-establishment of critical positive feedbacks is key to facilitating recovery. Theory predicts the primary drivers limiting system recovery shift from biological to physical as abiotic stress increases, but recent work has demonstrated that this seldom happens. We combined field and laboratory experiments to identify variation in limitations to coral recovery along an environmental stress gradient at Ningaloo Reef and Exmouth Gulf in northwest Australia. Many reefs in the region are coral depauperate due to recent cyclones and thermal stress. In general, recovery trajectories are prolonged due to limited coral recruitment. Consistent with theory, clearer water reefs under low thermal stress appear limited by biological interactions: competition with turf algae caused high mortality of newly settled corals and upright macroalgal stands drove mortality in transplanted juvenile corals. Laboratory experiments showed a positive relationship between crustose coralline algae cover and coral settlement, but only in the absence of sedimentation. Contrary to expectation, coral recovery does not appear limited by the survival or growth of recruits on turbid reefs under higher thermal stress, but to exceptionally low larval supply. Laboratory experiments showed that larval survival and settlement are unaffected by seawater quality across the study region. Rather, connectivity models predicted that many of the more turbid reefs in the Gulf are predominantly self seeded, receiving limited supply under degraded reef states. Overall, we find that the influence of oceanography can overwhelm the influences of physical and biological interactions on recovery potential at locations where environmental stressors are high, whereas populations in relatively benign physical conditions are predominantly structured by local ecological drivers. Such context-dependent information can help guide expectations and assist managers in optimizing strategies for spatial conservation planning for system recovery.