Abstract: As a representative oceanic reef system in the South China Sea (SCS), the Xisha Islands provide a unique natural laboratory for characterizing the vertical profile of hydrodynamic, light field, and nutrient within Mesophotic Coral Ecosystems (MCEs). Based on high-resolution vertical profiles across multiple transects, this study provides a comprehensive characterization of environmental gradients within these reef waters. The results indicate that the Xisha reef system is situated within a oligotrophic regime, with the vertical profiles being strongly governed by reef bathymetry and physical stratification. The mixed layer depth (MLD) ranges from 15 to 40 m, displaying a consistent spatial pattern of "shallower toward the reef and deepening offshore." Stratification promotes upper-layer stability and generally constrains the sustained vertical supply of nutrient-rich subsurface waters. Characterized by high transparency, these waters feature a euphotic zone depth (Z_eu) exceeding 79 m. This clarity allows for the preferential penetration of blue-green light (480-500 nm) into deeper water layers, providing the essential radiative energy for MCEs. A Deep Chlorophyll Maximum (DCM) generally occurs at 60-80 m (about 0.7-0.8 Z_eu), governed by the interplay between exponential light attenuation and the nutricline located at 60-90 m. This stratification highlights the phytoplanktonic adaptation to the trade-off between light and nutrient. Notably, subsurface dissolved oxygen maximum is observed at shallower depths (40-70 m). Overall, the vertical synergy of underwater light and nutrient supply within the 30-100 m, providing the critical environmental constraints for local MCEs. This research provides fundamental insights for understanding the biogeochemical dynamics and habitat architecture mechanisms in tropical reef ecosystems.

Key words: Xisha Islands, Mesophotic Coral Ecosystems (MCEs), underwater light field, Deep Chlorophyll Maximum (DCM), Vertical Profile