Seagrass meadows are important sources of reef island-building sediment

Low-lying coral reef islands are highly vulnerable to climate change impacts such as sea-level rise and flooding due to their low elevations and dependence on carbonate sediments produced by adjacent marine organisms. Changes in marine ecology alter rates and types of carbonate sediment production, affecting island resilience. While producers like parrotfish are increasingly understood, the capacity of seagrass meadows—especially their leaf epibionts (foraminifera, molluscs, serpulids, crustose coralline algae)—to generate reef island-building sand-sized sediments has been largely overlooked. Given that sand and gravel fractions best contribute to island building due to longer residence times on reef platforms, there is a critical knowledge gap regarding seagrass-derived sand-sized sediment production in reef island contexts. The study aims to quantify epibiont sediment production in a typical Maldivian seagrass meadow and determine the fraction suitable (sand-sized) for reef island building.

Prior work documents carbonate sediment production by various reef organisms, with parrotfish identified as major producers of island-building sediments in the Maldives. Seagrass habitats are known carbonate sources and widespread near reef islands, with epibionts on leaves contributing carbonate, often highlighted as mud-sized in earlier studies. However, sand and gravel are most relevant for island building; Maldivian islands are ~98% sand and gravel. Despite recognition of seagrass roles in carbonate cycling and recent estimates that tropical seagrass meadows can be sediment production hotspots, quantitative assessments of seagrass-derived sand-sized sediments for island building have been lacking. This study addresses that gap within the context of Huvadhoo Atoll, Maldives, where seagrass is sometimes removed for aesthetics.

Study area: a 1.1 km^2 seagrass meadow on the oceanward reef flat adjacent to Faathihutta island, Huvadhoo Atoll, Maldives (semidiurnal microtidal range ~1 m). Two species were present: Thalassodendron ciliatum and Thalassia hemprichii. Zonation and remote sensing: The site was divided into zones based on species composition, leaf densities, and percent cover using snorkel surveys and remote sensing (UAV and PlanetScope imagery; 3 m spatial resolution, acquired 8 April 2019). An unsupervised classification delineated zones; accuracy was validated with 180 ground-truth points (overall accuracy 98%). Field sampling: Along four oceanward–lagoonward transects, 100 sampling points were established (Apr–May 2019). At each, three 0.25 × 0.25 m quadrats were randomly placed (n=300). Within quadrats, species-level percent cover and total seagrass leaf counts were recorded; 27,528 leaves were counted to derive species-level leaf densities per m^2 per zone. Epibiont mass per leaf: At five random sampling points within each zone, ten mature leaves of each species (n=400; 50 leaves per species per zone) were analyzed using the weight loss after acidification method to estimate carbonate epibiont mass (g CaCO3 per leaf). Ten unencrusted leaves per species per zone were also acid-treated to correct for natural leaf mass loss. Epibiont CaCO3 per leaf was calculated as E_prod = A − [B + (A × C)], where A is dry mass of encrusted leaves (g), B is mass after acid treatment (g), and C is percent mass loss of unencrusted acid-treated leaves. Production rate calculation: Species-level CaCO3 production rates (kg CaCO3 m^-2 yr^-1) per zone were computed by multiplying epibiont mass per leaf by species-specific leaf densities (per m^2) and annual leaf turnover rates. To be conservative, the longest published turnover rates were used: 94 days (3.9 crops yr^-1) for T. ciliatum and 30 days (12.2 crops yr^-1) for T. hemprichii. Grain size and agitation experiment: Bulk epibiont samples were scraped from leaves at three sites per zone for each species (n=24). After removing organics with 5% sodium hypochlorite (24 h), grain size distributions were measured with a Malvern Mastersizer 2000. To simulate wave-driven transport and abrasion, samples were agitated in water for 21 days on an IKA HS 501 horizontal shaker at 106 oscillations per minute, then re-measured. Gravel-sized fractions (>2 mm) were excluded from Mastersizer analyses due to instrument limits. Sand-sized production was estimated by multiplying epibiont production rates by the post-agitation sand-sized percentage per species and zone. Scaling and statistics: Zone-level species production rates were summed and multiplied by zone areas from the classification to obtain total and sand-sized production for the site; cumulative errors were calculated via standard error propagation. Statistical analyses (ANOVA or Kruskal–Wallis where assumptions not met) were conducted in R.

• Site-wide epibiont carbonate production: 762,000 ± 90,000 kg CaCO3 yr^-1 across 1.1 km^2 (Results). Post-agitation sand-sized production: 482,000 ± 23,000 kg CaCO3 yr^-1. This sand-sized production approximates the volume needed to build the adjacent island of Faathihutta over ~18 years (island area 5,773 m^2; elevation 0.81 m).
• Abstract values reported: total 853,000 ± 90,000 kg CaCO3 yr^-1; sand-sized 541,000 ± 23,000 kg CaCO3 yr^-1 over the same 1.1 km^2.
• Zone-level production rates ranged from 0.22 ± 0.13 kg CaCO3 m^-2 yr^-1 (sub-Zone 1b) to 0.86 ± 0.21 kg CaCO3 m^-2 yr^-1 (Zone 3), driven by differences in leaf densities, species composition, epibiont mass per leaf, and turnover.
• Mass of epibionts per leaf ranged 0.05 ± 0.01 to 0.07 ± 0.01 g CaCO3; significantly higher on T. ciliatum than on T. hemprichii leaves (two-way ANOVA: F1,32 = 5.00; P = 0.03). Zone 2 had significantly higher epibiont mass per leaf (0.07 ± 0.01 g CaCO3; F3,32 = 4.21; P = 0.01).
• Grain size distributions: Pre-agitation mean 589 ± 504 µm with 78 ± 13% sand-sized; Post-agitation mean 237 ± 51 µm with 61 ± 10% sand-sized. Post-agitation distributions were much less variable (SD 51 µm), indicating approach toward endpoint grain sizes; no significant species or zone differences pre- or post-agitation (Kruskal–Wallis, P ≥ 0.08 pre; P ≥ 0.82 post).
• Conservative assumptions likely underestimate total production due to exclusion of non-epibiont producers, removal of >2 mm fraction in analysis, and use of longest turnover rates; using shortest published turnover rates, production could range from 1.0 ± 0.1 to 2.3 ± 0.1 kg CaCO3 m^-2 yr^-1 depending on zone.
• National upscaling for Maldives: 81,183 ± 18,421 tonnes CaCO3 yr^-1 produced by seagrass epibionts over 118 km^2; 51,353 ± 12,118 tonnes yr^-1 (63%) sand-sized (435 ± 103 g m^-2 yr^-1 sand-sized; average total 688 ± 156 g m^-2 yr^-1).

The study demonstrates that seagrass meadows can generate substantial quantities of sand-sized carbonate sediment suitable for reef island building, directly addressing the knowledge gap regarding their role in island sediment supply. Post-agitation grain size distributions suggest that epibiont-derived sediments reach or approach transport-stable endpoint sizes conducive to shoreline accumulation. While not all produced sediments will reach island shorelines (some retained within meadows or exported off-reef), retained sediments can contribute to vertical seabed accretion and potential attenuation of wave energy reaching islands, enhancing coastal resilience. The reported production rates likely understate total contributions due to conservative turnover assumptions, exclusion of gravel-sized fractions, and omission of other seagrass-associated producers (e.g., infaunal/epifaunal foraminifera, gastropods, urchins, and nursery-supported parrotfish sediment production). Spatial considerations indicate lagoonward meadows may be particularly important on eastern atoll rims where lagoonal wave energy is greatest, aligning with observed lagoonward island accretion. Although per-area production can be higher in narrow outer reef zones, the extensive spatial coverage of seagrass meadows means their integrated contribution to platform- and national-scale sediment budgets can be substantial. These findings elevate seagrass sediment generation as an ecosystem service alongside blue carbon storage and habitat provision, with implications for conservation and coastal management to avoid degrading meadows or interrupting sediment transport pathways.

This study provides, to the authors’ knowledge, the first quantitative estimates of sand-sized sediment production by seagrass epibionts in a reef island sediment supply context. Seagrass meadows can produce substantial volumes of carbonate sediment suitable for island building, potentially enhancing the physical resilience of reef islands to climate change impacts. Sediment production by seagrass epibionts represents a valuable and underrecognized ecosystem service that increases the natural capital value of seagrass meadows and strengthens the geomorphic case for their conservation. Future work should refine estimates by incorporating spatial variability in environmental conditions, seagrass cover and species composition, species-specific turnover rates, and transport dynamics to islands.

• Conservative turnover rates were used (longest published), likely underestimating production; using shorter turnover rates yields higher estimates.
• Grain size analyses excluded gravel-sized (>2 mm) fractions due to instrument limits, omitting some contributors (e.g., larger foraminifera, gastropods).
• Focused only on epibionts; did not include other seagrass-associated carbonate producers (infaunal/epifaunal organisms, parrotfish contributions from nursery use).
• Laboratory agitation (21 days at 106 oscillations per minute) may not perfectly mimic marine transport/abrasion rates; ongoing abrasion/dissolution in nature could alter sizes.
• Not all produced sediments will reach island shorelines; transport pathways and retention/export were not quantified.
• National upscaling did not account for spatial variability in environmental conditions, seagrass densities, or species composition due to data limitations.