Mesophotic Coral Ecosystems

39 / 100

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 39
MCEs occur along the entire Hawaiian Archipelago, which
consists of high volcanic islands and associated reefs,
submerged banks, atolls and seamounts spanning 2,500 km
from the Big Island (18.5°N) to Kure Atoll (28°N; Kahng and
Maragos 2006, Fletcher et al. 2008, Rooney et al. 2008; Figure
1). This archipelago provides a unique natural laboratory for
studying ecological and geological processes associated with
coral reefs. Located in the oligotrophic Central North Pacific
Ocean, the Hawaiian Archipelago has the most isolated
coral reef ecosystem in the world (Grigg 1988). As a result,
Hawai‘i exhibits relatively low diversity of coral species, but
exceptionally high levels of marine endemism, both in terms
of the percentage of species and their numerical abundance
(Eldredge and Evenhuis 2003, DeMartini and Friedlander
2004, Kerswell 2006, Grigg et al. 2008).
The Hawaiian Islands share a common geological history,
having been formed over the Hawaiian hotspot as the Pacific
plate moved northwest towards the Aleutian Trench (Grigg
1982, 1988, Fletcher et al. 2008, Rooney et al. 2008). Wide
insular island shelves at mesophotic depths surround most of
the islands and atolls, providing substantially more potential
habitat for MCEs than shallow-water coral reefs (Parrish
and Boland 2004, Locker et al. 2010, Rooney et al. 2010). At
mesophotic depths, large monospecific aggregations of corals
and macroalgae are commonly observed over spatial scales
of 10s to 100s of meters (Kahng and Kelley 2007, Kahng et al.
2010, Spalding 2012).
Common massive and branching shallow-water corals
(i.e., Porites, Pocillopora and Montipora) dominate the coral
community structure to 50–60 m (Grigg 1983, Kahng and
Kelley 2007, Rooney et al. 2010, Franklin et al. 2013). While
these shallow-water corals persist tomuch deeper depths, their
dominance on hard substrate appears to be replaced by other
megabenthic organisms, including green and red macroalgae
(e.g. Halimeda, Ulva , Udotea, Cladophora, Codium,
Avrainvillea, Peyssonnelia and non-articulated corallines),
plate corals (Leptoseris and Pavona), finely branched or plate-
like morphs of the coral Montipora capitata, in areas of high
current flow (Rooney et al. 2010), antipatharians (Antipathes,
Cirrhipathes, Myriopathes, Aphanipathes and Stichopathes)
and the invasive octocoral Carijoa spp. (Adey et al. 1982,
Kahng and Grigg 2005, Webster et al. 2006, Spalding 2012,
Luck et al. 2013, Wagner 2015).
Below 80 m, live benthic cover attenuates significantly, and
obligate phototrophs of Leptoseris spp. dominate the coral
community (Kahng 2006, Kahng and Kelley 2007, Rooney
et al. 2010; Figure 2). Similar vertical zonation has also been
recordedwithin themesophoticmacroalgae and antipatharian
communities (Agegian and Abbott 1985, Wagner 2015).
The 80–90 m depth range is also associated with a peak in
diversity and a change in composition within the macroalgae
community in the Main Hawaiian Islands (Spalding 2012).
Some common mesophotic species appear to be depth
specialists (i.e., observed only below a given depth). In
particular, Leptoseris hawaiiensis and two antipatharian
species (Aphanipathes verticillata and Stichopathes echinulata)
have been observed only below 80 m (Luck et al. 2013,
Pochon et al. 2015, Wagner 2015). Several macroalgae species
3.8. Hawaiian Archipelago, USA
Samuel E. Kahng, Hawai ‘i Pacific University, USA
Figure 1. Map of the islands, atolls and submerged banks of the Hawaiian Archipelago.
0 250 500 750Kilometres
Main Hawaiian I
slan
ds
N o r t
h w e s
t e r n H a w a i i a n
I s l a n d s
Maui
Hawai‘ i
O‘ahu Kauai Nihoa
Necker
Island
Brook Banks
Gardner
Pinnacles
Maro
Reef
Pioneer
Bank
French Frigate
Shoals
Pearl and
Hermes
Atoll
Kure
Atoll
Midway
Atoll
Lisianski
Island
‘Au ‘au channel
Lāna‘i
Maui
Moloka‘ i
Kaho‘olawe
Ni‘ihau
Johnston Atoll