Sustained coral reef growth in the critical wave dissipation zone of a Maldivian atoll

Coral reef degradation, driven by climate change and human activities, threatens coastal communities reliant on reefs for protection from ocean waves. Indo-Pacific reefs' protective function depends on the reef edge's elevation relative to sea level, which induces wave breaking. Wave energy dissipation further depends on reef flat depth and width. Reef structure and its protective function rely on net calcium carbonate accumulation, a balance between constructive (calcification) and destructive (erosion, dissolution) processes. Holocene reef growth rates have varied widely (1-20 mm/year) due to sea-level changes and ecological factors. Sea-level rise is predicted to reduce reef protection by increasing water depths and exacerbating shoreline erosion and flooding, particularly where reef growth lags behind sea-level rise or reef degradation occurs. This study aims to (1) introduce a method to directly quantify reef growth rates; (2) present data on contemporary reef growth from a critical wave-breaking zone; and (3) use these measurements to assess future reef growth and wave protection capacity. The findings challenge existing interpretations based on deeper fore-reef settings.

Previous studies on paleo-reef growth used geological reconstructions from reef cores, but these have limitations in evaluating current and future reef growth capacity because Holocene growth conditions differed from present-day conditions. Estimates of contemporary reef growth have relied on indirect census-based observations of living cover and their conversion to growth rates, using in-situ ecological surveys to estimate net carbonate budgets. These studies, while valuable for understanding carbonate budget dynamics, have limitations in estimating vertical reef growth and potential reef submergence. First, assumptions in converting carbonate deposition to linear accretion remain poorly validated; second, the role of detrital sediment is not directly assessed; third, data are often gathered from reef zones peripheral to the critical wave-breaking reef crest; and fourth, reef crest communities may respond differently to disturbances than deeper forereef counterparts. The growth of the reef crest and outer reef flat is crucial for evaluating changes in wave energy transformation, as reductions in crest growth can alter wave breaking patterns.

This study uses coral reef accretion frames (CRAFs) to directly measure changes in reef surface topography at four locations across the reef crest and outer reef flat of Huvadhoo atoll in the Maldives. CRAFs enable highly-resolved, repeatable measurements of changes in reef surface elevation. The CRAFs were installed in February 2018 and measured annually for three years (2018-2020). The study area includes: a crustose coralline algae (CCA)-dominated reef crest; a mixed CCA and coral zone on the outer reef flat; and an algal pavement on the central reef flat. Seven transects were measured at each site, totaling 693 measurements per site. A graduated scale was placed behind the measurement rods and photographed, and images were analyzed digitally to reconstruct the topography of the reef surface. The error associated with the measurement technique was evaluated through repeat measurements, resulting in an estimated CRAF measurement error of ±1.07 mm. Statistical analysis used one-way Kruskal-Wallis tests and post hoc Dunn tests to assess differences in reef elevation changes among sites. Estimates of reef productivity (G, kg CaCO3 m-2 y-1) were calculated from net reef growth values using a density value of 1576 kg/m3. These were then compared to previous census-based reef budget estimates. The measured reef accretion rates were compared against current and future projections of sea-level rise (using the RCP 4.5 and RCP 8.5 scenarios).

The CRAFs detected micro-scale changes in surface elevation and showed a dynamic reef flat topography. There was a statistically significant difference in measured changes in reef surface elevation among sites (Kruskal-Wallis test, H(3) = 521, p = 0.0001). The CCA and encrusting coral-dominated reef crest showed a net elevation increase of 13.2 mm over two years (6.6 ± 12.6 mm/year). Individual transect results ranged from -10.6 ± 27.8 mm/year to 17.0 ± 20.0 mm/year, with 93% of transects showing net accretion. Sites 2 and 3 (outer reef flat, coral-algal zone) showed minimal net change at Site 2 (-0.07 ± 6.1 mm/year) and net accretion at Site 3 (6.1 ± 20.3 mm/year, 3.1 ± 10.2 mm/year annually). Site 4 (reef pavement) showed negligible change (-0.5 ± 1.8 mm/year). Point-scale variations were considerable, ranging from a maximum of 134.8 mm to -92.0 mm in a single year. Analysis of point measurements revealed growth rates of key calcifying organisms, including nodular calcareous algae (up to 19.3 mm/year), encrusting red coralline algae (4.0–6.0 mm/year), *Pocillopora sp*. (up to 11.0 mm/year), and encrusting *Porites sp*. (6.0 mm/year). Significant losses in vertical structure (up to 57.0 mm) attributed to physical damage were observed, along with marked increases (up to 120 mm) in reef level around fissures, due to detrital infill or CCA/coral expansion. Estimated carbonate production rates at the reef crest ranged from 8.51 to 12.51 G (mean 10.39 G), comparable to values from other reef regions, highlighting the productivity of this zone despite a recent bleaching event. Outer reef flat production values ranged from −1.11 to 4.98 G (mean 2.35 G), and the algal pavement zone showed lower values (−2.69 to 1.15 G, mean −0.77 G).

This study provides direct, quantitative measurements of contemporary reef accretion at the critical wave-breaking zone, showing that the reef crest and outer reef flat are currently accreting, despite considerable spatial variability. The reef crest accretion rate (6.6 ± 12.6 mm/year) exceeds the current rate of sea-level rise in the archipelago (3.46 ± 0.25 mm/year) and is comparable to the RCP 4.5 scenario (6.9 mm/year). Outer reef flat accretion rates match current sea-level rise rates in areas with productive cover. The high growth rates of CCA and corals at the reef crest, even after a recent bleaching event, suggest that CCA may be an important calcifier maintaining vertical growth in this high-energy zone. Observed spatial variability highlights the importance of considering various eco-geomorphic zones when assessing reef growth and its impact on coastal protection. The study's findings contrast with previous studies suggesting a collapse in reef accretion following the bleaching event, emphasizing the importance of site-specific studies. The high production rates measured at the reef crest and outer reef flat, even after the 2016 bleaching event, suggests that the wave breaking zone may be relatively resilient to bleaching.

This study demonstrates the value of direct, high-resolution measurements of reef surface topography changes for understanding contemporary and future reef trajectories. The CRAF method provides quantitative measures of net reef accretion, enabling detailed examination of reef development processes. While further measurements are needed for broader extrapolation, the findings suggest that the reef crest may retain the capacity to maintain its wave protection function under moderate rates of sea-level rise. Continued monitoring is crucial to assess the impacts of environmental stressors and refine models predicting reef growth and wave dissipation.

The study's dataset comprises a small number of sample sites, limiting broad extrapolation. Extrapolation to other regions and reef types requires further investigation. The study period lacked significant storm events; therefore, storm impacts on reef accretion were not fully assessed. Additional environmental stressors (ocean acidification, ocean warming) may influence long-term reef accretion rates. The observed growth rates may not represent long-term trends.