Restoration depth mediates seasonal recalcitrant carbon retention capacity in tropical seagrass meadows

Seagrass restoration enhances carbon sequestration by promoting suspended particulate matter (SPM) capture, yet the effects of season and water depth on deposition dynamics in restored meadows remain unclear. This study investigated seasonal and depth-driven SPM dynamics in transplanted Enhalus acoroides meadows across tropical Indo-Pacific coastal intertidal zones (shallow: 0.6 m; medium: 0.9 m; deep: 1.1 m). Season emerged as a critical driver, with summer exhibiting 2.5-fold higher particulate deposition rates (0.13 ± 0.02 g DW m⁻² d⁻¹) compared to winter (0.05 ± 0.01 g DW m⁻² d⁻¹). Elevated seagrass leaf length (about 42 %) in summer drove 1.5–3.1 times greater deposition of particulate organic matter (POM), inorganic matter (PIM), and their carbon components (POC and PIC). Crucially, restoration zones prioritised stable carbon sequestration, retaining 2.1–2.6 times more recalcitrant/refractory organic carbon in summer than those in winter period. Water depth acted as a modulating factor though no significant differences, with medium zones (0.9 m) achieving peak deposition efficiency through optimal light availability (sustaining canopy growth) and moderated hydrodynamics (flow velocity increased by 51 % compared to deep zones). These findings suggest that medium-depth zones (0.9 m) emerge as optimal E. acoroides restoration targets to maximize current carbon sequestration efficiency, offering critical insights for climate-resilient blue carbon management.