Mesophotic Coral Ecosystems

3.8. Hawaiian Archipelago, USA Samuel E. Kahng , Hawai ‘ i Pacific University, USA

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

Kure Atoll

Pearl and Hermes Atoll

Midway Atoll

Moloka ‘ i

‘ Au ‘ au channel

Maui

Pioneer Bank

N o r t h w e s t e r n Lisianski Island

Lāna ‘ i

Maro Reef

Kaho ‘ olawe

Gardner Pinnacles

French Frigate Shoals

H a w a i i a n

Brook Banks

Necker Island

Nihoa

I s l a n d s

O ‘ ahu Kauai

Ni ‘ ihau

Maui

M ain H a wa iian I s lan d s

Hawai ‘ i

Johnston Atoll

0

250

500

750Kilometres

Figure 1. Map of the islands, atolls and submerged banks of the Hawaiian Archipelago.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 39