Quantitative Analysis of the Mesophotic Coral Ecosystem Bentos in the Northwestern Hawaiian Islands

Knight, Laura Jean
Wiegner , Tracy N.
Ostertag, Rebecca
Tropical Conservation Biology & Environmental Science
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Mesophotic coral reef ecosystems are light-dependent coral communities at tropical and some higher latitudes that occur from about 30 to 150 m deep, in the so-called “coral-reef twilight zone” or “deep reefs.” New emphasis has been placed on investigating mesophotic communities around the world. Advances in diving technologies have made it safer to conduct research at these depths. In this study of the mesophotic zone of the Northwestern Hawaiian Islands(NWHI), 391 photoquadrat images were examined (Nihoa 155, French Frigate Shoals 60, Laysan 57, and Kure 119) from a total of 23 transects (Nihoa 7, French Frigate Shoals 8, Laysan 3, and Kure 5). Transect depth ranged from 43.9 m on Nihoa to 96 m on Kure Atoll. Images were analyzed using Coral Net BETA. Fifty random points were generated on each photograph of the substratum, and the organism under each point was identified to the lowest taxonomic level possible. Fifty-eight taxa, six species of Chlorophyta (green macroalgae), nine Phaeophyta (brown macroalgae), 15 Rhodophyta (red macroalgae), 16 species of corals, and 12 non-coral invertebrates) were recorded in the mesophotic communities. Each island or atoll had a different suite of top ten living space-holders. The diversity of living taxa was highest at French Frigate Shoals and the lowest at Laysan. The percent similarity in mesophotic communities was greatest between Kure and Laysan (44%). The least percent similarity was found between French Frigate Shoals and Kure Atoll (29%). Despite the mesophotic zone receiving a limited amount of sunlight, substrata in the mesophotic zone are not barren, but rich in living organisms. This study found that algal turfs comprised of multiple species are the most abundant space-holding entity in the mesophotic benthic community. The low diversity of taxa documented in these mesophotic communities may be a function of the low number of species adapted to mesophotic conditions or the remoteness of the NWHI which limits the dispersal of species to the mesophotic zone. This study highlights the importance of distinct deepwater mesophotic communities. Each island or atoll seems to host a unique community of algae, coral, and non-coral invertebrates, all of which may be critical to supporting the rich ichthyofauna in the mesophotic zone.
Conservation biology, Coral Reefs, Mesophotic Coral Ecosystem, North Western Hawaiian Islands
54 pages
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