Environmental filtering shapes patch dynamics across isolated mesophotic reefs

Mesophotic coral ecosystems (MCEs; [~]30-150 m) are major but poorly understood benthic habitats. We used Autonomous Reef Monitoring Structures (ARMS) and integrated metabarcoding (mtCOI, 18S), image analysis, and hydrodynamic modeling across six mesophotic banks in the Gulf of Mexico 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 depth and turbidity predicted community dissimilarity up to tenfold better than geographic distance. Turbidity, driven by the benthic nepheloid layer (BNL), was the dominant filter, while 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. TeaserSuspended particle layers, not dispersal barriers, determines which species colonize mesophotic coral reefs on the TX-LA continental shelf.