Mesophotic Coral Ecosystems

History of mesophotic reef investigation Patrick L. Colin , Coral Reef Research Foundation, Palau

Scientific knowledge of mesophotic reefs and their resident species largely began in the Age of Exploration in the eighteenth and nineteenth centuries, when dredging and trawling revealed new mesophotic reef species. Pioneering ichthyologists, such as Felipe Poey in Cuba and Pieter Bleeker in Indonesia, produced surprisingly thorough surveys, unsurpassed until recent times. In the early and mid-twentieth century, knowledge of the geology and origin of coral reefs — and by inference, MCEs —grew rapidly. Geologic investigations into submerged reefs focused on the back-stepping of reefs, some of which developed into MCEs, under rising global sea levels at the end of the last ice age (Macintyre 1967, Harris and Davies 1989; Fig 2.12). After World War II, open-circuit scuba diving was adopted by scientists, and by the 1960s and 1970s collections were being made using compressed air at mesophotic depths down to approximately 70–75 m. In the Western Atlantic, early investigatorswere exploring Jamaican reefs (GoreauandGoreau 1973, Goreau and Land 1974, Lang et al. 1975) and documenting the carbonate framework producing sclerosponges (Hartmann 1969, Hartmann and Goreau 1970) and a diverse variety of deeper water Caribbean corals (Wells 1973). Work in the Indo- West Pacific also brought new deep-water species to the attention of scientists. Much of the work on the ecology of MCEs in Hawai‘i was undertaken to understand antipatharians (Grigg 1965) and other precious corals (Grigg 1984). In the Indo-Pacific and Caribbean, scientists also discovered that species diversity at depths below 40 m were similar between the two regions (Kuhlmann 1983). Some early coral reef field guides for the Western Atlantic region also included mesophotic fauna (Randall 1968, Bohlke and Chaplin 1968, Colin 1978) and today many mesophotic reef organisms, both fish and invertebrates, are in field guides with excellent in situ photographs (e.g. Veron 2000, Fabricius and Alderslade 2001, Allen and Erdmann 2014). Much of the interest in MCEs was inspired by the underwater photographers who first penetrated these depths, including Douglas Faulkner (Faulkner and Chesher 1979). Photographic documentation techniques have since become a mainstay of MCE research. The potential for nitrogen narcosis (and the risks of decompression “sickness”) and the need for decompression were recognized quickly in the early days of open-circuit scuba diving, but it was not until the advent of mixed-gas diving that depth and time limits could be extended, making MCEs more readily accessible. The ability to monitor and control the oxygen content of a breathing gas mixture resulted in the development of mixed-gas rebreathers — first for the military and later for civilian applications. Walter A. Starck II and John Kanwisher developed the first practical closed-circuit mixed- gas rebreather, the Electrolung, in the later 1960s (Starck 1969, Starck and Starck 1972). At the upper depths of the mesophotic

zone (30–40 m), the introduction of Nitrox (enriched oxygen air) diving in 1977 allowed increased bottom times compared with compressed air diving. In the last decade, use of mixed- gas rebreathers with galvanic oxygen sensors and computer technology for gas control and decompression computation has become increasingly common for scientific research (Pyle 1996b), and has made diving to the lower depths of the mesophotic zone (90–100 m) practical. Small research submersibles (Figure 2.13) have been used on many occasions to document mesophotic environments. The first notable reef projects were carried out in Hawai‘i in the late 1960s (Strasburg et al. 1968), and later in Belize (James and Ginsburg 1979) and Jamaica using the Nekton submersible in the 1970s. In the Pacific, a fishery resource study in 1967 provided the first report of dense mesophotic scleractinian coral communities in Japan (Yamazato 1972). In the Red Sea, submersibles allowed for the first studies on the ecophysiology of mesophotic corals and their distribution (Fricke and Schumacher 1983, Fricke and Knauer 1986). Other technological advances have improved our knowledge of MCEs. Multibeam sonar allowed the first detailed mapping of mesophotic areas, providing accurate depictions of slope and geomorphology. Small remotely operated vehicles or ROVs intended for relatively shallow water use (down to approximately 300 m depth) have also become widely available. Autonomous underwater vehicles (AUVs) provide new environmental information, often including otherwise hard-to-obtain time-series data.

Figure2.13. Small submersiblesmake it possible for researchers to study mesophotic coral ecosystems in situ for longer time periods than technical diving (maximumof 20minutes) permits. The author (Patrick Colin) pictured with Adrien “Dutch” Schrier off western Curacao (photo Barry Brown).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 18