Abstract
Mesophotic coral ecosystems (MCEs) continue to be understudied, especially in island locations spread across the Indo-Pacific Ocean. Pohnpei is the largest island in the Federated States of Micronesia, with a well-developed barrier reef, and steep slopes that descend to more than 1,000 m. Here we conducted visual surveys along a depth gradient of 0 to 60 m in addition to video surveys that extend to 130 m, with 72 belt transects and 12 roving surveys using closed-circuit rebreathers, to test for changes in reef fish composition from shallow to mesophotic depths. We observed 304 fish species across 47 families with the majority confined to shallow habitat. Taxonomic and trophic positions at 30 m showed similar compositions when compared against all other depths. However, assemblages were comprised of a distinct shallow (<30 m) and deep (>30 m) group, suggesting 30 m as a transition zone between these communities. Shallow specialists had a high probability of being herbivores and deep specialists had a higher probability of being planktivores. Acanthuridae (surgeonfishes), Holocentridae (soldierfishes), and Labridae (wrasses) were associated primarily with shallow habitat, while Pomacentridae (damselfishes) and Serranidae (groupers) were associated with deep habitat. Four species may indicate Central Pacific mesophotic habitat: Chromis circumaurea, Luzonichthys seaver, Odontanthias borbonius, and an undescribed slopefish (Symphysanodon sp.). This study supports the 30 m depth profile as a transition zone between shallow and mesophotic ecosystems (consistent with accepted definitions of MCEs), with evidence of multiple transition zones below 30 m. Disturbances restricted to either region are not likely to immediately impact the other and both ecosystems should be considered separately in management of reefs near human population centers.
Highlights
Studies of coral reef ecosystems are almost exclusively limited to depths shallower than 30 m, well above the lower depth limit of most photosynthetic coral (Kahng, Copus & Wagner, 2014; Menza, Kendall & Hile, 2008)
This zone lie the largely unexplored mesophotic coral ecosystems (MCEs), the light-dependent communities that typically range from depths of 30 m to over 150 m in tropical and subtropical regions (Kahng, Copus & Wagner, 2014)
circuit rebreather (CCR) are superior to open-circuit SCUBA for this type of study for several reasons: (1) they eliminate expelled gas into the environment and subsequently reduce noise that affects fish behaviors (Lindfield et al, 2014); (2) they greatly reduce gas consumption allowing extended bottom times and lower associated costs compared to open-circuit SCUBA; and (3) they function on a fixed O2 partial pressure, reducing decompression obligation and increasing the bottom time and number of transects on a single dive
Summary
Studies of coral reef ecosystems are almost exclusively limited to depths shallower than 30 m, well above the lower depth limit of most photosynthetic coral (Kahng, Copus & Wagner, 2014; Menza, Kendall & Hile, 2008). This zone lie the largely unexplored mesophotic coral ecosystems (MCEs), the light-dependent communities that typically range from depths of 30 m to over 150 m in tropical and subtropical regions (Kahng, Copus & Wagner, 2014). Information on the ecology and community composition of MCEs remains scarce (Kahng et al, 2010), based largely on studies from a few key locations with adequate technical support (Kahng, Copus & Wagner, 2014; Pyle et al, 2016; Wagner et al, 2014)
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