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_Coral Bleaching _
By: Clark
Coral reefs are the most biodiverse ecosystem on the planet. There are more
than 25,000 known species of organisms and countless others that have yet to
be identified (Helvarg, 2000). Reefs thrive on the shallow edge of tropical
seas, most often on the eastern edge of continents along warm water currents
that brush the coasts. Reefs cannot live in cold waters and are limited by
ocean depth and available sunlight. Coral is the foundation of the reef
community, providing a three-dimensional structure where thousands of species
of vertebrates and invertebrates live and feed. Some species of coral are
hard, while others soft. Some are branched, yet others are compact and
rounded. Coral is made up of large communities of tiny jellyfish like polyps.
These polyps absorb calcium from the sea water and secrete a hard limestone
skeleton. At night the polyps extend sticky, stinging tentacles from their
skeletons to capture and consume small floating organisms such as zooplankton.
Every coral has a two-stage life cycle: the larva, and the polyp. The larval
stage is free swimming, and the polyp is stationary. Ocean currents carry the
larva from the stationary parent polyp to any hard, clean, silt-free surface
where, if the conditions are perfect, the larva grows into a coral forming
polyp, never to move again (Levin, 1999). One of the most valuable resources
for coral polyps are algae. Some live on the coral skeletons, but one type in
particular, zooxanthellae, lives inside the tissue of the polyps.
Zooxanthellae makes up about half the weight of the fleshy polyps and are not
only a valuable food resource, but they are responsible for the brilliant
colors associated with coral. When coral looses these precious colors it turns
white, a phenomenon known as coral bleaching. The purpose of this paper is to
compare the notion of coral bleaching in scientific literature and studies to
those in the popular pressCoral bleaching is normally characterized by the
expulsion of the zooxanthellae algae, loss of algal pigmentation, or both.
Coral bleaching events have had serious effects on corals and reefs worldwide.
What is crucial to the understanding of zooxanthellae expulsion and bleaching
is how the density of zooxanthellae within the coral is changing, if at all,
under the prevailing range of environmental conditions (Gates and Edmunds,
1999). Over the last twenty years, there has been a dramatic increase in both
the frequency and intensity of coral bleaching events. Sixty major bleaching
events have been reported between 1960 and 1979, whereas only nine were
reported prior to 1979 (Huppert and Stone, 1998). Given the dependence of the
coral on this symbiotic algae, it is important to determine the cause of these
bleaching events. According to Helvarg (2000, p.12): "Coral reefs… are fragile
structures living within a narrow range of temperature, clarity, salinity and
chemistry. Even a slight increase in ocean temperature, or increased CO2… can
cause stresses such as bleaching… These stresses slow the rate of growth of
the corrals… With some 60 percent of the world’s coral reefs now losing
productivity, it’s becoming a global crisis and a scientific mystery."
Scientists have been trying to solve this "scientific mystery" by evaluating
each environmental condition that may be a cause for coral reef bleaching. A
number of explanations for coral bleaching have been proposed, including
unusually high seawater temperatures, high doses of ultraviolet light,
bacterial infection, and changes in salinity. Huppert and Stone (1998)
published a report on coral reef bleaching. They tested the role of the
ocean’s natural El Niño cycle and how "hot spots," concentrated areas of the
ocean that are abnormally warmer than average conditions, may affect coral
bleaching. They attempted to demonstrate how environmental conditions, in
particular temperature, determine the regularity in which hot spots occur and,
hence, the consistency of the bleaching events themselves. Huppert et al.
(1998, p. 1) suggests that: "Perhaps the most striking finding is the possible
existence of a ‘worldwide coral reef bleaching cycle,’ peaking every 3-4 yr…
[and] that this cycle is intimately tied with strong El Niño events, which
heat large areas of the… ocean." While Huppert and Stone maintain that the
warming of ocean waters could just be the result of nature’s El Niño cycle,
Winiarski (1998, p.4A) reported that global warming is to blame. In global
warming, the burning of fossil fuels discharges extreme levels of carbon
dioxide, trapping heat around the planet like a thick blanket. Winiarski
(1998) reports that reefs will survive only through a dramatic decline in the
consumption of fossil fuels. Warrick (1999, p.6) seems to stress that the
recent influx in bleaching and subsequent mortality is a combination of El
Niño and global warming. Warrick (1999) reported that in 1998 coral reefs
around the world appear to have sustained the most expansive and brutal
bleaching events in the modern record. He adds that the conditions in 1998
were aggravated by El Niño and that recent trends suggest that the threat to
coral reefs will only increase with time. The article continued in stating
that it is plausible that global warming could have affected such vast
bleaching concurrently throughout the widespread reef regions of the world.
"Thus the geographic extent, increasing frequency and regional severity of
mass bleaching events are likely a consequence of a steady rising baseline of
marine temperatures" (p.7) It is evident that there is not yet enough
information on the topic of coral reef bleaching to arrive at any concrete
conclusions about why it happens. Since it is a relatively new phenomenon it
is entirely fair to call it a scientific mystery. It is known that temperature
increase over a sustained period of time does play an active roll, but there
is some discrepancy in the cause of the elevated temperatures. Furthermore,
there are plenty of other environmental conditions such as high doses of
ultraviolet light and bacterial infection that may be limiting the ability of
the symbiotic algae zooxanthellae. Nevertheless, there is but little
scientific data to ascertain any definitive cause. Both scientific literature
and the popular press report that there is a problem and that temperature
increase is one of the conditions that causes coral bleaching. According to
Huppert et al. (1998) the cause is El Niño, but Helvarg (2000) suggests that
CO2 may be a legitimate reason, which it may be. The opposing viewpoints
between Huppert et al., reputable scientific authority, and Helvarg, a member
of the popular press, are not so much in opposition due to their cultural
status, but rather they are both reporting on an issue that has yet to have a
determinant cause. The same holds true for the views of Winiarski (1998) and
Warrick (1999). On the topic of coral reef bleaching, and since it is a new
notion, the popular press is probably more capable of expressing this
knowledge to the general public. Huppert et. al (1998) and Gates et al. (1999)
merely touch on the principles of coral bleaching before exploring the in
depth tests and models that were conducted in search of a very specific
fragment of the general subject. While on the contrary, authors such as
Helvarg (2000), Winiarski (19998) and Warrick (1999) have the benefit of being
able to describe what coral bleaching is and it’s probable causes in a much
more concise and to-the-point fashion.
_Bibliography _
References Gates, R.D., and Edmunds, P.J. 1999. The Physiological Mechanisms
of Acclimatization in Tropical Reef Corals. American Zoologist; Vol. 39, No. 1
(Feb. 1999), pp. 30-43. Huppert, A., and Stone, L. 1998. Chaos in the
Pacific’s Coral Reef Bleaching Cycle. The American Naturalist; Vol. 152, No. 3
(Sept. 1998), pp. 447-459. Levin, T. 1999. To Save a Reef. National Wildlife;
Vol. 37, No. 2 (Feb./Mar. 1999), pp. 20-29. Warrick, J. 1999. Warm Weather
Destroyed Corals in ’98, Report Says. Houston Chronicle; March 8, 1999, p. 6.
Winiarski, K. 1998. Coral in Peril as Reefs Suffer Worldwide. USA Today;
October 19, 1998, p. 04A.
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