From the sky to the deep sea: ecosystem structure and ecological integrity across the National Marine Sanctuaries of Bikar and Bokak, Marshall Islands

Remote oceanic atolls provide increasingly rare opportunities to examine relatively intact tropical marine ecosystems in the Anthropocene. However, most ecological studies remain focused on individual habitats, limiting understanding of cross-habitat linkages across terrestrial, pelagic, mesophotic, and deep-sea systems. We investigated ecosystem structure across the remote atolls of Bikar and Bokak in the northern Republic of the Marshall Islands to characterize biodiversity patterns, ecosystem structure and condition, across terrestrial-to-deep-sea gradients within the recently established National Marine Sanctuaries of Bikar and Bokak. Integrated ecological surveys conducted in 2023 included terrestrial habitats, seabird communities, coral reefs, pelagic ecosystems, mesophotic reefs, and deep-sea environments. Surveys incorporated benthic and reef fish assessments, baited remote underwater video systems, mesophotic submersible observations, deep-sea camera deployments, vegetation surveys, and seabird and sea turtle assessments. Multivariate analyses, generalized linear mixed models, and constrained ordination approaches were used to evaluate ecosystem structure and differences between atolls. Both atolls supported ecosystem characteristics consistent with relatively intact oceanic systems, including extensive seabird colonies, mature native vegetation, predator-rich reef fish assemblages, structurally complex mesophotic habitats, and diverse deep-sea communities. Reef fish assemblages were dominated by herbivores, sharks, and other upper trophic-level predators, while mesophotic and deep-sea surveys documented multiple apparent new regional records and several provisionally identified taxa, highlighting the limited exploration of deeper habitats in the northern Marshall Islands. Despite their proximity, the atolls differed in several ecosystem components. Bokak supported higher reef-fish biomass and more coral-dominated benthic communities, whereas Bikar exhibited greater macroalgal cover, more extensive Pisonia grandis forest, higher Polynesian Rat encounter rates, and concentrated sea turtle nesting activity. These component-specific contrasts demonstrate substantial ecological heterogeneity between neighboring remote atolls. As relatively intact tropical marine ecosystems become increasingly uncommon worldwide, Bikar and Bokak provide complementary ecological baselines for evaluating future change across terrestrial-to-deep-sea gradients.