Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

While the attenuation of light and wave stress with increasing

depth are factors influencing community structure (reviewed

in Kahng et al. 2010), thermal regime at the seafloor also

changes significantly with increasing depth. Given Hawai‘i’s

exposure to internal tides,MCEs with southwesterly exposures

are subject to semidiurnal oscillations of the thermocline and

propagation of internal waves along the insular island shelves

(Merrifield et al. 2001, Merrifield and Holloway 2002). High-

resolution monitoring of thermal regime from 90–150 m in

the ‘Au‘au Channel, between the islands of Lāna‘i and Maui,

reveals that the lower mesophotic is decoupled from the

predictable seasonality of sea surface temperature (Figure 3)

and commonly experiences fluctuations of 5–7 °C within a

diel cycle (Figure 4; Kahng et al. 2012a).

Due in part to the optically clear waters in Hawai‘i, obligate

zooxanthellate corals (e.g.

Leptoseris hawaiiensis

at 153 m)

and benthic macroalgae (e.g.

Cladophora

sp. at 212 m) have

been reported from exceptional depths (Kahng and Maragos

2006, Kahng et al. 2012b, Spalding 2012).Themaximumdepth

for zooxanthellate corals appears to occur at progressively

shallower depths at higher latitudes along the archipelago

(Table 1). Peak abundance of corals also occurs at shallower

depths in the Northwestern Hawaiian Islands compared with

the Main Hawaiian Islands at the lower latitudes (Rooney et

al. 2010). For fishes in the Northwestern Hawaiian Islands,

levels of endemism on MCEs appears to increase with latitude

(Kane et al. 2014). Given the gradient in thermal regime

with latitude along the archipelago (Kahng 2006), the depth

limit for warm-water benthic organisms (both phototrophic

and heterotrophic) are likely limited by lower temperatures

at the northernmost islands (Kahng et al. 2012a). The lower

mesophotic habitat at the northern end of the archipelago is

almost certainly temperate and not subtropical.

Figure 3.

Temperature-depth contour fromOctober 2006 to August 2008 in the

‘

au Channel, between the islands of Lāna

‘

i and Maui. The

top bar represents sea surface temperature over the same time frame. Temperature recorded every 30 min at 90–150 m (reproduced from

Kahng et al. 2012a; see Figure 1 for location).

Figure 4.

High resolution temperature-depth contour from 5–8 November 2006 in the

‘

au Channel, between the islands of Lāna

‘

i and

Maui. Temperature recorded every 90 seconds at 90–110 m (see Figure 1 for location).