Mesophotic Coral Ecosystems

Figure 3. Temperature-depth contour fromOctober 2006 to August 2008 in the ‘ Au ‘ 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 ‘ au Channel, between the islands of Lāna ‘ i and Maui. Temperature recorded every 90 seconds at 90–110 m (see Figure 1 for location).

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.

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)

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 41