Environmental filtering shapes patch dynamics across isolated mesophotic reefs.

Mesophotic coral ecosystems (MCEs; ∼30 to 150 meters) are major but poorly understood benthic habitats. We used Autonomous Reef Monitoring Structures (ARMS) and integrated metabarcoding (mtCOI and 18S), image analysis, and hydrodynamic modeling across six mesophotic banks in the Texas-Louisiana Shelf to test whether community assembly is governed by environmental filtering or dispersal limitation. Local environmental conditions explained nearly twice as much compositional variance as geographic effects. Differences in water visibility, an inverse proxy for benthic nepheloid layer (BNL) intensity, and depth predicted community dissimilarity up to 10-fold better than geographic distance. The BNL was the dominant filter, whereas depth effects were weaker and taxon specific. Hydrodynamic simulations revealed dispersal is variable but not limiting. These findings identify the BNL as a key physical driver linking shelf oceanography, biodiversity, and ecosystem function. Suspended particle dynamics associated with BNLs merit integration into conservation planning as critical mediators of ecological connectivity in mesophotic and other patchy reef systems globally.