Abstract
Although the existence of coral-reef habitats at depths to 165 m in tropical regions has been known for decades, the richness, diversity, and ecological importance of mesophotic coral ecosystems (MCEs) has only recently become widely acknowledged. During an interdisciplinary effort spanning more than two decades, we characterized the most expansive MCEs ever recorded, with vast macroalgal communities and areas of 100% coral cover between depths of 50–90 m extending for tens of km2 in the Hawaiian Archipelago. We used a variety of sensors and techniques to establish geophysical characteristics. Biodiversity patterns were established from visual and video observations and collected specimens obtained from submersible, remotely operated vehicles and mixed-gas SCUBA and rebreather dives. Population dynamics based on age, growth and fecundity estimates of selected fish species were obtained from laser-videogrammetry, specimens, and otolith preparations. Trophic dynamics were determined using carbon and nitrogen stable isotopic analyses on more than 750 reef fishes. MCEs are associated with clear water and suitable substrate. In comparison to shallow reefs in the Hawaiian Archipelago, inhabitants of MCEs have lower total diversity, harbor new and unique species, and have higher rates of endemism in fishes. Fish species present in shallow and mesophotic depths have similar population and trophic (except benthic invertivores) structures and high genetic connectivity with lower fecundity at mesophotic depths. MCEs in Hawai‘i are widespread but associated with specific geophysical characteristics. High genetic, ecological and trophic connectivity establish the potential for MCEs to serve as refugia for some species, but our results question the premise that MCEs are more resilient than shallow reefs. We found that endemism within MCEs increases with depth, and our results do not support suggestions of a global faunal break at 60 m. Our findings enhance the scientific foundations for conservation and management of MCEs, and provide a template for future interdisciplinary research on MCEs worldwide.
Highlights
Tropical coral reefs are compelling subjects for a wide range of scientific investigations because they provide an optimal combination of high diversity, extensive existing data, robust information infrastructure, large potential for the discovery of new taxa, and opportunities to gain new insights into fundamental ecological dynamics (Reaka-Kudla, 1997)
Our intention in this work is to provide a broad characterization of mesophotic coral ecosystems (MCEs) across the Hawaiian Archipelago based on more than two decades of interdisciplinary and collaborative research, with emphasis on a seven-year effort to document MCEs in Hawaii largely funded by National Oceanic and Atmospheric Administration (NOAA)
When information has previously been published, we provide appropriate literature citations, and when information is presented for the first time, we indicate it as such
Summary
Tropical coral reefs are compelling subjects for a wide range of scientific investigations because they provide an optimal combination of high diversity, extensive existing data, robust information infrastructure, large potential for the discovery of new taxa, and opportunities to gain new insights into fundamental ecological dynamics (Reaka-Kudla, 1997). They are among the most severely threatened ecosystems on Earth (Pandolfi et al, 2003; Knowlton et al, 2010). There are potentially thousands of species that have yet to be discovered and scientifically described from deeper coral reef habitats (Pyle, 1996d; Pyle, 2000; Rowley, 2014) and the basic ecology and population dynamics of these communities, as well as their connectivity with shallow reefs, are just beginning to be explored
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