Seasonal dynamics of seagrass-mediated sediment stabilization in a temperate eelgrass meadow: field evidence from the Yellow Sea

Seagrass meadows stabilize coastal sediment through canopy attenuation of hydrodynamic forces and rhizome mechanical anchoring, yet the full-year temporal dynamics of this effect across contrasting hydrodynamic conditions remain poorly documented, and static cross-sectional designs cannot distinguish seagrass actively driving sediment change from fine-grained substrates passively attracting colonization. We conducted 14 consecutive months of monthly paired field observations at three Zostera marina sites along a hydrodynamic exposure gradient in the Yellow Sea, China, tracking seagrass biomass, sediment properties, surface elevation, and water optical environment, with hydrodynamic intensity and benthic microalgal biomass controlled as covariates in linear mixed-effects models. The difference in median grain size between vegetated and bare areas (ΔD50) tracked the seasonal rhythm of aboveground biomass strictly, with summer peaks of 55, 47, and 29 µm and winter minima of 17, 13, and 11 µm at the low-, intermediate-, and high-energy sites; the treatment effect changed by approximately 5% after covariate control. This strict temporal coupling supports seagrass actively driving sediment stabilization rather than passively colonizing fine-grained substrates. A non-zero winter residual ΔD50 coincided with belowground biomass remaining at 75–80% of peak values, indicating that rhizome anchoring maintains baseline stabilization independently after canopy senescence. Cumulative elevation confirmed this pattern, with net deposition of +11 mm at the low-energy vegetated site versus net erosion of −9 mm at the high-energy bare site. Water transparency differences correlated strongly with leaf area index (R² = 0.77–0.89, p < 0.001), demonstrating that sediment stabilization simultaneously improves the benthic light environment. The seasonal amplitude of stabilization decreased with increasing hydrodynamic exposure, while winter erosion vulnerability was greatest at the high-energy site. These findings advance causal attribution of seagrass sediment stabilization from static association to temporal-dynamics-level evidence, and provide a basis for temporally targeted conservation — particularly restricting anthropogenic disturbance during winter at high-exposure meadows.