Mesophotic Coral Ecosystems

Table of Contents

1. Regional Variation in Mesophotic Coral Ecosystems

   1. Frontmatter

   2. 2. Bermuda

      Gretchen Goodbody-Gringley, Timothy Noyes, Struan R. Smith

      Abstract

      Bermuda’s subtropical coral reefs represent the northernmost reef system in the Atlantic Ocean, and consist of a large lagoon with thousands of patch reefs, bounded along the edge by a shallow annular rim reef tract that drops quickly to mesophotic coral ecosystems (MCEs) encircling the entire platform and two adjacent shallow seamounts. Although extensive investigations on Bermuda’s MCEs are lacking, several historical studies, conducted via dredging (late 1800s), followed by more recent investigations (1970s–2000s) using traps, deep diving, submersibles, and remotely operated vehicles, have occurred, and thus, a diverse collection of information has accumulated on MCEs. These studies reveal reefs dominated by scleractinian corals that decrease in abundance and diversity from upper to lower MCEs towards habitat dominated by fossil biogenic reefs and rhodolith beds with low topographic complexity. Fish and algal communities thrive on MCEs in Bermuda, and it is suggested that depth may serve as a refuge from fishing pressure for many species. However, invasive lionfish (Pterois spp.) are documented in high abundance on MCEs in Bermuda, which may threaten the health of these ecosystems. Here we discuss research, albeit limited, conducted on MCEs in Bermuda including descriptions of the physical environment, benthic habitat, and general ecology, with particular attention to the biodiversity of major taxonomic groups, and provide suggestions for ecosystem management and conservation.

   3. 3. The Bahamas and Cayman Islands

      Marc Slattery, Michael P. Lesser

      Abstract

      As shallow coral reefs worldwide are increasingly degraded by natural and anthropogenic stressors, mesophotic coral ecosystems (MCEs; ~30–150 m) represent a potential refugia that may act as seed banks for coral reef resilience. This is particularly true in the Caribbean Basin where phase shifts from coral dominance to either algal or sponge dominance have been reported with increasing frequency. However, coral reefs of the Bahamas and Cayman Islands are less impacted than other Caribbean reefs, and they offer an opportunity to assess the ecology of MCEs and the connectivity between shallow and mesophotic coral reefs. The MCEs of the Bahamas and Cayman Islands are composed of a shallow sloped upper zone (~30–60 m) and a vertical lower zone (60–100+ m). The upper MCE zone has similar biodiversity and percent cover to nearby shallow reefs, but the lower MCE zone is dominated by sponges, and it includes many species not found on shallow reefs. The ecological importance of the mesophotic sponges is unequivocal; they provide habitat for a variety of species and play a significant role in benthic-pelagic coupling via filtration of bacterioplankton, and their symbiotic microbes are sources of nitrogen cycling. Moreover, our data indicate that sponge diversity on MCEs in the Bahamas and the Cayman Islands is more similar to one another than the sponge diversity among shallow and mesophotic reefs in each region. Threats to shallow reefs (e.g., climate change and invasive species) may also impact MCEs; conservation will require mitigation of these stressors.

   4. 4. Pulley Ridge, Gulf of Mexico, USA

      John K. Reed, Stephanie Farrington, Andy David, Stacey Harter, Shirley A. Pomponi, M. Cristina Diaz, Joshua D. Voss, Keith D. Spring, Albert C. Hine, Villy H. Kourafalou, Ryan H. Smith, Ana C. Vaz, Claire B. Paris, M. Dennis Hanisak

      Abstract

      Pulley Ridge is a limestone ridge that extends nearly 300 km along the southwestern Florida shelf in the eastern Gulf of Mexico. The southern terminus of Pulley Ridge supports a mesophotic coral ecosystem (MCE) at depths of 59–105 m and is the deepest known photosynthetic coral reef off the continental United States. The biodiversity consists of 95 species of macroalgae, 92 demosponges, 18 octocorals, 17 scleractinian corals, 9 antipatharian corals, and 86 fishes. Twenty managed fishery species occur at Pulley Ridge including red grouper, and since 2010 the lionfish population has dramatically increased. The dominant scleractinian corals are plate like corals of the family Agariciidae (Agaricia spp. and Helioseris cucullata), Montastraea cavernosa, Madracis spp., and Oculina diffusa. The percent cover of benthic biota averaged 49.9% over all regions of Pulley Ridge, and macroalgae were dominant (46.5% cover). Scleractinian corals averaged 1.5% cover and sponges had 1.2% cover. In the past 10 years, the Pulley Ridge MCE had a substantial loss of scleractinian coral. The percent coral cover on the Main Ridge dropped from 12.8% in 2003 to 0.9% by 2012–2015, a 93% loss of coral. However, recent surveys show the majority of corals to be relatively healthy; only 1.21% of the colonies counted (38,368) showed signs consistent with “white syndromes” disease. The prevalence of disease on Pulley Ridge is relatively low compared to the Caribbean. The factors causing the decline of the coral communities at Pulley Ridge between 2003 and 2012 are unknown.

   5. 5. The Mesoamerican Reef

      Erika Gress, Joshua D. Voss, Ryan J. Eckert, Gwilym Rowlands, Dominic A. Andradi-Brown

      Abstract

      At over 1000 km in length, the Mesoamerican Reef (MAR) is a near-continuous reef system within four different countries: Mexico, Guatemala, Belize, and Honduras. MAR is the largest reef system in the Northern Hemisphere and is comprised of fringing and barrier reefs and offshore atolls, which extend into deeper water where mesophotic coral ecosystems (MCEs; 30–150 m depth) are found. Scientific studies of MCEs in the MAR began in the 1970s, and despite a rapid increase in marine research throughout the region in recent years, MCE ecological research has been restricted to a few locations and has been focused on hard (scleractinian) corals and fish communities. Hard corals have been documented at a maximum depth of 102 m in the MAR. However, hard corals do not represent the dominant benthic community at mesophotic depths in most cases. The benthic organisms providing structural habitat on many of the known MAR MCEs are octocorals, sponges, black corals, and calcareous macroalgae. Studies on MAR fish communities showed that a large proportion of fish species are found on both shallow reefs and MCEs. The limited data available suggests that MCEs are likely widespread along the MAR. There is evidence of the negative effects of fisheries, sedimentation, harvesting of black corals, and invasive lionfish on MAR MCEs. Improved identification and increase of biological and ecological studies of MCEs, coupled with an extension of scope to include mesophotic habitats in managed areas, should be undertaken to enhance their protection in the region.

   6. 6. Discovery Bay, Jamaica

      Phillip Dustan, Judith C. Lang

      Abstract

      Mesophotic coral ecosystems occur on fore-reef slopes (~25 to 55 m) and near-vertical deep fore reefs (~55 to ~122 m) off the north Jamaican coast. In the 1960s, T.F. Goreau and colleagues found corals and calcareous green algae (Halimeda) thriving to depths of ~70 m and coralline sponges even deeper. Boring sponges and calcareous algae produced prodigious quantities of carbonate muds and sands, some becoming lithified into reef rock and the remainder draining into deeper water. Low-light flattening of corals facilitated sediment shedding and stabilized dislodged colonies. Huge pinnacle reefs with frame work-building Orbicella dominated the upper fore-reef slope (~25 to 45 m) at Discovery Bay. Foliaceous agariciids monopolized some lower slopes and the upper deep fore-reef. Submersible-based explorations of the deep fore-reef cliff in 1972 confirmed active framework construction by corals at ~55 to 70 m and by coralline sponges and crustose coralline algae at ~70 to 105 m. Species of deep reef fishes occurred below ~60 to 90 m. The base of the cliff at ~91 to 145 m consisted of lithified sediments and debris. Non-coralline sponges and algae were the dominant benthos from 53 to 120 m in a 1984 submersible study. Fore-reef slope corals have declined greatly since the 1980s, and their skeletons are largely overgrown with algae. Contributing factors include enhanced herbivore losses from overharvesting (fish) or disease (Diadema), repeated bleaching-mortality events, increased trapping of sediments, degraded water quality, hurricanes, and coral diseases. Apart from identified sponges collected with open circuit trimix scuba in the 1990s, the post-1984 condition of the deep fore reef is unknown.

   7. 7. Puerto Rico

      Richard S. Appeldoorn, Monica Alfaro, David L. Ballantine, Ivonne Bejarano, Hector J. Ruíz, Nikolaos V. Schizas, Wilford E. Schmidt, Clark E. Sherman, Ernesto Weil

      Abstract

      Mesophotic coral ecosystems (MCEs) in Puerto Rico cover a potential area of 2180 km² or 38% of the total area from the shoreline to 150 m. MCEs occur primarily along the upper insular slope of the Puerto Rico-Virgin Islands platform. Patchy MCE development occurs along the insular slope, with distribution strongly related to geomorphology. Shallow mesophotic platforms can support well-developed coral communities dominated by the Orbicella annularis species complex, but deep mesophotic platforms are more poorly developed and consist primarily of algae and sponges, with patchy occurrences of corals, dominated by Agaricia spp. Macroalgae, with 185 taxa, are the dominant component of MCEs, both in species richness and percent cover. The dominant algal components below 40 m are calcified encrusting red algae including Corallinales and Peyssonneliales species. Twenty-seven scleractinian corals and two hydrocorals were recorded within MCEs. Dominant corals in shallow MCEs were Orbicella spp., Siderastrea siderea, and Porites astreoides, but by 50 m Agaricia dominates, particularly A. undata, A. lamarcki, and A. grahamae. Over 100 fish species have been recorded from mesophotic depths, with Chromis insolata being the most common and abundant species. High vertical connectivity appears to occur within fishes, with a significant component potentially dependent on shallow, nearshore nursery areas. For all taxa, species richness and abundance decreased with depth, and changes in community structure were observed, with noted breaks occurring at ~45 and ~60 m. Primary local threats to Puerto Rico’s MCEs are overfishing, invasive species, and land-based anthropogenic inputs that increase turbidity and sedimentation.

   8. 8. The United States Virgin Islands

      Tyler B. Smith, Marilyn E. Brandt, Viktor W. Brandtneris, Rosmin S. Ennis, Sarah H. Groves, Sennai Habtes, Daniel M. Holstein, Elizabeth Kadison, Richard S. Nemeth

      Abstract

      Mesophotic coral ecosystems (MCEs) at 30–100 m depth are the dominant hard bottom habitat in the US Virgin Islands of the northeastern Caribbean Sea. Well-developed MCEs surpass the extent of shallow coral reefs by almost a factor of three. These habitats range from high coral cover structural reefs to low coral cover hard bottoms on banks, slopes, and walls. The most common communities in the United States Virgin Islands (USVI) are star coral (Orbicella franksi) banks that are concentrated in a relatively large area (187 km²) south and southwest of St. Thomas in depths of 30–43 m. Wall and slope coral communities in upper (30–60 m) and lower MCE (60–100 m) depths are dominated by lettuce corals (Agaricia spp.) Most MCEs are heavily covered in macroalgae, particularly Lobophora variegata. Corals are species rich in many upper MCE habitats (20–28 species) compared with shallow reefs (31), but not in upper and lower MCE slopes and walls (6–8). Sponges are species rich (24–53 species) in the MCEs of the USVI and may contribute much to nutrient cycling. Fishes are also species rich across habitats (45–151 species), but often their biomass is dominated by a few species, particularly on edges with high abundances of planktivores and piscivores. Despite their importance to the ecology and economy of USVI, MCEs are threatened by a variety of local and global stressors and disturbances that are causing declines in coral cover.

   9. 9. Bonaire and Curaçao

      Pedro R. Frade, Pim Bongaerts, Carole C. Baldwin, Arthur C. Trembanis, Rolf P. M. Bak, Mark J. A. Vermeij

      Abstract

      Bonaire and Curaçao are oceanic islands surrounded by coral reefs on their leeward sides extending steeply down to mesophotic depths (30–150 m). Easy access from shore, as well as the geopolitical context of the two islands, has made the mesophotic coral ecosystems (MCEs) of Bonaire and Curaçao among the most well studied in the world. MCE research has been conducted in the region since the 1970s, and ongoing research employs state-of-the-art exploration technologies such as manned submersibles, autonomous underwater vehicles, and mixed-gas, rebreather technology. Mesophotic coral communities in Bonaire and Curaçao are typically dominated by agariciids, with Agaricia lamarcki and A. grahamae covering a substantial proportion of the substrate at upper mesophotic depths (30–60 m), while the lower mesophotic (>60 m) consists predominantly of A. grahamae and A. undata. Although much of the habitat is dominated by sediment with only patchy coral growth, in some locations coral assemblages can cover up to 100% of the seafloor down to depths of 70–85 m. Recent biodiversity studies, mostly focusing on fishes, sponges, and corals, documented only limited overlap between shallow and mesophotic reef communities, and that MCE biodiversity is strongly structured by depth. Since MCEs in Bonaire and Curaçao harbor highly specialized communities facing specific threats deriving from their proximity to urbanized land, these ecosystems warrant new management policies and conservation measures. These measures should protect the whole extent of these reefs, which rank among the healthiest in the Caribbean region.

   10. 10. Brazil

       Ronaldo Bastos Francini-Filho, Viviana Márquez Velásquez, Marianna Barbosa da Silva, Marcos Rogerio Rosa, Paulo Yukio Gomes Sumida, Hudson Tercio Pinheiro, Luiz Alves Rocha, Carlos Eduardo Leite Ferreira, Carlo Leopoldo Bezerra Francini, Ricardo de Souza Rosa

       Abstract

       Indirect evidence for the occurrence of mesophotic coral ecosystems (MCEs) in Brazil dates back to the 1960s. Only in the last 10 years have Brazilian MCEs been studied systematically, through the use of new tools such as trimix technical diving (open and closed circuit), remotely operated vehicles (ROVs), drop cameras, submersibles, and sidescan sonar. Brazilian MCEs occur along an extensive latitudinal gradient, from the Amazon Reef in the north (5° N) to the Vitória-Trindade Seamount Chain in the south (21° S). Fisheries data and in situ unpublished observations indicate that MCEs also occur further south (24° S), where scleractinian corals, octocorals, and reef fishes are commonly found over rock bottoms between 30 and 70 m. The primary research topics published in the last decade include habitat mapping, benthic and fish assemblage structure, biodiversity surveys, microbial abundance and function, ecosystem assessment, evolution, and conservation. A conservative estimate indicates that at least 25 species of elasmobranchs, 275 teleost fishes, and 476 sessile benthic species (234 algae, 166 sponges, and 76 anthozoan cnidarians) occur in Brazilian MCEs. The primary reef builders are coralline algae (both encrusting and free-living nodules) and the scleractinian coral Montastraea cavernosa. Benthic assemblages are generally dominated by sponges, black corals, and octocorals. Fish assemblages are dominated by planktivorous fishes, while piscivorous species, particularly jacks (Carangidae), are also abundant at mesophotic depths. Overfishing, mining, and pollution are among the main threats to Brazilian MCEs.

   11. 11. The Red Sea: Israel

       Gal Eyal, Raz Tamir, Netanel Kramer, Lee Eyal-Shaham, Yossi Loya

       Abstract

       The mesophotic coral ecosystems (MCEs) of Eilat, in the Northern Red Sea, are among the best-studied worldwide, as demonstrated by the high number of publications from the region. Nonetheless, Eilat’s MCEs remain relatively unexplored compared to its shallow reefs. Its MCEs host diverse benthic communities that are potentially linked ecologically to shallow reefs. Here, we summarize the history of MCE research and compare the shallow and mesophotic reefs using long-term biotic and abiotic data. Eilat’s MCEs exhibit lower fluctuations in temperature, light, sedimentation, and a decreased frequency of shore-related disturbances than adjacent shallow reefs, supporting the hypothesis that key environmental parameters become more stable with increasing depth. However, nutrient concentrations are more variable in MCEs than nearby shallow reefs. We provide a novel definition of the upper (30–80 m) and lower (80–160 m) mesophotic zone boundaries in Eilat, based on the degree of light penetration, as well as the relative abundance of major fauna and flora. Scleractinian coral diversity increases with depth, as well as the abundance of specialist taxa. Corals (93 spp.) comprise the major organisms contributing to living benthic cover. A mass coral-bleaching event took place in 2015 that exclusively affected MCEs, and we discuss the event’s potential mechanisms and consequences for shallow vs. mesophotic coral assemblages. Protection and regulations of MCEs are needed to maintain and support these unique ecosystems.

   12. 12. The Chagos Archipelago

       Dominic A. Andradi-Brown, Zena Dinesen, Catherine E. I. Head, David M. Tickler, Gwilym Rowlands, Alex D. Rogers

       Abstract

       The Chagos Archipelago, located in the central Indian Ocean and officially known as the British Indian Ocean Territory, contains some of the most remote reefs in the Indian Ocean. The Chagos Archipelago is comprised of a series of atolls, including the largest atoll in the world, the Great Chagos Bank. Records from surveys of mesophotic coral ecosystems (MCEs; reefs 30–150 m depth) in Chagos stretch back to 1905, with more extensive work conducted in the 1970s and post-2010. Coral and fish communities vary considerably with depth and among habitat types. Coral cover generally declines with increased depth across the shallow reef to MCE depth gradient, though in several locations close to 100% scleractinian coral cover has been observed on MCEs. Consistent with earlier studies, we identify five coral species as indicative of Chagos MCEs. Recently collected fish community data are analyzed to illustrate, for the first time, patterns in reef fish species richness, abundance, biomass, and trophic groups across a shallow to upper-MCE depth gradient (0–60 m). Fish species richness, abundance, and biomass declined with increased depth, while richness, abundance, and trophic group patterns were also influenced by habitat type (seaward versus lagoonal reef). To date, the vast majority of MCE research in Chagos has focused on upper mesophotic depths. We recommend future work consider the full MCE depth range within the Chagos Archipelago.

   13. 13. Ryukyu Islands, Japan

       Frederic Sinniger, Saki Harii, Marc Humblet, Yohei Nakamura, Hideo Ohba, Rian Prasetia

       Abstract

       Mesophotic coral ecosystems (MCEs) in Japan are currently only reported from the Ryukyu Islands and remain poorly known compared to shallow reefs in the region. MCEs in the Ryukyus were first examined in the 1960s and are among the earliest-studied MCEs globally. MCEs in the region support a high diversity of flora and fauna due to a combination of factors including the warm poleward-flowing Kuroshio Current, the complex geomorphology and diversity of habitats, and the proximity to the Coral Triangle. Recent research into the biology and ecology of mesophotic corals has examined questions such as ecological connectivity between mesophotic and shallow corals and revealed that several coral species span shallow to lower mesophotic depths (>60 m). Observations of reproduction in Seriatopora hystrix and Acropora tenella suggest that the propagule release period of both species is broadly synchronous in shallow and mesophotic corals, although the reproductive season is considerably shorter in mesophotic corals. MCEs in the Ryukyu Islands face stressors similar to shallow coral reefs such as climate change and rapid coastal development. However, the knowledge required to understand the response of MCEs to these threats is sparse. Despite evidence that at least some MCEs appear capable of recovering from disturbances (such as typhoons), investigations on how coastal development affects MCEs and how to mitigate threats to MCEs are urgently needed.

   14. 14. Taiwan

       Vianney Denis, Derek Soto, Stéphane De Palmas, Yu Ting Vicky Lin, Yehuda Benayahu, Yusheng Michael Huang, Shao-Lun Liu, Jian-Wen Chen, Qi Chen, Nicolas Sturaro, Ming-Jay Ho, Yeng Su, Chang Feng Dai, Chaolun Allen Chen

       Abstract

       Taiwan straddles tropical and subtropical latitudes in the North Pacific Ocean. While shallow-water coral reefs in Taiwan have been studied for many years, mesophotic coral ecosystems (MCEs) have recently enjoyed a surge of interest because they are hypothesized to provide a refuge for some reef species in distress. The distribution of MCEs in Taiwan is influenced by numerous biophysical and environmental variables including substrate availability, temperature, light irradiance, and sedimentation. In 1991, an early survey of the diversity of MCEs off Taiwan’s southern coast highlighted sedimentation and the lack of suitable substrates as important factors restricting reef communities at depths below 30 m. The steeply sloping east coast of Taiwan supports well-developed and more accessible MCEs and has therefore been the focus of recent research. Preliminary surveys have documented 18 macroalgal, 95 scleractinian, 33 octocoral, 2 antipatharian, 14 sponge, and 76 fish species. With the exception of a few scleractinians recorded for the first time and possibly restricted to mesophotic waters, the majority of species have a wide bathymetric distribution in Taiwan. However, low-light conditions at mesophotic depths in the north restrict the distribution of photosynthetic organisms to shallow waters. Only one zooxanthellate scleractinian species was found at depths below 40 m where benthic communities are dominated by fan- and whip-shaped octocorals. Historically, MCEs have received little research interest in Taiwan, and most current research is focused on descriptive studies. Future research should examine the ecological importance of these habitats and the roles they play in reef survival.

   15. 15. The Philippines

       Patrick C. Cabaitan, Timothy Joseph R. Quimpo, Edwin E. Dumalagan Jr., Jeffrey Munar, Mary Ann C. Calleja, Ronald Dionnie D. Olavides, Kevin Go, Ritzelle Albelda, Dominic Cabactulan, Erin Joy Capdos Tinacba, Doctor Ma. Angelique A., Cesar L. Villanoy, Fernando P. Siringan

       Abstract

       The Philippines is situated within the Coral Triangle marine biodiversity hotspot and supports highly diverse coral reef communities. However, Philippine reefs are exposed to many natural and anthropogenic disturbances. Consequently, mesophotic coral ecosystems (MCEs) have drawn increasing interest because of their potential significance as refugia for many reef species. MCEs in the Philippines occur in a variety of settings, reflecting the wide diversity of reef habitats within the archipelago. MCEs remain poorly studied compared to shallow reefs, but preliminary investigations show that MCE sites support diverse ecological communities. Here, we describe the physical features, biodiversity, and current condition of three MCE sites in the Philippines that occur in different environmental settings: surge-exposed fringing reefs at Patnanungan, turbid fringing reefs at Abra de Ilog, and the oceanic atolls of Apo Reef. Patnanungan is dominated by rubble fields with high macroalgae cover, an indication of destructive fishing and overexploitation of herbivorous fishes and invertebrates. Abra de Ilog is exposed to turbid waters due to two nearby rivers, and as a result, hard corals are not found below 30 m. Apo Reef, an oceanic atoll located within a marine park, supported higher abundance and diversity of benthic and mobile megafauna at mesophotic depths than the other two sites. MCEs are probably common throughout the Philippines, and future research should investigate additional sites, include long-term monitoring of factors driving spatial and temporal patterns of biodiversity, and investigate the potential importance of Philippine MCEs as refugia.

   16. 16. Palau

       Patrick L. Colin, Steven J. Lindfield

       Abstract

       Palau, an island archipelago located in the western Pacific warm pool, has over 500 km of outer reefs with steep slopes that host mesophotic coral ecosystems (MCEs). This low-latitude reef system is impacted by strong seasonal variations in oceanic conditions, particularly in the depth range of MCEs. El Niño brings low water temperatures with rapid variation, shallow thermoclines, phytoplankton blooms, and cold-water bleaching to MCEs, while La Niña has warm temperatures, deep thermoclines, and warm-water bleaching. MCE species diversity for Palau is relatively well-known from 20 years of collections using submersibles and mixed-gas diving. We have observed changes in MCEs through typhoon impacts and seasonal water temperature anomalies. The structure of MCEs varies around the main island group of Palau, with the east coast MCEs being dominated by rubble transported downslope due to typhoons with sparse corals, and the west/southwest MCEs are more protected, host a more diverse assemblage and higher cover of sessile invertebrates, and are periodically affected by coral bleaching. In addition, MCEs are also found at the remote oceanic islands and atolls of Palau’s southwest Islands and within the deep lagoons of the main island group. Through our observations and measurements of oceanographic conditions around Palau, we suggest temperature is an important factor limiting coral growth on MCEs, while typhoons also have major impacts on the structure of MCEs through sedimentation and debris moving downslope on the steep outer reefs.

   17. 17. Pohnpei, Federated States of Micronesia

       Sonia J. Rowley, T. Edward Roberts, Richard R. Coleman, Heather L. Spalding, Eugene Joseph, Mae K. L. Dorricott

       Abstract

       The mesophotic coral ecosystems (MCEs) of the Senyavin Islands (Pohnpei Island, and neighboring atolls Ant and Pakin) in the Federated States of Micronesia have received little research attention until recent years. These vibrant, environmentally dynamic ecosystems harbor a reservoir of biodiversity, with species and interactions new to science. Depths of ≥90 m have up to 20 °C annual variance. A strong El Niño event in 2016 resulted in a bloom-forming cyanobacteria smothering the upper MCEs of Pohnpei (25–65 m). Conditions persisted into 2017 with extensive coral bleaching and reef degradation with associated smothering by bloom-forming cyanobacteria and algae in the shallows. The initial bloom signature of 2016 at depth may, therefore, serve as a projected indicator of shallow reef health. Of the 160 reef-building scleractinian corals reported, 28 spanned the full depth range (0–45 m). Differences in irradiance due to geomorphology, as well as reef health, determined the depth transition between two primary benthic groups: photosynthetic scleractinians and filter-feeding azooxanthellate gorgonians, 60 m on low-relief atoll reefs and 45 m at high-relief walls and degraded reefs. Of the 109 gorgonian corals reported, 19 spanned the full depth range (0–140 m) with 70 morphospecies specific to lower mesophotic depths. Similarly, fish assemblages partitioned between shallow and mesophotic depths, characterized by herbivores and planktivores, respectively. Continuously growing marine resource exploitation and terrestrial runoff are heavily influencing reef health. The MCEs of Pohnpei are, thus, unique, yet vulnerable to the exacerbating stresses of man.

   18. 18. Papua New Guinea

       Ken Longenecker, T. Edward Roberts, Patrick L. Colin

       Abstract

       Papua New Guinea (PNG) hosts a unique range of geological features and is located in the Coral Triangle, the world center of marine biodiversity. However, working in PNG poses considerable logistical challenges in addition to those typically associated with mesophotic coral ecosystem (MCE; light-dependent reefs from 30 to ~150 m) research. Although its MCEs are poorly documented, they are likely extensive. Water temperatures at 90 m (23.3–26.3 °C) are cooler and less variable annually than those at 15 m (24.2–31.0 °C) but tend to be more variable on a daily basis and are within the range necessary to support coral growth. Limited studies have documented at least 213 coral species (40% of PNG’s coral fauna) and 73 sponge species (14% of PNG’s sponge fauna) at mesophotic depths. MCE fish communities are generally composed of taxa typical of shallow coral reefs; however these appear to be less abundant at mesophotic depths. Because biological surveys have been conducted in only a few locations, checklists of PNG’s mesophotic species are likely to grow with additional research. The major threat to PNG’s MCEs is likely terrigenous sediment associated with the country’s chief industries of forestry, mining, and agriculture. However, the effects of global climate change, i.e., warmwater coral bleaching, have also been observed at mesophotic depths.

   19. 19. Northwest Australia

       Andrew Heyward, Ben Radford

       Abstract

       The northwest continental shelf of Australia supports a series of emergent reefs toward the western edge and a chain of submerged shoals extending eastward along the Sahul Shelf. Hard corals and calcareous algae are major components of the benthos in the upper mesophotic regions (30–60 m) on both reefs and shoals. These habitats transition to heterotrophic-dominated communities at around 60 m depth in clear water at shelf-edge locations. While mixed hard coral communities of moderate diversity are commonly encountered, characteristic upper mesophotic coral habitats include dense areas of foliaceous species from the families Agaricidae and Acroporidae in depths of 40–60 m. Extensive fields of fungiid corals can be found occupying unconsolidated substrates at similar depths on both reefs and shoals. The alternate ecological stages of otherwise geomorphically similar shoals provide evidence of shoal scale temporal stochasticity in recruitment and disturbance events. Green calcareous algae of the genus Halimeda can be abundant and are a major sediment producer in shoal bioherms. Taxonomic studies have been focused on the shallow reefs of the region, with collection of voucher specimens from mesophotic depths lacking from the majority of reef and shoal locations.

   20. 20. The Great Barrier Reef and Coral Sea

       Tom C. L. Bridge, Robin J. Beaman, Pim Bongaerts, Paul R. Muir, Merrick Ekins, Tiffany Sih

       Abstract

       The Coral Sea lies in the southwestern Pacific Ocean, bordered by Australia, Papua New Guinea, the Solomon Islands, Vanuatu, New Caledonia, and the Tasman Sea. The Great Barrier Reef (GBR) constitutes the western margin of the Coral Sea and supports extensive submerged reef systems in mesophotic depths. The majority of research on the GBR has focused on Scleractinian corals, although other taxa (e.g., fishes) are receiving increasing attention. To date, 192 coral species (44% of the GBR total) are recorded from mesophotic depths, most of which occur shallower than 60 m. East of the Australian continental margin, the Queensland Plateau contains many large, oceanic reefs. Due to their isolated location, Australia’s Coral Sea reefs remain poorly studied; however, preliminary investigations have confirmed the presence of mesophotic coral ecosystems, and the clear, oligotrophic waters of the Coral Sea likely support extensive mesophotic reefs. Although mesophotic reefs in the GBR and Coral Sea are among the best-studied globally, most research has focused on only a few sites, and research effort dedicated to mesophotic coral ecosystems remains negligible compared to shallow-water reefs. Despite the lack of ecological data from most mesophotic reef habitats, precautionary management approaches that explicitly considered latitudinal and cross-shelf gradients in the environment resulted in mesophotic reefs being well-represented in no-take areas in the GBR. In contrast, mesophotic reefs in the Coral Sea currently receive little protection.

   21. 21. Fiji

       Richard L. Pyle

       Abstract

       The nearly 1000 islands, islets, atolls, and reefs of Fiji are located in the southwestern tropical Pacific Ocean, east of Vanuatu and New Caledonia, and west of Samoa and Tonga. The Fiji islands include a rich variety of habitats and consistently clear, calm, and warm offshore waters. Shallow coral reefs in Fiji have been subject to a great deal of research, but research on mesophotic coral ecosystems (MCEs) has been limited primarily to fishes. A series of expeditions from 2001 to 2010 using mixed-gas, closed-circuit rebreathers resulted in the discovery of many new species. Data downloaded from the Global Biodiversity Information Facility (GBIF) reveal a clear pattern of undersampling on MCEs for all taxonomic groups. Patterns of diversity and species richness among fishes are similar to those for reef fishes in general. A series of controlled rotenone stations at depths of 0–120 m at one particular site suggest that fish diversity on MCEs may approximate that of shallow reefs and that as many as 30% of the total ichthyofauna may consist of undescribed (and undiscovered) species. Very little is known about the ecology of MCEs in Fiji. Fijian MCEs face the same general threats as adjacent shallow reefs, in addition to global-scale threats facing reefs worldwide (i.e., climate change, ocean acidification, and overfishing of food fishes). The geographic location, variety of habitats, excellent diving conditions, and logistical infrastructure suggest that Fiji is an excellent location to conduct additional MCE research in the Pacific.

   22. 22. American Samoa

       Anthony D. Montgomery, Douglas Fenner, Randall K. Kosaki, Richard L. Pyle, Daniel Wagner, Robert J. Toonen

       Abstract

       Over a century of study in American Samoa has built a foundation of coral reef ecology within the region. However, this work has been restricted to shallow coral reefs (SCRs; <30 m) until recently, where a few studies have started describing American Samoa’s mesophotic coral ecosystems (MCEs). MCEs are defined as coral reef communities with zooxanthellate corals and associated biotic assemblages between 30 and 150 m depth. Mapping efforts within the territory have documented habitat characteristics for SCRs, as well as MCEs. We estimate that American Samoa has 451.5 km² of marine habitat between the shoreline and 150 m depth. Mesophotic depths represent 357.5 km² (79%) of the total area. Approximately 56 km² (12.4%) of the marine habitat above 150 m is under various levels of protection through a system of local, territorial, and federal marine protected areas. Of this, 21.7 km² (6%) includes mesophotic depths. With only a handful of studies conducted and the majority of MCEs in American Samoa unexplored, there remain significant information gaps in understanding the basic biodiversity and ecology of the region. There are over 300 species of scleractinian corals known from American Samoa, and approximately 110 species at mesophotic depths, representing over one-third of the total diversity. Approximately 1013 fish species have been recorded from American Samoa (0–150 m), including 5 new records and 4 potentially new species from MCEs. Other anthozoan corals are currently being studied, but most invertebrate and algal communities at mesophotic depths remain uninvestigated.

   23. 23. Cook Islands

       Richard L. Pyle, Joshua M. Copus, Gerald McCormack

       Abstract

       The Cook Islands are located in the southern Pacific Ocean, east of Fiji and west of the Society Islands. They consist mostly of coral atolls in the north and high basaltic islands in the south. The islands have a narrow fringing reef, a shallow-reef area several hundred meters wide sloping gently to a depth of about 30 m, and a precipitous drop-off extending to 200 m or more. Research on fishes in mesophotic coral ecosystems (MCEs) in Rarotonga, the largest and most populous island, were documented during expeditions from 1989 to 2012 using conventional SCUBA and technical diving, resulting in the discovery of many new species. Very little is known about other groups in MCEs, but data downloaded from the Global Biodiversity Information Facility (GBIF) reveals a clear pattern of undersampling on MCEs for all taxonomic groups. Patterns of diversity and species richness among fishes are similar to those for reef fishes in general, but differ in several ways, including an underrepresentation of Gobiidae (likely due to sampling bias) and an overrepresentation of Acanthuridae. The primary threats to Cook Islands MCEs are the same general threats that apply to shallow reefs, as well as all reefs at all depths worldwide (i.e., climate change, ocean acidification, and overfishing of food fishes). Over the course of more than 20 years of direct, albeit qualitative, observations in Rarotonga, there have been no major changes in the overall health, abundance, and diversity of organisms on shallow or MCE habitats.

   24. 24. French Polynesia

       Michel Pichon

       Abstract

       French Polynesia is composed of 118 high islands and atolls stretching in a vast oceanic expanse in the central Pacific Ocean, from just below the equator to almost 30° S. It is composed of five archipelagos, which all differ in terms of their geological origin and history, environmental conditions, and reef development. The Tuamotu Archipelago is entirely comprised of atolls, whereas fringing or barrier reefs are found in the Society, Gambier, and Austral Islands (only one atoll), and coral reef accretion is almost nonexistent in the Marquesas. Coral reef studies have concentrated mostly on the Society and Tuamotu Archipelagos, and, except for occasional observations, been restricted to a depth of 30–40 m on the reef slopes. Our knowledge of mesophotic coral ecosystems is therefore limited, particularly for the deeper zone, below 80 m depth. The scleractinian mesophotic fauna is highly diverse compared to shallow reefs, with 38 genera and 96 species recorded. This fact is likely to be a consequence of water clarity, allowing the photophilic shallow coral species to extend deeper than usual. In some areas, substratum cover by Pachyseris speciosa reaches values of 80% at depths of ≥70 m, and a typical “deep” mesophotic assemblage dominated by Leptoseris spp. has also been recognized. Quantitative data on other major biotic components of the mesophotic assemblages are either lacking or insufficient to allow their characterization as depth generalists or depth specialists.

   25. 25. The Hawaiian Archipelago

       Heather L. Spalding, Joshua M. Copus, Brian W. Bowen, Randall K. Kosaki, Ken Longenecker, Anthony D. Montgomery, Jacqueline L. Padilla-Gamiño, Frank A. Parrish, Melissa S. Roth, Sonia J. Rowley, Robert J. Toonen, Richard L. Pyle

       Abstract

       The Hawaiian Archipelago is one of the largest and most isolated island chains in the world, and its marine ecosystems are well-studied. Research on Hawaiian mesophotic coral ecosystems (MCEs) began in the 1960s and has intensified during the past decade. In Hawai‘i, rich communities of macroalgae, corals and other invertebrates, and fishes inhabit MCEs and are associated with increased water clarity and decreasing average current strength with depth. Extensive calcified and fleshy macroalgal beds are found both in discrete patches, dense beds, and meadows over both hard and soft substrates. Several species of corals typical of shallow reefs extend to depths of ~60 m. The dominant corals below 60 m are in the genus Leptoseris, which can form extensive coral reefs spanning tens of km². Few octocoral species inhabit shallow reefs and upper MCEs (30–70 m) but are diverse at the deepest range of MCEs (>130 m). Sponges do not represent a major structural component of MCEs. Many species of fishes occur on both shallow reefs and MCEs, but MCEs harbor more endemic species (up to 100% endemism). Several new species of macroalgae, corals and other invertebrates, and fishes have recently been documented. Over 60% of the territorial waters surrounding the archipelago are protected as the Papahānaumokuākea Marine National Monument; however, no specific protections exist for MCEs. Generally, threats affecting Hawai‘i’s shallow reefs also affect MCEs to varying degrees. MCEs may be more insulated from some threats but more vulnerable than shallow reefs to others (e.g., water clarity).

   26. 26. Isla del Coco, Costa Rica, Eastern Tropical Pacific

       Jorge Cortés

       Abstract

       Mesophotic coral ecosystems (MCEs), at ocean depths between 40 and 150 m, have been found throughout the tropics but are different in the Eastern Tropical Pacific (ETP). This observation is based on submersible dives that have explored several ETP mesophotic areas. The ETP is characterized by a shallow thermocline (approximately 50 m depth), reduction in light availability at deeper depths due to the accumulation of particulate matter at the thermocline, cold temperatures, low oxygen concentrations, and low aragonite saturation state (Ω_arag) that may explain the absence of zoothanthellate corals below 40 m. Isla del Coco is located 500 km off the Pacific coast of Costa Rica. The MCEs of this island have been explored using the DeepSee submersible. There is a sharp change in benthic fauna, in both soft and hard bottoms, at approximately 50 m depth. The combination of the shallow thermocline bringing cold and low pH waters to near the surface plus the turbidity (low light) at the thermocline leads to a situation where zooxanthellate corals do not generally thrive below about 15–25 m depth (maximum around 40 m) and do not build reefs in deeper waters. The substrate between 50 and 150 m is densely covered with red algae (down to 90 m), octocorals, black corals, azooxanthellate corals, and calcareous hydroids. This chapter is the first synthesis of mesophotic environments of any ETP site and characterizes the MCEs of Isla del Coco National Park.

   27. 27. Chile and the Salas y Gómez Ridge

       Erin E. Easton, Matthias Gorny, Ariadna Mecho, Javier Sellanes, Carlos F. Gaymer, Heather L. Spalding, Jaime Aburto

       Abstract

       In Chile, light-dependent benthic taxa have been observed to ~280 m with the presence of zooxanthellate cnidarians forming mesophotic coral ecosystems (MCEs) to depths of at least 120 m at Rapa Nui (Easter Island) and Salas y Gómez. Evidence from dredge samples suggests MCEs in the southeast Pacific once extended along the Salas y Gómez Ridge as far east as ~85° W, but MCEs in Chile were virtually unexplored prior to 2010. Despite providing critical structural habitat for numerous flora and fauna, including commercially important fishes, basic information about their geographic distribution and community structure are lacking. Here, we review the state of knowledge on these communities and present preliminary results from the most recent surveys at Rapa Nui. Surveys at mesophotic depths (30–280 m) have revealed ecosystems dominated by crustose coralline algae with occasional patches of rhodoliths, undiscovered biodiversity, the first reports of wide-ranging Pacific species, and reports of potential new species in multiple taxa including fishes and echinoderms. Preliminary observations suggest fish communities change with depth and time of day, some habitats support commercially important and juvenile fishes, and MCEs and fisheries at Rapa Nui are in decline due to local, artisanal fishing practices. The recent designation of no-take marine parks in Chile may help to conserve these habitats, but additional information is required to support management decisions and future conservation efforts at Rapa Nui, Salas y Gómez, and seamounts within the Chilean Exclusive Economic Zone.