Mesophotic Coral Ecosystems

6.3.4. Ocean acidification

6.3.5. Tropical storms

Rising levels of carbon dioxide in the atmosphere — caused in large part by the burning of fossil fuels — has led to an increase in the levels of carbon dioxide in the ocean. Upon absorption of carbon dioxide, seawater becomesmore acidic and its carbonate ions less abundant. As corals and other calcifying organisms require calcium carbonate to build skeletons and shells, increasing ocean acidification may inhibit growth (Langdon and Atkinson 2005, Albright et al. 2010, Fabricius et al. 2011). Ocean acidification can also impact organisms in other ways, such as the ability of fish to detect predators (Munday et al. 2014) and a decrease in coral settlement rates (Doropoulous et al. 2012). Perhaps the most consistent and pronounced effects of ocean acidification observed on coral reef ecosystems are enhanced rates of bioerosion (whereby hard substrata is eroded by living organisms; Andersson and Gledhill 2013). Little information exists regarding the effects of ocean acidification on MCEs. One study, which examined the precious coral, Coralliumrubrum , canbe used as an example of what could happen to mesophotic corals as it occurs at mesophotic depths in theMediterranean Sea. In controlled studies simulating ocean acidification conditions anticipated by the end of the century, C. rubrum exhibited reduced calcification and polyp activity (Cerrano et al. 2013). It has been suggested that calcification in the Mediterranean Sea may have already declined (by 50 per cent) as a consequence of anthropogenically-induced ocean acidification (Maier et al. 2012). In some regions, precious corals are a component of MCEs; therefore, it is plausible that these populations will be directly impacted by ocean acidification over the course of the century.

Hydrodynamic disturbances associated with storms (hurricanes in the Atlantic and Eastern Pacific, typhoons in the North Pacific and Indian Ocean and cyclones in the South Pacific) affect many coral reef regions, and play a significant role in structuring shallow reefs (Gardner et al. 2003, De’ath et al. 2012). Water velocities from storm waves (maximum orbital velocities) decline exponentially with depth, and MCEs are therefore afforded some protection from hydrodynamic disturbances (e.g. Woodley et al. 1981). However, organisms living in the upper mesophotic zone (30–50 m) may experience direct impacts from storms (White et al. 2013). Indirect effects of storms, such as debris avalanches, can affect MCEs (Harmelin-Vivien and Laboute 1986), while very severe storms can damage reefs to depths of at least 70 m (Bongaerts et al. 2013a). The typical plating and foliose morphologies of many mesophotic coral species leave them prone to degradation following physical disturbance. For example, significant impacts to large foliose coral communities combined with a large increase in rubble were detected on MCEs off Okinawa following a typhoon in 2012 (White et al. 2013). Submerged banks not exposed to breaking waves are likely to be less vulnerable than lower reef slopes (Roberts et al. 2015). In any case, predicted changes in the location, frequency and particularly the intensity of storms expected as ocean temperatures rise (IPCC 2013) will likely affect MCEs.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 74