Ocean Chemistry Has Been Rapidly Changing

    Effects of the Acidification of the Oceans

    In today's ocean and under the currently observed extremely rapid rate of acidification, not only shallow-water
    coral reefs are at risk. Since the aragonite saturation horizon is moving toward the surface at a rate of 1–2 m per
    year, it is believed that deepwater coral reefs, probably the most biodiverse system at greater ocean depths, may be
    sooner at risk than their brethren in the photic zone. Projections suggest that by the end of the century, 70% of
    deepwater reefs could be situated in a zone of aragonite undersaturation, while today more than 95% are situated in
    the supersaturated zone. This trend is disconcerting and might severely compromise the functioning of these
    ecosystems.

    Clearly, acidification is as big a challenge for the conservation of coral reefs as any other factor. It appears that the
    only management action possible is a more responsible use of fossil fuels since we cannot otherwise regulate how
    much CO2 enters the oceans. While the prospects of ocean acidification are certainly a potential threat to the
    future viability of corals, much remains to be learned about its exact mechanisms with regards to coral health and
    long-term effects on reef building. Nonetheless, some scientists fear acidification could cause a mass extinction of
    coral reefs.

    Bleaching: The Result of Changes in Ocean Chemistry
    A startling result of changes in ocean chemistry is the idea that it may have triggered the evolution of the
    corallimorpharia—essentially naked corals without a skeleton. Some studies have suggested  that every dramatic
    change in climate and/or ocean chemistry has had some evolutionary consequence—and herein lies the true lesson.
    Since humans are inadvertently  manipulating ocean chemistry, we may expect major biotic upheaval. So what, if
    any, are the signs indicating that we might be steering toward a systemic modern reef crisis?


















    Bleaching and Coral Mortality
    A major and apparently very recent threat to coral reefs, with the potential of negating success to all conservation
    efforts, is bleaching and associated coral mortality. Dinoflagellate symbionts of the genus Symbiodinium, referred
    to as “zooxanthellae,” live within coral tissues. They exist in what is an obligatory association for the host coral,
    but not for the algae, which contribute photosynthates and aid calcification.

    Stress caused by high temperature or irradiance damages the symbionts' photosynthetic system, leading to
    overproduction of oxygen radicals that damage the symbionts and their hosts. As a result, the symbionts can be
    expelled or die, turning the coral white since the yellow-brown pigmentation of the symbionts is lost—this
    phenomenon is referred to as bleaching.

    A variety of nonphotosynthetic pigments inside the corals may not be diminished during bleaching and corals can
    appear in a variety of attractive, mortality-masking pastel colors. Bleaching events, when they occur, are usually
    not confined to corals alone, but can also affect numerous other organisms, such as gorgonians, soft corals,
    anemones, foraminifera.

    Since at least one of the primary culprits of coral-reef bleaching appears to be elevated temperature, it comes as
    little surprise that in a rapidly warming world the number of coral-reef bleaching events has risen dramatically
    since the early 1980s. The frequency and scale of coral bleaching over the past few decades have been
    unprecedented, with hundreds of reef areas bleaching at some point, and occasionally even entire ocean basins
    affected.

    Bleaching is often variable and patchy over micro (mm to cm) to meso (km) scales. This can be explained by
    fluctuations in environmental conditions, spatial heterogeneity of reef surfaces, genetic differences in hosts or
    symbionts, and differences in environmental history. Bleaching has been reported from almost every coral-reef
    region and wherever corals occur (even if not reef building, such as in the Mediterranean).


    WHEN THINGS HEAT UP



















    Corals and other reef organisms with zooxanthellae live very close to their upper thermal tolerance limits, which
    makes them susceptible to heat (∼1.0 to 1.5°C above seasonal maximum mean temperatures). At high
    temperatures and light, the lipid composition of thylakoid membranes in the symbiont changes and degrades. Also
    increased nitric acid synthase accompanies bleaching.

    In general, bleaching results from accumulated oxidative stress on the thylakoid membranes of symbiont
    chloroplasts as a result of damage to Photosystem II, which causes degradation and expulsion of the symbionts
    from host tissue. Protective mechanisms involve enzymatic antioxidants that degrade reactive oxygen species, and
    also the xanthophyll cycle can dissipate excess absorbed energy. While other stressors, like low temperatures, can
    also cause bleaching, light/heat interactions cause the majority of events on tropical reefs.

    Coral bleaching is patchy both on the scale of reefs and individual corals. This is a result of interaction between
    environmental stressors and the patchy distribution and/or zonation of different Symbiodinium within and among
    coral species. Within the coral, different types of zooxanthellae are found.

    Since these can respond differently to environmental stressors, the distribution of symbiont diversity within and
    among coral colonies and species can influence patterns of bleaching, and the proportion of the symbiont clades
    may change following a bleaching event. Symbiodinium in clade D (particularly D1a) are resistant to elevated
    temperature conditions and can remain much longer in coral-host tissues than other clades.

    Thus, the heat resistance of corals may indeed be linked to the type of zooxanthellae they harbor. suggested in their
    “adaptive bleaching hypothesis” that changes in algal symbiont communities following bleaching might be a
    mechanism allowing coral adaptation to environmental change—a point still very much in debate.  

    Bleaching events are predicted to recur more rapidly due to global warming. Bleaching is episodic, with the most
    severe events typically accompanying coupled ocean–atmosphere phenomena, such as the ENSO, which result in
    sustained regional elevations of ocean temperature. Bleaching episodes have resulted in catastrophic loss of coral
    cover in some locations and have changed coral community structure in many others, with a potentially critical
    influence on the maintenance of biodiversity in the marine tropics.

    Bleaching has also been found to increase coral diseases, the breakdown of reef framework by bioeroders, and the
    loss of critical habitat for associated reef fish and other biota. Secondary ecological effects, such as the
    concentration of predators on remnant surviving coral populations, have also accelerated the pace of decline in
    some areas.

    Many reefs with high coral cover also continued to decline after a bleaching event (Cook Islands, U.S. Virgin
    Islands). Other reefs with low cover regenerated rapidly (Arabian Gulf recovered from 0% to 42% in 9 years;
    American Samoa recovered from 6% to 40% in 4 years).

    Scientists do find that even with repeated and severe bleaching mortality, there may be at least limited recovery,
    given enough asexual regeneration or connected populations. However, changes in community structure must be
    expected at high bleaching recurrence. In particular, Acropora dominance may be compromised—model
    predictions and empirical observations seem to conform.
    The species documented by with most potential for successful regeneration were mostly broadcast spawners. This
    may be due to a different life-history strategy, with larvae spending more time in the water column than those of
    brooders and dispersing further from the parent, thus reducing the extinction debt. While recruitment is important,
    the maintenance of reef framework is key for the conservation of biodiversity associated with corals.

    Clearly, coral bleaching, largely caused by global warming, is a major challenge for the conservation of coral reefs.
    It is unclear whether bleaching can be managed, but emphasis is put on attempting to minimize additional stressors,
    since bleaching is known to facilitate the outbreak of diseases and to weaken corals.

    References

    Gardner, T.A. 2003. Long-term region-wide declines in Caribbean corals. Science, pp. 958–960.

    Kerswell, A. P.  2003. Effects of hypo-osmosis on the coral Stylophora pistillata: nature and cause of low-salinity
    bleaching, Marine Ecology, pp. 145–154.

    van Oppen, M.J.H. 2009. Coral bleaching, pp. 139–158.
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