A comprehensive investigation of mesophotic coral ecosystems in the Hawaiian Archipelago

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.

Keywords: Amino acid isotopic composition; Closed-circuit rebreathers; Endemism; Hawaiian Archipelago; Mesophotic coral ecosystems; Refugia.

Figure 1. Map of the Hawaiian Archipelago.

Source Imagery: Landsat.

Figure 2. Location of study areas.

Inset shows remote camera survey (TOAD) track locations, and sites for “John,” “Frank,” and “Tele 1” and “Tele 2” data moorings. MHI imagery from Landsat, USGS.

Figure 3. Research divers place a dome over a set of corals 89 m deep.

Research divers Ken Longenecker (left), Dave Pence (center) and Christina Bradley (right) place a dome over a set of corals 89 m deep as part of an experiment to determine coral feeding patterns, while pilot Terry Kerby and science observers Brian Popp and Andrea Grottoli watch on from the HURL submersible Pisces V. Photo: RL Pyle.

Figure 4. Collecting samples using the Pisces submersible manipulator arm.

Photo: HURL.

Figure 5. Colony of Leptoseris sp. being stained with Alizarin Red for growth rate studies.

Photo: HURL.

Figure 6. Ross Langston demonstrating the videogrammetry technique for estimating fish sizes.

A video camera fitted with parallel lasers (A); superimposes a measurement scale on target fish (B–D). Photos: H Bolick, K Longenecker and R Langston.

Figure 7. Generalized diagram of major components of MCEs in the ‘Au‘au Channel, Hawaiian Islands.

Illustration by RL Pyle.

Figure 8. Temperature data from “John” and “Frank” moorings.

Temperature data from “John” and “Frank” moorings, comparing seasonal and daily fluctuations in water temperature at each of eight different depths off the ‘Au‘au Channel from August 2008 to July 2009. Graphs represent the average daily temperature (A) and the daily standard deviation (SD) (B) at each depth. The thin black line below each depth trace in (B) represents SD = 0, and the thin black line above represents SD = 1; the greater the distance of the color line from the black line below (SD = 0), the more dynamic the daily temperature. SD is based on n = 72 temperature values/day for data recorded at 84 and 123 m, and n = 36 for other depths.

Figure 9. One-year temperature profile in two MCE habitat types at Kaua‘i and Maui.

Branching coral (Montipora; A) habitat was at approximately 57 m and black coral (Antipathes, B) habitat was at 34 to 62 m.

Figure 10. Current magnitude profiles.

Sontek 250 kHz Acoustic Doppler Profiler profiles of current magnitude in cm s⁻¹, with overlap shown in the black line between the deeper Frank mooring and shallower John mooring. Broken down by seasons to show detail (A, Autum; B, Winter; C, Spring; D, Summer).

Figure 11. Comparison of Leptoseris-dominanted MCE habitats.

(A) Kaua‘i and (B) Maui, showing the close similarity in general structure.

Figure 12. Heterogeneous reef fish distribution on Leptoseris reefs in the ‘Au‘au Channel.

Reef fish distribution on Leptoseris reefs in the ‘Au‘au Channel was heterogeneous, with large areas nearly devoid of fishes (A) punctuated with areas of high fish diversity and abundance (B). The fishes seen in the distance in (A) represent a separate localized area of high abundance. All but two of the fishes visible in (B) belong to endemic species (Endemics: Chaetodon miliaris, Pseudanthias thompsoni, Sargocentron diadema, Dascyllus albisella, Holacanthus arcuatus, Centropyge potteri; Non-endemic: Forcipiger flavissimus, Parupeneus multifasciatus). Photos: HURL.

Figure 13. Total number of macroalgal species (over all sites combined) found at each depth surveyed.

Depth of occurrence is based upon collections and visual observations when species level identifications were verified. Shallower depths (40–60 m) were collected by mixed-gas divers while depths ≥70 m were collected by submersibles. See Spalding (2012) for collection locations. Data are included in the “AlgaeData” (Supplemental Information 2; Tab 1) worksheet of the Raw Data file.

Figure 14. Fish and macroalgal species changeover at 10-m depth intervals.

The degree of fish (n = 445) changeover (A) and macroalgal (n = 72) species changeover (B) at 10-m depth intervals. Values of each bar represent the number of species with a maximum known depth limit within 10 m above each depth interval plus the number of species with a minimum known depth limit within 10 m below each interval, expressed as a percentage of the total species present at the interval. A high value indicates a more substantial break, and a low value represents a less substantial break. Data are included in the “AlgaeData” ((Supplemental Information 2; Tab 1)) and “FishData” (Supplemental Information 2; Tab 2) worksheets of the Raw Data file.

Figure 15. Proportion of endemic reef fish species in mesophotic fish communities of the NWHI.

Figure 16. Proportion of endemic coral reef fish species across the tropical Indo-Pacific by island/region.

Sources: Randall, 1998; Moura & Sazima, 2000; Allen, 2008; Floeter et al., 2008. Kane, Kosaki & Wagner, 2014; this study.

Figure 17. Temperature log from stained coral.

Figure 18. Comparison of fish assemblage trophic structure between shallow and mesophotic reefs in the NWHI.

NIH, Nihoa; FFS, French Frigate Shoals; MID, Midway Atoll; PHA, Pearl and Hermes Atoll; KUR, Kure Atoll.