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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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
History of mesophotic reef investigation
Patrick L. Colin
, Coral Reef Research Foundation, Palau
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).