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Abstract

Kelp forests are among the world’s most productive marine ecosystems, yet they are experiencing widespread decline due to a combination of local and global stressors, raising concerns about the continued loss of these critical habitats. As more mesophotic kelp forests are uncovered in tropical and subtropical regions, they have been proposed either as highly vulnerable to climate-driven marine heatwaves or as potential deep-water climate refugia for shallow biodiversity. However, the ecology of mesophotic kelp forests and their associated biodiversity remains largely understudied. This thesis presents the first comprehensive investigation of mesophotic kelp forests in the Galápagos Archipelago. Straddling the equator, the archipelago hosts diverse marine environments shaped by converging currents, complex volcanic topography, and strong seasonal temperature variability amplified by El Niño and La Niña events. The presence of kelp forests across three distinct locations and depths enabled assessment of physical, ecological, and temporal drivers across variable environmental gradients, making it an ideal system for studying these understudied ecosystems. Using innovative remotely operated vehicle surveys and in situ oceanographic monitoring, I document novel species and taxonomic characterizations, evaluate their resilience and potential as climate refugia, and assess their biodiversity. First, I characterize the novel Eisenia kelp populations, discovered at 45-77 m depth during my time as a research scientist with the Charles Darwin Foundation, and use phylogenetic and morphological analyses to confirm a possible new species alongside the previously known endemic Eisenia galapagensis. Second, surveys across the cold and warm season reveal successful reproduction and recruitment across all kelp populations and that environmental conditions, particularly depth and oceanographic setting, influence population structure and thermal stability. Third, I assess fish and sessile macrofaunal assemblages establishing that mesophotic kelps host rich and dissect communities with many undescribed taxa, 13 species of commercial importance, and eight species of conservation concern. Fish assemblages transition from planktivore- to piscivore-dominated with depth, with reduced herbivory corresponding with high macroalgal diversity despite the low light conditions. These findings highlight the uniqueness of Galápagos mesophotic kelp forests, their potential as climate refugia, and the need for site-specific conservation strategies.