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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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6.3.4.
Ocean acidification
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.
6.3.5.
Tropical storms
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.