Abstract: Abstract: The spatiotemporal heterogeneity of sediment grain size in coral reef areas directly governs the habitat of reef-building corals, while its response to both normal waves and extreme wave events is critical for quantifying ecosystem resilience. However, quantitative linkages between this process and hydrodynamic forcing remain poorly constrained, hindering the assessment and prediction of geomorphological stability and ecological health under hydrodynamic disturbances. Analysis of 13-month monthly sediment samples with concurrent hydrodynamic data from Weizhou Island's southwestern reef flat demonstrates fine-sand dominance. Significant spatial heterogeneity exists with larger, more variable grains southwest contrasting smaller, better-sorted sediments west-northwest. The sorting coefficient exhibits opposing seasonal phases between summer-autumn and spring-winter. Empirical Orthogonal Function analysis indicates the first median grain size mode arises from coupled wave direction and reef topography where strong southwesterly waves coarsen southwestern sediments while local currents fine western and northwestern zones, with typhoons amplifying this pattern. The second mode's spatial asynchronicity reflects wave-current regulation wherein wave energy fluctuations trigger fine-particle resuspension or settling combined with seasonal current-driven northwest accumulation and southeast offshore transport. These processes collectively drive periodic domain-wide coarsening-fining state transitions. Sediment transport heterogeneity is modulated by wave nonlinearity and current velocity-direction interactions. Reef flat width and substrate roughness further influence sorting efficiency through wave energy dissipation regulation. These findings offer mechanistic insights into hydrodynamic-sedimentary feedback in coral reef systems.

Key words: coral reef, Weizhou Island, calcareous sediments, grain-size characteristics, hydrodynamic