Mesophotic Coral Ecosystems

Table of Contents

1. Environments, Biodiversity, and Ecology of Mesophotic Coral Ecosystems

   1. Frontmatter

   2. 28. The Mesophotic Coral Microbial Biosphere

      William Leggat, Sarah Gierz, Alejandra Hernandez-Agreda, Tracy Danielle Ainsworth

      Abstract

      Dynamic symbioses between the host, photosynthetic eukaryotes, and diverse prokaryotes are the foundation of corals’ ecological successes across highly diverse ocean habitats. This may be especially so in mesophotic corals which face unique environmental challenges when compared to shallow-water corals, for example, light levels that drive photosynthesis are significantly reduced. The success of reef-building corals in the mesophotic zone provides insights not only into the capacity of these populations to act as refugia but importantly the biological mechanisms that underpin coral success across divergent environmental regimes. The necessity in answering the fundamental questions—How do corals succeed in diverse habitats, and what is the contribution of all members of the metaorganism to coral function?—is underscored by uncertainty regarding the ability of reef corals to acclimate to rapid environmental changes. “Whole-system” or “whole-organism” studies provide a new direction for biological research in understanding the complexity of coral reef ecosystems and provide the capacity with which to unravel the biological basis of coral reef health and reef sustainability into the future. In this context, the mesophotic zone is of particular importance. Here, we review advances in understanding of the coral holobiont in the mesophotic zone, focusing on both eukaryotic and prokaryotic symbionts, and compare and contrast this to our understanding of shallow-water corals. In addition, we outline the challenges that remain in applying new technologies to advancing our understanding of these highly complex and valuable marine ecosystems.

   3. 29. Macroalgae

      Heather L. Spalding, Gilberto M. Amado-Filho, Ricardo G. Bahia, David L. Ballantine, Suzanne Fredericq, James J. Leichter, Wendy A. Nelson, Marc Slattery, Roy T. Tsuda

      Abstract

      Macroalgae in mesophotic coral ecosystems are generally understudied compared to corals and fishes yet may be more abundant than coral-dominated reefs given their lower depth limits (> 200 m) and ability to grow over soft and hard bottom habitats. These assemblages are abundant and diverse globally, with changing species composition with increasing depth. Ubiquitous macroalgal assemblages include the red algal rhodolith beds and nongeniculate and Peyssonneliales assemblages; green algal Halimeda beds, meadows, and bioherms and Caulerpa spp. beds; and brown algal Lobophora spp. or Distromium spp. beds, Sargassum spp., and kelps. The use of molecular techniques is elucidating macroalgal diversity and rates of endemism, and molecular data and phylogenetic analyses often show strong cryptic diversity or pseudodiversity when compared with morphoanatomical analyses. Mesophotic macroalgae are important as habitat and may serve as seedbanks or refugia for ecosystem resilience following environmental stress. Invasive algal blooms may be deleterious, particularly with the removal of native herbivores or increasing nutrients. Geomorphologically, calcified species such as rhodoliths and Halimeda spp. are significant global producers of calcium carbonate. Abiotic factors influencing the abundance and distribution of mesophotic macroalgae include temperature, water clarity, nutrients, and currents. In general, threats include rhodolith mining, oil spills, sedimentation, ocean acidification, invasive species, bottom trawling, and eutrophication. The impacts of global warming at mesophotic depths are unknown. Future studies should focus on collections for molecular analyses to evaluate population-level dynamics and connectivity between shallow and mesophotic depths and in situ manipulations to determine competitive interactions and ecophysiological processes in these low-light environments.

   4. 30. Symbiodiniaceae Genetic Diversity and Symbioses with Hosts from Shallow to Mesophotic Coral Ecosystems

      Tamar L. Goulet, Matthew Q. Lucas, Nikolaos V. Schizas

      Abstract

      Multiple members of the phylum Cnidaria (e.g., corals, octocorals, and sea anemones) and other organisms such as mollusks, foraminiferans, and sponges associate with unicellular dinoflagellates belonging to the family Symbiodiniaceae. These symbioses are often obligatory, and for shallow coral reefs, form the foundation of the ecosystem. This chapter presents the current knowledge of Symbiodiniaceae genetic distinction and the ramifications of different Symbiodiniaceae genotypes on the host-Symbiodiniaceae entity, the holobiont. Since the early 1990s when molecular techniques using polymerase chain reaction (PCR) enabled rapid resolution of Symbiodiniaceae, both within and between host species, knowledge on Symbiodiniaceae diversity has grown exponentially. Subsequently, application of multiple molecular techniques and genetic markers enabled the analysis of Symbiodiniaceae diversity from broad genera (formerly cladal) groupings to the individual genotype. Previous lack of a standard Symbiodiniaceae nomenclature, however, led to naming redundancies and utilization of the same terminology to discuss different levels of taxonomic resolution. This ambiguity has now been addressed. Knowledge of Symbiodiniaceae genetic diversity enables understanding and puts in context the host-Symbiodiniaceae genotypic combination. Deciphering the holobiont’s ecology, including the holobiont’s responses to environmental conditions brought about by global climate change, requires knowledge of Symbiodiniaceae identity, as does contemplating the applicability of the deep reef refugia hypothesis. This review accentuates the current meager knowledge of Symbiodiniaceae genetic diversity in mesophotic coral ecosystems in general, which is confined to scleractinian and antipatharian coral hosts, and the lack of data on Symbiodiniaceae genotypes in other symbioses.

   5. 31. Large Benthic Foraminifera in Low-Light Environments

      Willem Renema

      Abstract

      Large benthic foraminifera (LBF) are an important component of low-light, mesophotic tropical marine environments, including coral ecosystems. LBF occur from nearshore, shallow coastal environments experiencing high-terrestrial runoff to the deep-shelf edge in transparent, oceanic waters. Here, I compare the LBF in both these low-light habitats. In both reef-associated and interreef environments, species show differing tolerance to both light intensity and terrestrial influx. In interreef environments, LBF can alter the benthic environment from muddy to coarse carbonate grains. Their depth distribution is truncated by seasonal variability in water transparency, particularly for the deepest-living species. This is because shallower-dwelling species are more likely to experience suitable environmental conditions throughout the year and can position themselves in microhabitats experiencing higher light irradiance during periods of low light intensity, thus managing to maintain their symbionts. In contrast, deep-living species are less flexible because light intensity in deep water is ubiquitously low; consequently, deep-dwelling LBF live predominantly on or very close to the water-seafloor interface. In coastal environments, zonation or habitat fractionating, i.e., the differentiation of assemblages on the reef slope, increases from nearshore to offshore reefs, primarily due to species-specific differences in tolerance to nutrients.

   6. 32. Sponges

      Shirley A. Pomponi, M. Cristina Diaz, Rob W. M. Van Soest, Lori J. Bell, Linnet Busutil, Deborah J. Gochfeld, Michelle Kelly, Marc Slattery

      Abstract

      Sponges are dominant, but poorly understood, components of mesophotic coral ecosystems (MCEs). Herein, we review the current understanding of mesophotic reef sponges focusing on their biodiversity, ecology, and threats, and comparing this to shallow reef sponges. The few studies of MCEs report a large number of new species, for which their contribution to ecosystem services and our understanding of sponge biodiversity and evolution are unknown. Major threats to MCE sponges are similar to threats to deep-water sponge communities and to mesophotic corals: fishing activities, pollution, and climate change, as well as in the Caribbean, invasive lionfish predation on herbivorous fishes that can result in overgrowth of algae that smother corals and sponges. The current geographic, habitat, and sampling biases prevent a full understanding of mesophotic sponge biodiversity and their ecological roles. Future studies must include not only massive sponges, but also the rare and harder to collect encrusting sponges. It is premature to draw global patterns of diversity and distribution for mesophotic sponges, since MCEs have not been studied worldwide, and geomorphological features vary within regions, causing species distributions to be highly variable.

   7. 33. Biodiversity of Reef-Building, Scleractinian Corals

      Paul R. Muir, Michel Pichon

      Abstract

      Zooxanthellate scleractinian corals are moderately well-known for shallow reef habitats, but not for mesophotic depths (>30 m) that are relatively difficult to access. Mesophotic habitats are light-limited, with different hydrodynamics and sedimentation processes, which result in growth forms that are often difficult to classify using traditional schemes based largely on shallow reef specimens. We analyzed published data and museum records, using specimen-based records to minimize classification issues, finding 53 mesophotic species in the western Atlantic Ocean (85% of total species) and 338 in the Indo-Pacific (45%). Only four species were recorded exclusively below 30 m depth, while the great majority were common shallow reef taxa. Over 96% of western Atlantic and 82% of Indo-Pacific genera and most coral lineages were represented below 30 m depth. In the Indo-Pacific, species and genus richness varied widely between regions and were significantly correlated with shallow reef species richness. Overall, species richness decreased steadily with increasing depth, with little evidence for distinct faunal boundaries: 157 species occurred ≥60 m and 31 deeper than 100 m, with species occurrence only moderately related to phylogeny. Our knowledge of mesophotic biodiversity is rapidly changing as more regions are documented and new molecular techniques suggest taxonomic revisions and resolve deepwater cryptic species. We conclude that mesophotic scleractinian fauna are largely a subset of shallow scleractinian fauna, comprising a significant proportion of coral species and most genera, with the potential to play a significant role in lineage preservation and the future of coral reefs.

   8. 34. Reef-Building Corals of the Upper Mesophotic Zone of the Central Indo-West Pacific

      Emre Turak, Lyndon DeVantier

      Abstract

      Species composition, habitat preferences, and community structure of zooxanthellate scleractinian corals in the upper mesophotic zone (UMZ, 30–60 m depth) of the central Indo-West Pacific (IWP) are described. A total of 340 species were recorded from 287 sites from the Andaman Sea across the Coral Triangle to Micronesia and Solomon Islands. This represents almost half of the reef-building coral species present in the central IWP, yet is far from complete. Species were categorized as either UMZ specialists (9 spp.), depth generalists (57 spp.), or with habitat preferences for the lower reef slope (63 spp.), mid-lower slope (172 spp.), or shallow slope (39 spp.). The five most common species in the UMZ were depth generalists (massive Porites spp.) or species that more commonly occur on mid-lower or lower reef slopes above the UMZ, such as Pachyseris speciosa, Acropora granulosa, Oxypora lacera, and Seriatopora hystrix. Only one of the ten most common corals, Acropora elegans, is an UMZ specialist. Percentages of species occurring in the UMZ ranged widely among genera, being highest in Euphyllia (100%), Leptoseris (92%), and Cycloseris (90%) and lower for speciose IWP genera Acropora (47%), Montipora (53%), and Porites (46%). Five broad, mid-lower slope coral communities extended into the UMZ, where seven coral assemblages were defined, each with greater or lesser biogeographic fidelity. We assess our findings in relation to the life histories of the species present and place these in the broader context of the “deep reef refugia” hypothesis.

   9. 35. Sexual Reproduction of Scleractinian Corals in Mesophotic Coral Ecosystems vs. Shallow Reefs

      Tom Shlesinger, Yossi Loya

      Abstract

      Corals utilize sex-derived diversity to adapt to environmental changes and to occupy new ecological niches. With major declines in coral reefs worldwide and calls for ecosystem-based management, understanding how environmental gradients affect coral reproductive performance over a species’ range and within a demographically relevant timescale is critical. The study of coral reproduction is a mature field with the reproductive aspects of more than 450 species recorded. However, the vast majority of coral reproduction studies have been on shallow reefs, while knowledge of reproduction in mesophotic coral ecosystems (MCEs) is sparse. This knowledge gap hinders our ability to assess the resilience and functionality of MCEs and to understand ecosystem-scale connectivity. Environmental factors that influence coral reproduction, such as light, temperature, and disturbances, can vary dramatically with depth. Sexual reproduction has evolved, partly, to address environmental pressures. We, therefore, expect that environmental parameters can influence reproductive patterns and success. There is currently insufficient information to allow conclusions to be drawn regarding the effects of mesophotic depths on the phenology of coral reproduction, other than it might differ from shallow reefs. Nonetheless, it appears that reproductive performance decreases with depth, with most of the species studied so far in MCEs having exhibited either reduced fecundity or reduced oocyte size compared to shallower populations. Here, we summarize the current knowledge on mesophotic coral reproduction and propose several hypotheses regarding the changes in coral reproductive traits across depth and their implications for population connectivity and persistence. Additionally, we highlight crucial knowledge gaps and recommend future research.

   10. 36. Coral Sclerochronology: Similarities and Differences in the Coral Isotopic Signatures Between Mesophotic and Shallow-Water Reefs

       Tsuyoshi Watanabe, Takaaki K. Watanabe, Atsuko Yamazaki, Shiori Yoneta, Kohki Sowa, Frederic Sinniger, Gal Eyal, Yossi Loya, Saki Harii

       Abstract

       Coral sclerochronology is a powerful tool for understanding environmental and ecological changes on coral reefs. Geochemical, isotopic, and skeletal density banding analyses along the major growth axis of massive coral skeletons from tropical shallow-water reefs have been used successfully to reconstruct decadal- to centennial-scale histories of climate and coral growth with annual to seasonal resolution. However, little is known about how coral sclerochronological approaches could capture environmental and/or physiological changes in mesophotic coral ecosystems (MCEs), which occur at depths ranging from 30 to 150 m. We compared the oxygen and carbon isotopes and growth records of coral from upper MCEs with those from adjacent shallow reefs by examining Porites corals collected at 4 and 40 m from Okinawa, Japan, and from 5 and 50 m water depths from the Gulf of Eilat, Red Sea, Israel. Porites corals in MCEs exhibited low calcification rates, but still recorded distinct seasonal to interannual variability in oxygen and carbon isotope signals consistent with coral isotopic records on shallow reefs. The amplitudes of the seasonality in the isotopic records in corals from MCEs were larger than those expected from seasonal environmental variations and those recorded in shallow reefs, indicating that coral growth rate and/or physiological changes affected skeletal isotopic composition. Our results suggest that sclerochronological records have great potential for reconstructing environmental and ecological characteristics of MCEs. The isotopic records of MCE corals are also more influenced by physiological processes such as symbiotic photosynthesis, calcification rates, and trophic levels than their shallow-water counterparts.

   11. 37. Antipatharians of the Mesophotic Zone: Four Case Studies

       Marzia Bo, Anthony D. Montgomery, Dennis M. Opresko, Daniel Wagner, Giorgio Bavestrello

       Abstract

       About 63% of the known antipatharian genera occur at mesophotic depths (30–150 m), with the majority extending into the deep sea. Along the continental shelf and offshore sites, antipatharians tend to increase in diversity and abundance with depth, reaching a peak at mesophotic depths due to favorable environmental factors enhancing their settlement and growth and biotic factors associated with lower levels of competition for space. A review of taxonomic and ecological studies for shallow and mesophotic antipatharians is presented for four regionally based case studies, three in the tropics (1) Central Indo-Pacific, plus adjacent sections of the Western Indo-Pacific, (2) Eastern Indo-Pacific (primarily Hawaiʻi), and (3) the Caribbean Sea) and one at temperate latitudes in the Mediterranean Sea and adjacent sections of the Northeast Atlantic. The mesophotic fauna is mainly represented by the families Antipathidae, Aphanipathidae, and Myriopathidae. The most diverse community is found in the Central/Western Indo-Pacific, followed by the Caribbean Sea. The tropical antipatharians are represented by shallow species that extend their distribution into the upper mesophotic zone (30–60 m), while the temperate antipatharians consist of deepwater (> 150 m) species that extend upward into the lower part of the mesophotic zone. Black corals in mesophotic coral ecosystems can be habitat-forming components of benthic assemblages on hard substratum. They have an enormous potential for hidden biodiversity and play an important ecological role for the broader marine ecosystem. The threats to antipatharians consist of demersal fishing activities and coral harvesting, which may be highly destructive to these poorly understood systems.

   12. 38. Octocorals of the Indo-Pacific

       Yehuda Benayahu, Tom C. L. Bridge, Patrick L. Colin, Ronen Liberman, Catherine S. McFadden, Oscar Pizarro, Michael H. Schleyer, Erez Shoham, Bastian T. Reijnen, Michal Weis, Junichi Tanaka

       Abstract

       Mesophotic coral ecosystems (MCEs), which comprise the light-dependent communities of corals and other organisms found at depths between 30 and ~150 m, have become a topic that increasingly draws the attention of coral reef researchers. It is well established that after the reef-building scleractinian corals, octocorals are the second most common group of macrobenthic animals on many shallow Indo-Pacific reefs. This chapter reviews the existing knowledge (e.g., species composition and depth of occurrence) on octocorals from selected Indo-Pacific MCEs: Okinawa (Japan), Palau, South Africa, the northern Red Sea, and the Great Barrier Reef (Australia). For all reefs, zooxanthellate taxa are not found below 65 m. We, therefore, suggest that physiological constraints of their symbiotic algae limit the depth distribution of zooxanthellate octocorals. More studies of lower MCEs (60–150 m) and their transition to deepwater communities are needed to answer questions regarding the taxonomy, evolutionary origins, and phylogenetic uniqueness of these mesophotic octocorals. New findings on mesophotic octocoral sexual reproduction indicate a temporal reproductive isolation between shallow and mesophotic octocoral populations, thus challenging the possibility of connectivity between the two populations. The existing data should encourage future studies aimed at a greater understanding of the spatiotemporal features and ecological role of mesophotic octocorals in reef ecosystems.

   13. 39. Gorgonian Corals

       Juan A. Sánchez, Luisa F. Dueñas, Sonia J. Rowley, Fanny L. Gonzalez-Zapata, Diana Carolina Vergara, Sandra M. Montaño-Salazar, Iván Calixto-Botía, Carlos Edwin Gómez, Rosalinda Abeytia, Patrick L. Colin, Ralf T. S. Cordeiro, Carlos D. Pérez

       Abstract

       Mesophotic gorgonian corals comprise a polyphyletic group of octocorals mostly with a proteinaceous branching axial skeleton. Dense assemblages of gorgonian corals usually dominate the seascape in mesophotic coral ecosystems (MCEs). In this chapter, we review the mesophotic gorgonian coral biodiversity, followed by a synthesis of the ecological implications of inhabiting this environment, as well as the threats that these communities face. MCEs include ~87 gorgonian corals genera distributed worldwide, where the Indo-Pacific (65) is almost twice as diverse as the Caribbean and Gulf of Mexico (37) and Brazil (23), whereas the Tropical Eastern Pacific has only eight genera. We discuss several predictions on the nature of mesophotic gorgonian corals in areas such as microbial endosymbiosis to understand the health, ecology, and evolution of these assemblages. A notable colonization of shallow-water species into MCEs in the Caribbean suggests that colonizing deeper environments promotes ecological divergence. MCEs are not immune to the influence of natural events such as tropical storms and/or anthropogenic encroachment from coastal development, pollution, global climate change, ocean acidification, and overfishing; yet, gorgonian corals in general appear resilient to many of these threats.

   14. 40. Fishes: Biodiversity

       Richard L. Pyle, Randall K. Kosaki, Hudson T. Pinheiro, Luiz Alves Rocha, Robert K. Whitton, Joshua M. Copus

       Abstract

       Fishes are an important component of coral reef ecosystems, and in comparison to other marine phyla, the taxonomy of fishes is relatively robust. Some of the earliest explorations of mesophotic coral ecosystems (MCEs) involving both submersibles and rebreather diving focused on fishes. Since 1968, over 400 publications have documented fishes on MCEs, ~75% of which were published since 2011. Most fish species inhabiting MCEs belong to families and genera typical of shallow coral reefs, and many new species remain to be discovered and described. Species richness generally peaks at a depth of 30 m and declines with increasing depth. The composition of the fish communities on MCEs includes a mixture of species restricted to MCEs and species with broad depth ranges. Patterns of species turnover and composition vary depending on geographic location, ecological characteristics, and method of study. Nearly 70% of MCE fish research has occurred within the tropical western Atlantic and Hawaiʻi. Not enough is known about global distributions to infer broad biogeographical patterns, but there seems to be higher representation by endemic species and individuals on MCEs, and the eastward attenuation of diversity of shallow Pacific reefs does not appear to apply to fishes within MCEs. Analyses of nearly 900,000 occurrence records of reef fishes at depths of 1–200 m reveal patterns of diversity that are mostly consistent with controlled studies. Future work should emphasize basic exploration and documentation of diversity in under-sampled geographic regions and hypothesis-driven studies in areas where logistics facilitate MCE research.

   15. 41. Disease Problems

       Ernesto Weil

       Abstract

       Worldwide ecological deterioration of coral reefs is mostly caused by disease-induced mass mortalities linked to thermal anomalies, and aided by local anthropogenic stressors. Mesophotic coral ecosystems (MCEs; 30–150 m) are found deeper where temperatures are cooler, in low light, and mostly offshore. These characteristics are proposed to protect MCEs (“deep reef refugia” hypothesis) from shallow-water threats (e.g., thermal stress and pollution). The most commonly reported mesophotic health problem is thermal-induced bleaching, which is now more widespread due to global climate change (GCC). The oldest deep bleaching report (90 m) is from 1989, in the Caribbean, but recent reports indicate bleached corals below 120 m in Grand Cayman. Cold-water intrusions and turbidity can also cause mesophotic coral bleaching. Not much is known about biotic diseases and potential drivers in MCEs. “White syndromes” (WS) seem to be the most common in Puerto Rico and the US Virgin Islands. Overall, 9 of the 28 common MCE scleractinian species have been observed with disease signs similar to shallow WS and white plague disease. A new disease termed “intercostal mortality syndrome” affected 19% of the colonies of Orbicella, Siderastrea, and Agaricia spp. in Hind Bank. MCE surveys showed coral community disease prevalence in Puerto Rico varied between 0% and 15%, with a mean of 6%. This chapter presents a summary of what is known about disease threats to MCEs and the disruptive potential of GCC-induced changes in seawater thermal dynamics on species susceptibility, and how this could affect the protection these deeper environments may provide.

   16. 42. Light, Temperature, Photosynthesis, Heterotrophy, and the Lower Depth Limits of Mesophotic Coral Ecosystems

       Samuel E. Kahng, Derya Akkaynak, Tom Shlesinger, Eric J. Hochberg, Jörg Wiedenmann, Raz Tamir, Dan Tchernov

       Abstract

       The attenuation of light with increasing depth, along with reduced exposure to wave stress, plays an important role in vertically structuring coral reef communities. Benthic photosynthetic organisms exhibit different depth distributions and abundance patterns which cause changes in community composition of associated reef fauna. This vertical zonation in coral reef community structure suggests special adaptations in response to the changing environmental regime with depth including changes in light intensity, light spectrum, and angular distribution. At the lower depth limits of mesophotic coral ecosystems (MCEs), both light and temperature can become limiting factors with the latter playing an important role at higher latitudes. The available evidence indicates that different species can exhibit distinct and sometimes opposing photophysiological adaptations with increasing depth. Some zooxanthellate corals appear to maximize ambient light utilization at the expense of efficiency, while others appear to maximize efficiency. Coral holobiont adaptations to mesophotic depths include changes in colony morphology, algal symbionts, pigment physiology, skeletal properties, and metabolic strategy. Given the scarcity of physiological studies at depths >60 m, the current understanding of how obligate zooxanthellate corals and other light-dependent organisms can inhabit such a broad depth distribution is far from complete. This chapter summarizes the ecologically relevant aspects of light and temperature regimes of MCEs, as well as the depth-related photophysiological and adaptive strategies of coral holobionts.

   17. 43. Bioerosion

       David K. Weinstein, Rebecca L. Maher, Adrienne M. S. Correa

       Abstract

       The study of bioerosion, a widespread process greatly impacting reef biodiversity, structural complexity, and sediment production, has largely focused on shallow-water reefs with no review of this process in deeper environments. In this first synthesis of bioerosion literature for mesophotic reefs (subtropical and tropical ecosystems in low-light conditions at depths of ~30 to 150 m), we show that the distribution of key bioeroder taxa, their abundances, and overall bioerosion rates are considerably different on mesophotic reefs compared to their shallow-water counterparts. In particular, carbonate grazing and phototrophic microboring rates decline with depth from shallow to mesophotic reefs. In the absence of significant erosive action by grazers, sponges are hypothesized as the primary long-term bioeroders on lower mesophotic reefs (60–150 m) and possibly on some upper mesophotic reefs (30–60 m). Given these factors, we postulate that mesophotic reef substrates experience slower bioerosion rates and lose less carbonate than shallower reefs over the same timeframe. This likely stems from differences in photosynthetically active radiation and other factors such as temperature, sedimentation, bioeroder food abundance and quality, substrate characteristics, and exposure time for bioerosion. There is a critical need to document mesophotic bioeroders via taxonomic inventories, as well as quantify their bioerosion rates across mesophotic depths in terms of specific bioeroder guilds using experimental substrates. These data will aid management efforts to maintain positive net carbonate budgets on mesophotic reefs, ensuring that sufficient three-dimensional structure is available to support biodiversity at mesophotic depths.

   18. 44. Geology and Geomorphology

       Clark E. Sherman, Stanley D. Locker, Jody M. Webster, David K. Weinstein

       Abstract

       Geomorphology and geological processes exert fundamental controls on the occurrence, distribution, and makeup of mesophotic coral ecosystems (MCEs). Two broad geomorphic categories are shelves and slopes. Shelves include outer portions of continental and insular shelves that dip gently into mesophotic depths before reaching the shelf break and have very low gradients (<1°). Other low-gradient habitats include tops of isolated banks. Slope habitats extend from platform breaks down into adjacent basins and can be divided into low-gradient slopes (<30°), steep slopes (~30 to 70°), and walls (>70°). On shelves, MCEs are best developed on positive relief features elevated above the surrounding seafloor. In slope settings, MCE development is typically favored on steep irregular slopes, where coral cover is concentrated on steep-sided buttresses and sediment is channelized into narrow chutes. Relict features related to past sea levels are critically important MCE habitats on both shelves and slopes. Coral and coralline algae remain the primary frame builders in MCEs. However, accretion at mesophotic depths is likely very slow, such that they form only thin biostromal veneers over relict substrates. Sediments in MCEs are dominantly autochthonous skeletal sands and gravels. Although fluxes of sediments to the seafloor in MCEs are typically lower than in shallow reefs, sedimentary dynamics still play an important role. Low-gradient seafloor has an increased potential for accumulation of sediment detrimental to MCEs. In slope settings, downslope bed-load transport of sediment can be orders of magnitude higher than vertical fluxes and likely exerts an important influence on MCEs.