Supporting resilience-based coral reef management using broadscale threshold approaches

doi:10.21203/rs.3.rs-4867751/v1

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Supporting resilience-based coral reef management using broadscale threshold approaches

https://doi.org/10.21203/rs.3.rs-4867751/v1

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Managing coral reefs to maintain ecosystem function and maximise resilience requires identification of resilience indicators and clear ecological reference thresholds for reef managers to maintain or aim for. In the absence of local resilience-based targets, reef managers can conduct local-scale resilience assessments by collecting data on resilience indicators and comparing them to recently established broadscale thresholds which have been defined by incorporating large spatial variability. This study documents the application of these broadscale threshold approaches to kick-start resilience-based management at Aldabra Atoll UNESCO Marine World Heritage Site. Aldabra’s seaward coral reefs conformed to the expected resilience of a well-managed and remote marine reserve. All but one reef met or exceeded thresholds for each of the five assessed resilience indicators and fell within the ‘recover’ strategy of the management strategy analysis. Combining the individual indices into a synthetic index of resilience gave an atoll-level estimate of reefs having an 87.5% ‘chance of recovery’ post-disturbance. Reef resilience predictions largely aligned with our data on post-bleaching coral trajectories. We recommend additional broadscale threshold categories that could be defined and included in future assessments, and suggest local factors that need to be considered to fine-tune the assessments at site-level.

Earth and environmental sciences/Ecology/Ecosystem ecology

Biological sciences/Ecology/Ecosystem ecology

Earth and environmental sciences/Environmental sciences/Environmental impact

Earth and environmental sciences/Ocean sciences/Marine biology

Earth and environmental sciences/Climate sciences/Climate change/Climate change impacts/Environmental health

Earth and environmental sciences/Climate sciences/Ocean sciences/Marine biology

Earth and environmental sciences/Environmental sciences

ecological thresholds

resilience indicators

coral reef conservation

coral bleaching

coral reef recovery

UNESCO World Heritage

In the face of climate change and lagging political responses to reducing greenhouse gas emissions, even the best protected coral reefs are transforming rapidly^1–3. Coral reef conservation and management goals therefore need to shift focus from maintaining intact systems to preserving ecosystem functions^4,5. Managing reefs to maintain ecosystem function and maximise resilience—the ability to resist and recover from disturbances⁶—is complex and requires clear articulation of management aims (i.e., to maximise resilience of what to what?), identification of resilience indicators and clear reference thresholds for reef managers to maintain or aim for^4,5,7.

Shifting to resilience-based reef management requires that targets be defined a priori, but defining resilience-based targets depends on availability of historical local or regional data about marine communities and natural variability^8,9. Reef management authorities often lack these data, or the capacity and resources to identify localised targets^10,11. It is feasible, however, for reef managers with reef monitoring capacity to conduct local-scale resilience assessments by collecting data on resilience indicators and comparing them to recently established broadscale thresholds which have been defined by incorporating large spatial variability. Although there are inherent problems in applying general values to a particular reef site, managers can adopt them as temporary ‘thresholds of potential concern’ until more site-specific data can be acquired^12,13.

Broadscale parameters established for predicting the recovery potential of live coral cover include thresholds for reef depth, structural complexity, herbivore biomass, juvenile coral density¹⁴, and have also been developed to define minimum resident reef fish biomass in no-take zones¹⁵. Another approach is to investigate the composition and organisation of reef communities which can explain energy flows through the ecosystem and therefore be used to evaluate ecological condition and expected vulnerability to climate change. For example, Graham et al.¹⁶ quantified the expected fish trophic distributions across a gradient of human disturbance. This provided specific predictions of biomass distributions between trophic levels defined for different levels of human disturbance, from which managers can compare their own local distributions^16–18. Darling et al.¹⁹ produced one of several recent guides for reef conservation management under climate change (see also reviews by Boon and Baxter²⁰, Harvey et al.²¹ and McLeod et al.^4,22), but it is the only approach that provides meta-analysis derived indicator thresholds that can readily be applied by managers at the local scale. The metrics of this assessment (percent cover of framework corals, prior heat stress, and magnitude of heat stress) combine information on key ecological processes: structural complexity, carbonate production, reef growth and past thermal stress to allocate reefs to one of three reef management strategies, i.e., protect, recover or transform.

Despite reef resilience assessments becoming more common, they rarely present structured processes to guide and justify indicator selection²³, and there are – to our knowledge – no published examples of the use of the above broad-scale thresholds being used by reef managers to assess site-level resilience and guide management. This study trials the application of these broadscale threshold approaches for resilience-based management at Aldabra Atoll, in the small island large ocean state of Seychelles. We use reef data collected in 2014 and 2015 (prior to bleaching at Aldabra in 2016) to: (1) assess relative resilience of Aldabra’s seaward coral reef sites by evaluating the position of the atoll's seaward reefs in broadscale analyses as described in Graham et al.¹⁴ and MacNeil et al., and along known disturbance gradients as in Graham et al.¹⁶; (2) combine the different scores for the resilience indicators in relation to the thresholds into a 'unique status assessment' for each site; and (3) classify Aldabra's seaward coral reefs into management strategies according to Darling et al.’s¹⁹ portfolio. We discuss the resilience assessments in the context of the 2014–2017 global bleaching event on Aldabra’s live coral cover and coral recovery trajectories between 2016 and 2022, and how these assessments/indicators can be applied elsewhere.

Study site

Aldabra, a UNESCO Marine World Heritage Site, is a remote atoll in the Western Indian Ocean within the Republic of Seychelles, located ca. 400 km northwest of Madagascar (Fig. 1). Aldabra hosts the largest reef system in Seychelles, which, due to its protection and remoteness, has limited local stressors. The atoll has been managed by a public trust, the Seychelles Islands Foundation (SIF) since 1979 and was designated as Special Reserve in 1981. Aldabra is uninhabited except for 10–20 SIF staff based at the research station, who monitor the atoll's ecosystems and biodiversity year-round. Ongoing management efforts on Aldabra include maintenance of the no-take protected area (2559 km² since 2018; formerly 439 km²) while allowing limited tourism and a small-scale subsistence fishery (only for SIF staff), and restoring ecosystems (e.g. invasive alien species eradications²⁴; native species re-introductions²⁵; marine debris removal²⁶). Due to Aldabra’s remoteness and history of protection, its coral reefs are thought to be relatively resilient, with potential to advance knowledge of natural ecosystem variation in the absence of significant direct stressors²⁷.

Data collection and processing

Coral reef monitoring projects have been conducted sporadically at Aldabra since the 1960s with varied methodologies^28–33 but a long-term annual reef monitoring programme was established by SIF in 2014. The 12 permanently fixed Aldabra Reef Monitoring (ARM) sites represent each of Aldabra’s coastlines with varying oceanographic conditions (Fig. 1) and data on fish (a subset of species) and benthic communities is collected along permanent transects annually^1,27,34,35 (Table S1).

In December 2015, additional surveys were conducted to obtain data on reef structural complexity, coral recruit density, total fish biomass and total herbivore biomass to supplement existing data (Table S1). We assessed reef resilience and future management strategies for the Aldabra reef monitoring sites using data collected in 2014 (percent framework corals) and 2015 (structural complexity, total fish biomass, total herbivore biomass, coral recruit density; Table S1).

At each of the eight sites, perpendicular to the reef slope, three 50-m long transects were laid at 5 m depth and one 50-m long transect at 15 m depth. On each transect, data on fish community, structural complexity and coral juvenile density were collected.

The fish community was surveyed by recording number, identity (to species) and size (to nearest cm) of all fish > 8 cm by a single observer along the entire 50-m transects with a transect width of 4 m. At the start of every survey day, fish size estimation was calibrated using PVC pipes of known lengths. Count data was converted to biomass (kg/ha) using published length-weight relationships^36–42. Structural complexity was visually estimated using the 6-point scale of Wilson et al.⁴³ at the start of each transect. The number, identity and size of juvenile coral colonies (< 5 cm diameter) was recorded in five replicate 0.25-m² quadrats per transect.

Benthic photoquadrat surveys were conducted at each site along the permanent transect on two 10-m long sub-transects (10 m gap between sub-transects) on both sides of the tape measure as described in Koester et al.²⁷. The benthic photos were analysed using Coral Point Count with Excel extensions⁴⁴ by identifying the benthos at 16 randomly assigned points per image as described by Cerutti et al.¹.

Study approach and data analysis

All statistical analyses were conducted using R (version 4.1.2⁴⁵). Whilst acknowledging that depth is an important factor influencing reef communities, particularly fish assemblages⁴⁶, data from the three 5 m and one 15 m transects were pooled as only one transect was surveyed at 15 m depth at each site (see ‘Data collection and processing’).

1. Comparisons with published recommended thresholds

Values of four reef characteristics (total reef fish biomass [including only reef fish following MacNeil et al.¹⁵], herbivorous fish biomass, coral juvenile density and structural complexity) were compared against reference thresholds from MacNeil et al.¹⁵ and Graham et al.¹⁴ with two-tailed one sample t-tests (Table 1).

Table 1

Components of the resilience and future management assessment showing corresponding ecosystem processes and functions.
Approach	Method	Metric & year of data collection	Process of interest^a	Indicative of^a	References	Analysis
Resilience Assessment	Comparison with published recommended reference thresholds	Total fish biomass (1013 kg ha^− 1 of diurnally active, non-cryptic resident reef fish fish > 10 cm of 20 families); data collected in 2015	Productivity	Community functions	MacNeil et al.¹⁵	Two-tailed one-sample t-tests against reference thresholds Scoring of t-test outcomes Scores used in 'one out, all out' method and synthetic index
		Herbivore biomass (177 kg ha^− 1); data collected in 2015	Herbivory	Primary production, community functions	Graham et al.¹⁴
		Coral juvenile abundance (>6.2 per m²); data collected in 2015	Coral recruitment	Reef recovery (gain of primary habitat)	Graham et al.¹⁴
		Structural complexity (> 3.1); data collected in 2015	Productivity, active reef growth	Habitat availability, benthic community landscape, enhanced coral settlement	Graham et al.¹⁴
	Comparisons based on trophic pyramids distributions	Trophic levels of diurnally active, non-cryptic, reef-associated fish; data collected in 2015	Productivity	Human impact (cf. unexploited systems)	Graham et al.¹⁶	Comparison of trophic pyramids with those of “fished” and “unfished” seascapes as per Graham et al.¹⁶ and the classic trophic pyramid as per Trebilco et al.¹⁸
Management Assessment	Comparison to established threshold values of heat stress	Water temperature (satellite derived⁵³)	Physiological stress	Significant thermal stress	Darling et al.¹⁹	Assessing local DHW value during the 2014–2017 global bleaching event against 4 DHW threshold	Classification of sites into management strategy portfolio of Darling et al.¹⁹: protect, recover or transform.
Management Assessment	Comparison against the recommended minimum reference value (10%)	Percent cover of competitive and stress-tolerant corals (framework corals); data collected in 2014	Coral calcification	Active reef growth (gain of primary habitat)	Darling et al.¹⁹	T-test against reference threshold
^a also with reference to Brandl et al.’s⁷⁶ eight core ecosystem processes

T-test outcomes for each reef characteristic were scored for Aldabra overall and for each site. A score of 1 was given if the observed mean was significantly higher than the reference threshold; zero if the observed mean was not statistically different from the reference threshold; and − 1 if the observed mean was significantly lower than the reference threshold (i.e., target was not met).

To combine the different reef characteristics into a 'unique status assessment', two methods were used: the 'one out, all out' method and calculation of a 'synthetic index' (see also Halpern et al.⁴⁸) that we developed to incorporate responsiveness to data quality and real differences in coral reef health. The 'one out, all out' method was used against all four characteristics whereby if any characteristics scored − 1, the area did not meet the target⁴⁷. The synthetic index attempted a more nuanced investigation into the data and was calculated by normalising the scores (u) for all four characteristics (tfb: total fish biomass, hbm: herbivorous fish biomass, juv: coral juvenile abundance, rug: rugosity):

$$\:I=\left(\frac{\sum\:\left({u}_{tfb},{u}_{hbm},{u}_{juv},{u}_{rug}\right)+4)\:}{4+4}\right)\times\:100$$

The synthetic index therefore provides us with a score out of 100 that reflects the level of resilience based on all four characteristics.

2. Trophic pyramid structure comparisons

Following the method outlined by Graham et al.¹⁶ and using fish biomass data (kg/ha) we: (1) assigned trophic levels at the species level using Fishbase^41,49; (2) calculated relative distribution of biomass among five trophic levels to create trophic pyramids for Aldabra overall and each site individually; and (3) compared these trophic pyramids to the ‘fished’ and ‘unfished’ seascapes of Graham et al.¹⁶ and the classical trophic pyramid (see Trebilco et al.¹⁸). The five trophic levels, as suggested by Graham et al.¹⁶, are: 1: 2.0–2.5, 2: 2.5–3.0, 3: 3.0–3.5, 4: 3.5–4.0 and 5: 4.0–4.5. To enable direct comparison with Graham et al.¹⁶, we excluded any fish families that are not part of the 17 target families listed by Graham et al.¹⁶.

3. Management strategy classification

We used coral community data (pre-bleaching percent coral cover) and degree heating weeks (DHW) data accessed from the National Oceanic and Atmospheric Administration⁵⁰ to evaluate the ecological condition of Aldabra’s reef sites and their exposure to climate change following Darling et al.¹⁹ (Table 1). We then classified Aldabra’s reef sites based on the ‘protect’, ‘recover’ and ‘transform’ management strategies outlined by Darling et al.¹⁹. These strategies were to: (1) ‘protect’ functioning reefs that were associated with limited exposure to recent bleaching-level thermal stress (DHW < 4°C-weeks) and maintained coral cover above 10%; (2) ‘recover’ reefs that have recently maintained cover above 10% but were exposed to severe potential bleaching stress in 2014–2017; and (3) for reefs with coral cover below net-positive carbonate budgets (< 10% hard coral cover), societies may ultimately need to ‘transform’ away from reef-dependent livelihoods¹⁹.

Ecological condition was represented by the potential for a net-positive carbonate budget before the 2014–2017 bleaching event¹⁹. To assess ecological condition we: (1) assigned life history strategies (‘competitive’, ‘generalist’, ‘stress-tolerant’, ‘weedy’) at the genera and growth form level following Darling et al.⁵¹, using the Coral Traits Database⁵² (https://coraltraits.org/); (2) calculated percent cover of (summed) competitive and stress-tolerant corals (hereafter, ‘framework’ corals); and (3) compared Aldabra data against the threshold value (10% cover¹⁹) using two-tailed one-sample t-tests.

Climate change exposure was represented by thermal stress during the 2014–2017 bleaching event¹⁹. To assess climate change exposure we: (1) extracted daily DHW data available from NOAA Coral Reef Watch⁵³ between two coordinates at Aldabra (coordinate one: -9.575; 46.075, coordinate two: -9.225; 46.675) for the period January 2014 to December 2017 through the ERDDAP data server via the Pacific Islands Ocean Observing System (PacIOOS) website (https://pae-paha.pacioos.hawaii.edu/erddap/griddap/dhw_5km.html); (2) calculated maximum DHW estimated for each year; and (3) compared these Aldabra data against the established threshold for thermal stress exposure (4 DHW °C-weeks¹⁹).

The two indices (ecological condition and climate change exposure) were then placed into Darling et al.’s ¹⁹ management strategy portfolio, which recommends a ‘protect’ strategy if reefs were exposed to thermal stress below DHW 4°C-weeks during 2014–2017 and had > 10% cover of framework corals, a ‘recover’ strategy if reefs were exposed to severe bleaching-level stress (DHW > 4°C-weeks during 2014–2017) and had > 10% cover of framework corals, and a ‘transform’ strategy if reefs were exposed to severe bleaching-level stress and had < 10% cover of framework corals.

Recommended threshold comparisons

Comparing the observed values versus recommended thresholds for four reef characteristics across all eight sites indicated high likelihood of resilience on the seaward forereef slopes of Aldabra using both the ‘one out, all out’ method, and the synthetic index (Table 2). Reef characteristics met recommended thresholds — structural complexity was not significantly different from the recommended threshold (t = -0.13125, df = 31, p > 0.05; Table S2), while total and herbivorous fish biomass and juvenile coral colony density were significantly greater than recommended (t = 3.3138, df = 31, p < 0.01; t = 3.8482, df = 31, p < 0.01, t = 5.5305, df = 30, p < 0.001, respectively; Table S2). Combining the individual indices into a synthetic index of resilience gave an atoll-level estimate of reefs having an 87.5% chance of recovery post-disturbance (Table 2).

Table 2

Unique status assessments considering four reef characteristics (total fish biomass; herbivorous fish biomass; juvenile coral density; structural complexity) overall and at each site. Numbers represent outcomes from the two-tailed one-sample t-tests (pooling three ‘5 m depth’ and one ’15 m depth’ transects: df = 3 at the site-level; df = 31 overall); observed mean was significantly higher (1), lower (-1) or not statistically different (0) from the recommended threshold. Columns represent assessment outcomes using the 'one out, all out' method (Pass/Fail based on all characteristics) and the 'synthetic index' expressed on a scale of 0–100%.
Site	Total fish biomass	Herbivorous fish biomass	Juvenile coral density	Structural complexity	One-out, all-out	Synthetic Index score (%)
Overall	1	1	1	0	Pass	87.5
ARM01	0	0	1	1	Pass	75
ARM02	0	0	1	1	Pass	75
ARM03	0	0	0	0	Pass	50
ARM04	0	0	0	0	Pass	50
ARM05	0	0	0	0	Pass	50
ARM06	1	0	1	0	Pass	75
ARM08	0	0	0	0	Pass	50
ARM12	0	0	0	-1	Fail	37.5

Investigating individual sites showed similar patterns to the overall assessment (Table 2). Seven (of the eight) sites displayed high likelihood of resilience using the ‘one out, all out’ method and the synthetic index (i.e., index score ≥ 50%). For the three reefs (ARM01, ARM02, ARM06) with an index score of 75% we can be confident that they would be likely to recover following a disturbance.

Site ARM12 met thresholds for three of the four reef characteristics: total reef fish biomass, herbivorous fish biomass and juvenile coral density (t = 1.2099, df = 3, p > 0.05; t = 1.6439, df = 3, p > 0.05; t = 0.55641, df = 3, p > 0.05, respectively; Table S2). However, the structural complexity threshold was not met (t = -9.3531, df = 3, p < 0.01; Table S2), resulting in failure using the ‘one out, all out’ method. Combining the individual indices into a synthetic index of resilience gave a site-level estimate of 37.5% chance of reef recovery post-disturbance (Table 2).

Trophic pyramid structure comparisons

The overall trophic pyramid of the reef fish community compared favourably to Graham et al.’s ¹⁶ concave unfished pyramid shape (Fig. 2, Figure S1). At the site-scale, seven of the eight pyramids showed higher proportions of upper trophic level biomass than expected based on a classic biomass pyramid ¹⁸, approaching Graham et al.’s¹⁶ concave shape. Community biomass was greater than the recommended 650 kg/ha at the atoll-level (mean biomass = 1846 ± 255 SE kg/ha) and for all sites (mean biomass ranged from 986 ± 157 kg/ha at ARM01 to 4267 ± 756 kg/ha at ARM06; Table S2).

Management strategy classification

All but one of Aldabra’s seaward coral reefs fell within the ‘recover’ strategy of Darling et al.’s¹⁹ management portfolio: predicted maximum heat stress was at > 6 DHW in April 2016 (Figure S2) and the cover of competitive and stress-tolerant coral species did not differ from the 10% threshold at all sites except ARM05 (Table S3). Site ARM05 therefore fell within the ‘transform’ strategy, whilst all other sites fell within the ‘recover’ strategy.

By using multiple approaches to evaluate site-level resilience, we were able to establish that in 2015 (pre-bleaching) Aldabra’s seaward coral reefs conformed to the expected resilience of a well-managed and remote marine reserve. For each of the assessed resilience indicators (total and herbivorous fish biomass, trophic fish biomass distributions, coral juvenile density, and structural complexity), the reefs met or exceeded multiple thresholds. Combining the individual indices into a synthetic index of resilience gave an atoll-level estimate of reefs having an 87.5% ‘chance of recovery’ post-disturbance. In addition, within Darling et al.’s¹⁹ management strategy analysis, all but one of Aldabra’s seaward coral reefs fell within the recover strategy, meaning they are predicted to recover despite exposure to severe bleaching stress. Crucially, the resilience predictions mostly align with post-bleaching data on Aldabra’s reef dynamics (Table S4; Figure S3), validating the application of these approaches.

Resilience assessment

In 2015, both total (986–4267 kg/ha) and herbivore (264–958 kg/ha) fish biomass at all seaward coral reefs sites on Aldabra met or exceeded recommended thresholds (total fish biomass: 1000 kg/ha¹⁵; herbivore biomass: 177 kg/ha¹⁴; Fig. 3). These biomass figures are equivalent to other remote and protected sites, like the Chagos Archipelago⁵⁴, but align more closely with the biomass recorded around the inhabited atoll of Diego Garcia, where subsistence fishing occurs, than the uninhabited and unfished Northern Atolls⁵⁵. This could indicate that even a population of < 1 person/km² on Aldabra may have an impact on resident fish biomass via the Aldabra subsistence fishery, as also suggested by Fournier-Carnoy⁵⁶. Despite this, we found that Aldabra’s fish biomass trophic pyramids have a concave structure (except ARM08), where most of the biomass is located in the top (predator) and bottom (herbivores) trophic levels. This indicates energy-balanced communities¹⁶ and is similar to other remote and protected locations (e.g. Mexico⁵⁷). Aside from the threat of climate driven changes in fish community structure and biomass^58–60, the present state of relative health in fish biomass and trophic structure at Aldabra is threatened by increasing poaching (SIF, unpubl. data). With fishers travelling longer distances⁶¹ and dwindling stocks elsewhere, Aldabra’s fish biomass is highly attractive, so greater surveillance and enforcement of the reserve will be a key management strategy to continue to meet these resilience thresholds. The Aldabra subsistence fishery could also shift efforts from predominantly bottom fishing which targets reef fish, to trolling for pelagic species.

The high herbivorous fish biomass found across Aldabra’s reef sites promotes the availability of suitable coral larvae settlement substrates¹⁴. As such, coral juvenile density at all reef sites in this study (7.0–25.8 coral juveniles/m²) met or exceeded the recommended resilience indicator threshold¹⁴ of 6.2 coral juveniles/m². All sites also met or exceeded the recommended threshold for structural complexity, except ARM12. This site is located in the south-east of the atoll and exposed to high and persistent wave energy⁶². Reefs in such conditions naturally consist of more stable coral growth forms (e.g. encrusting and massive⁶³), impacting the structural complexity score of the reef. The site met all other thresholds, highlighting site-specific factors that can now be considered and incorporated when defining (or refining) the local thresholds used for local resilience-based management.

Predicted resilience vs post-disturbance trajectories

Our research presents a snapshot of one point in time, but studies of Aldabra’s reefs from as early as the 1960s align with our findings. The north-west reef front of Aldabra was described as having the most ‘luxuriant coral growth’^28,29,32. This corresponds with our findings of high structural complexity and a net positive carbonate budget at all sites except ARM12 and ARM05 in the south-east and east, and the high resilience index scores at sites on the north-west coast. The south to south-east reef fronts (sites ARM08, ARM12), were assumed to have undergone catastrophic cyclone damage in the years preceding the 1960s surveys^28,29,32, and both sites are naturally exposed to stronger waves than the northern coast, shaping benthic community composition⁶². The fact that our 2015 surveys show similar patterns to those described 50 years ago, despite the impacts of the 1998 global coral bleaching event⁶⁴, demonstrates the long-term resilience of several areas of reef and supports the accuracy of both the assessment methods and resulting resilience scores.

Contemporary data, collected annually at Aldabra since 2014, allow preliminary checks of our predicted resilience from this study against observed changes in ecosystem community and processes following the 2016 bleaching event, which peaked at Aldabra in April 2016. Overall, this event caused a reduction of relative hard coral cover of 54%^1,27, with substantial variation across sites (14–73% reduction; Table S4, Figure S3). By early 2022, six years post-bleaching, relative hard coral cover at these sites reached 43–107% of the pre-bleaching cover in 2014 (Table S4). This highlights that live coral recovery potential (one component of ecosystem resilience) varied across sites, and in some cases was surprising compared to our estimates of predicted resilience. For example, our 2015 resilience assessment highlighted ARM12 as potentially less resilient than the other reefs, but this was one of the two sites that reached or exceeded pre-bleaching hard coral cover values by 2022 (Table S4, Figure S3). Conversely, site ARM05, which met all resilience indicator thresholds, experienced relatively low hard coral recovery, only reaching 43% of the pre-bleaching cover by 2022. This lower recovery trajectory was attributed to rapid expansion of the calcareous green algae Halimeda spp., which is more abundant on the eastern seaward reefs of Aldabra²⁷, attributed to an increase in hydrodynamic energy from west to east^32,65. Potentially we need to consider some additional indicators such as Halimeda specific thresholds within future resilience assessments for Aldabra.

Overall, while trends in live coral cover followed expectations, we can now describe additional site-specific characteristics likely affecting resilience/recovery potential. For example, at Aldabra, we can be less concerned by lower complexity scores in areas exposed to high and persistent wave energy but consider additional indicators to track the extent of Halimeda spp., especially on eastern reefs. Aldabra’s daily temperature regimes are known to influence fine-scale resilience patterns, especially when comparing the lagoonal vs. seaward coral reefs. Aldabra’s lagoon, which has a much greater daily temperature fluctuation saw less mortality and faster recovery from the 2016 bleaching event²⁷. Localised nutrient input from seabird colonies via guano deposits may also influence the resilience potential of adjacent reefs around islands⁶⁶. At Aldabra these inputs are patchy⁶⁷ due to tidal changes, strong currents and because seabird colonies (Fig. 1) are restricted by the presence of invasive mammals. Both of these factors (temperature regimes and levels of seabird-derived nutrients) should be considered as additional broadscale resilience indicators to future assessments for resilience-based management, although these proposed indicators require more resources to acquire site-level data than the indicators used in this study.

Management assessment

All but one of Aldabra’s seaward reef sites in this study fell within the recover strategy under Darling et al.’s¹⁹ approach. The management goal in this strategy is to move reefs back above the 10% threshold of framework coral cover as quickly as possible following climate impacts. Active management strategies to achieve this include minimising local stressors (e.g., invasive species control on islands, pollution control, reduced tourism activities, reduced fishing pressure) and conducting active restoration (e.g., coral gardening or other reef restoration techniques, coastal habitat restoration). Two Aldabra reef monitoring sites with high synthetic index scores (ARM01, ARM06) – our proxy for resilience – are located closest to local human activity and potential stressors: the research station, the food security and the tourism zones (Fig. 1). These findings suggest that the current level of human activity in the vicinity of these sites has minimal impact on site-level reef resilience.

Other local stressors identified by managers include plastic pollution and invasive alien mammals. An estimated 500 tonnes of plastic pollution have accumulated along Aldabra’s coastline²⁶. Evidence suggests that plastic could impact coral health and subsequent resilience⁶⁸. SIF already undertakes active management of this issue through regular clean-ups, although limited resources hinder removal at scale²⁶. Invasive alien mammals are known to suppress seabirds and their nutrient inputs on islands⁶⁹. Eradicating introduced rats and cats from Aldabra would kickstart restoration of a more diverse breeding seabird community across the atoll. This in turn is expected to substantially increase the quantity and distribution of seabird-derived nutrient subsidies entering near-shore reef systems. These nutrient subsidies are now understood to enhance reef resilience through faster coral growth, higher recruitment rates and higher reef fish biomass^66,69–72. As a strategy to enhance coral resilience at Aldabra, a rat and cat eradication is therefore of highest priority and an eradication feasibility assessment for the atoll is currently underway.

Usefulness of the approaches used

Despite efforts to make the concept of resilience-based management operational to reef managers and conservationists (e.g. Graham et al.¹⁴, Maynard et al.⁷³), uptake has been slow. The application of several approaches to assess coral reef resilience at Aldabra has been largely successful, with reef resilience predictions mostly aligning with observed post-bleaching coral trajectories. As reef managers, we found the exercise highly valuable for Aldabra—the largest reef system in Seychelles and an important coral larvae source for the region^74,75. Combining resilience indicators into the synthetic index was an effective way of summarising multiple results at monitoring site level, but the variability of index scores between sites meant an island-level score was less valuable from a site management perspective.

It would be a valuable exercise to conduct similar resilience-based management assessments across Seychelles and other island groups where data is available. Such reef resilience mapping could be combined with recent coral reef connectivity data^74,75 to strengthen and inform reef system management strategies, define national-level resource and conservation priorities and feed into monitoring national contributions to meeting the Kunming-Montreal Global Biodiversity Framework objectives. With the development of appropriate tools, such as analysis and tracking tools via an open access app, resilience-based management approaches are likely to be extremely important for identifying and protecting the reefs most likely to survive repeated bleaching events. With bleaching events becoming more frequent and intense, applying these broadscale thresholds can provide reef managers with tangible resilience targets to maintain, improve or aim for, and help guide and implement adaptive resilience-based management actions for coral reefs.

Competing interests

The authors declare no competing interests.

Funding

The development and set-up of the Aldabra Reef Monitoring (ARM) programme was funded by the Global Environmental Facility (GEF Project ID 3925). The annual ARM surveys were funded by the Seychelles Islands Foundation. Open Access publishing of this article was funded by the University of Bremen.

Author Contribution

AJB and AK contributed equally to this work and are joint first authors. AJB: Conceptualization, Methodology, Validation, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration. AK: Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Funding acquisition. NB: Conceptualization, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition. PH: Methodology, Investigation, Data Curation, Writing - Review & Editing, Funding acquisition. RW: Formal analysis, Data Curation, Writing - Review & Editing. FFD: Writing - Review & Editing, Supervision, Project administration, Funding acquisition. KCS: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration.

Acknowledgement

We thank the Seychelles Islands Foundation (SIF) and their current and former staff members for their work and support. Specifically, we extend our appreciation to the many Aldabra staff who over the years have contributed to marine data collection and planning. We are grateful to M. Pratchett, K. Nash and N. Graham who provided valuable comments on earlier versions of this manuscript.

Data Availability

The data on fish diversity, abundance, and biomass utilised in this research is accessible on Figshare (DOI: https://doi.org/10. 6084/m9.figshare.22792580.v1). The data on coral juvenile abundance and diversity is available in the supplementary files of Koester et al. 2021 (PLOS ONE 16(12): e0260516. https://doi.org/10.1371/journal.pone.0260516). All other data used for this research is available from the corresponding author on reasonable request.

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Supporting resilience-based coral reef management using broadscale threshold approaches

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Version 1

Abstract

Figures

Introduction

Methods

1. Comparisons with published recommended thresholds

2. Trophic pyramid structure comparisons

3. Management strategy classification

Results

Discussion

Declarations

Competing interests

Funding

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1