Coral reefs are one of the biodiversity hotspots of the planet with an estimated 25 percent of all marine life forms (Tickell 1999). They are comparable to the tropical rainforests in diversity and play vital roles in the saline environment. They provide physical protection from ocean waves and tropical storms and house a large variety of fish, crustaceans, bryozoans, mollusks, worms, and many other animals (Sea World Education Department 2001). They are also important economically and nutritionally for many countries around the world. For example, coral reefs have commercial value through fishing and tourism. Reef fishes alone provide 60 percent of the animal protein for Indonesia etc (Ryan 2001).
Surprisingly, these reefs are built from one of the simplest and smallest animals, the coral. This animal is generally composed of a two to three celled polyp surrounded by a calcareous external skeleton, which is secreted by the organism (BBSR-TCEB 2001). Many coral live symbiotically with algae called zooxanthellea, which provides it with a large portion of its nutrition (Madl and Yip 1999). Over time new corals grow over the remains of dead coral and forming the amazing coral reefs of today.
In recent years threats to coral reefs have caused scientists to raise an alarm about the decline of coral species. Impacts to corals, and thus coral reefs, have occurred both directly such as from mining for construction, destructive fishing practices, and accidental kicking from scuba divers; and indirectly through nutrient loading, temperature changes, and overfishing (Tickell 1999). Loss of corals can occur in different ways. Corals can be killed directly by many of the devastating human practices used in coral reefs. Coral bleaching can also take place when the zooxanthellea are expelled from the corals, removing their coloration from the corals and practically starving the corals to death (Brown 1997). Another way that corals loss happens is through its replacement by algae in a phase-shift of a reef ecosystem triggered by an accumulation of stresses and disturbances (McManus et al. 2000). This paper identifies and discusses the types of threats to corals that have caused declines in coral reef ecosystems in recent years and introduces some potential threats that have not yet been fully researched or documented.
-ANTHROPOGENIC DISTURBANCES-
Many of the threats to corals are due to human impact on their environment. As the human population increases, it depends more on the ocean for sources of food and ocean shores for living spaces. This exploitation and encroachment disrupts, and in some cases destroys, the coral ecosystem.
Overfishing:
Overfishing coral reefs has led to decreasing populations of fishes and to shifts in species composition. Large predatory species have declined or have disappeared, while the remaining herbivorous fishes have decreased in size (Hughes 1994). The loss of herbivory then results in an increased growth of algae, one of the corals' main competitors, on the reefs. With little to hold them in check, the algae and sponges overtake the reef and crowd out the corals, thus ending the upward growth of the coral reef, as erosion rates surpass the accretion rates of reefs (Richmond 1993) (Hallock et al. 1993). Not only does the removal of fish disrupt the coral ecosystem, but the method of fishing also makes large impacts. Improper use of anchors and fishing nets with weights attached to the bottom also cause damage to corals.
Damaging Fishing Techniques:
In some parts of the world damaging fishing techniques, such as dynamiting and cyanide poisoning, are still employed. These techniques kill much more than the fish that are targeted and impact the long-term recovery of coral reef ecosystem. Dynamiting not only has the short term effects of killing any mature fish within a 50-70 meter radius, but it also kills untargeted animals such as invertebrates (plankton on which the fish feed), juvenile fish, fish larvae, and eggs resulting in longer recovery time in fish populations (Guard and Masaiganah 1997). Blasting also destroys large sections of hard coral reducing fish habitat as well. Guard and Masaignanah (1997) report that dynamiting has been used to kill marine fish in Tanzania since the 1960's and is still used today.
Since enforcement has stepped up on dynamite fishing in recent years, some fishermen are using a more silent approach to fishing, cyanide poisoning. This technique also allows for catching fish live, which reaps higher returns in the market. A squirt of cyanide by a diver can stun large fish long enough to transport it to the surface. However, that same squirt kills the corals, smaller fish, and invertebrates in the vicinity of the large fish (Dayton 1995).
Nutrient Overloading:
With populations increasing along coastlines, human outfalls into the ocean such as sewage, pesticides, and runoff have impacted corals. Stress to the coral reef ecosystem, through increased nutrients in the water, has caused many problems. Nutrients in what was once a nutrient poor environment, gives faster-growing algae, sponges, and tunicates a competitive advantage over the slower-growing corals (Hallock et al. 1993). In addition, increased phytoplankton growth and increased suspended sediments reduce the light transmission of waters surrounding the reefs causing slower growth in corals. Sewage can also increase coral mortality through bacterial infection. Sabellid and serpulid worms, which eat into the reef structure, also flourish with sewage present (Richmond 1993).
Snorkeling and SCUBA Divers:
With increased popularity of snorkeling and SCUBA diving, more people are getting close-up looks at corals. Sometimes, divers intentionally come in contact with reefs, but many times accidental contacts are made with corals. These contacts can damage corals through breakage, abrasion, or crushing. Medio et al. (1997) conducted a survey in Ras Mohammed National Park, Egypt, and found that 62 percent of intentional contacts were made with hand and 71 percent of unintentional contacts were made by divers' fins. They also found that after briefing divers on the impacts of contact with the corals, intentional contact decreased dramatically, but that the unintentional contacts did not alter.
Carbon dioxide:
Since the beginning of the Industrial Revolution, fossil fuels have been burned and have increased the carbon dioxide CO2 in the air. Until recently, not much research has been done on the effect of CO2 coral reef growth. Pennisi (1998) reports on new findings that indicate that that if increases in CO2 reduce the acidity of seawater, then less carbonate will be available for corals to utilize in reef development. Tests have been conducted which show that at lower concentrations of carbonate ions (170-98 percent saturation), coral reef growth declines by three to six times the amount of growth for current concentration of seawater (390 percent saturation). Global warming is also an issue connected to increased CO2 (Pennisi 1998). Temperature changes in coral environments is discussed in the next section.
-NATURAL DISTURBANCES-
Although many would argue that human caused disturbances are also natural, I include in this category impacts to corals that may have occurred in the absence of humans on the earth. This include stresses on corals caused by temperature changes, diseases, solar radiation, dust, and weather events such as hurricanes.
Temperature changes:
Corals tend to grow best from 23¡C to 26¡C, but are known to survive fluctuations from 17¡C to 32¡C (Madl and Yip 1999). Thus, when temperature changes occur, local populations of corals are stressed and can decrease through bleaching longer through slowed growth and lower reproductive rates. El Ni–o and La Ni–a events provide sudden changes in climate and suface water temperature. Much documention of coral decline during the latest El Ni–o event in 1997-1998 shows how destructive these natural events can be. The International Society of Reef Studies claimed that it was the "most geographical widespread bleaching every recorded" (Holden 1998). Monitoring by the Goabal Coral Reef Monitoring Network claim that 11 percent of the world's known reefs had been destroyed prior to 1998. However, another 16 percent had been severely damaged by the El Ni–o event, with some coral reefs not likely to bounce back (Normile 2000). Our current trend in global warming is also concerning scientists. Even though it is happening at a slow rate, human development and pollution may not allow for colonization of new areas as temperature regions shift.
Diseases:
Many diseases impact coral communities. A whole paper could be written about all of them. Many of the diseases are caused or initiated by bacteria or fungi, while some causes of diseases are still unknown. One of the fastest destroying diseases that does not involve a pathogen is Shut-Down-Reaction. A single scratch to a stressed coral causes this unique disease. Disintegration of the coral then follows at rates averaging 10 cm/hour (Madl and Yip 1999). Strangely, this can be contagious as parts of the initial coral comes in contact with other stressed colonies of coral. A list of known coral diseases follow: White Band Disease, Black Band Disease, Bacterial Infection, White Plague, White Pox Disease, Red Band Disease, Black Overgrowing Cyanophyta, Black Aggressive Band, Fungal Infection, Rapid Wasting, Lethal Orange Disease, Yellow Band Disease, Dark Spot Disease, Skeleton Eroding Band, and Peyssonnelia (Madl and Yip 1999).
Solar radiation:
Coral bleaching can also be triggered by high solar radiation. It is still unknown if specific wavelengths are responsible, but those suspected are UVR (280-400nm) and PAR (400-700 n) (Brown 1997). Recent research has shown that three major factors affect the levels of UVR that coral receive. These are atmospheric ozone, weather patterns, and attenuation of UVR in the water column. Solar radiation and temperature caused bleaching are closely related and are difficult to distinguish in the field (Brown 1997). Global warming, and the resulting changes in weather patterns, may effect the level of coral irradiation in the future.
Dust:
Some researchers are hypothesizing that dust originating from the Sahara Desert and blowing west across the Atlantic may be impacting the corals found in the Caribbean. The dust carries nutrients (see nutrient discussion) and helps broadcast bacteria, viruses, and fungi which may cause algal infestation or promote diseases or coral bleaching (USGS 2000)
Hurricanes:
Weather events such as hurricanes or typhoons can cause heavy damages to coral populations; however, they also can act as a marine equivalent of a forest fire. Hurricanes remove sediment that has been threatening to bury corals and open up areas for new settlement of corals (Hughes 1994). However, in a stressed coral system, algae can recover faster and overtake areas that were once dominated by corals (McManus et al. 2000).
-CONCLUDING REMARKS-
The threats to corals that are compiled in this paper are far from a complete list of threats. However, the goal was to cover some of the most notable threats and introduce some controversial ones. More research and coral reef monitoring are needed to better understand the effects of anthropogenic and natural disturbances on the reef ecosystem. Many individual threats are still not well understood and the interactions of disturbances on corals will more difficult to understand.
Until we can better understand these threats to coral reefs, we can strive to reduce human impacts on the reef ecosystem. We can promote better management of coral reefs, by eliminating destructive fishing practices and working towards a more sustainable level of fishing from the reefs. Improvements in agricultural practices and sewage treatment can also help reduce the nutrient and sedimentation problems associated with runoff from land. But most importantly on our excursion to the Florida Keys and the Bahamas, we need to be careful divers and do not intentionally or accidentally damage corals.
References
Brown, B.E. 1997. Coral bleaching: causes and consequences. Coral Reefs. Vol. 16: 129-138.
Connel, J.H. 1997. Disturbance and recovery of coral assemblages. Coral Reefs. Vol. 16: 101-113.
Dayton, L. 1995. The killing reefs. New Scienteist. Vol. 148: 14-15.
Guard, M., and M. Masaiganah. 1997. Dynamite fishing in southern Tanzania, geographical variation, intensity of use and possible solutions. Marine Pollution Bulletin. Vol. 34(10):
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Hallock, P., F.E. Muller-Karger, and J.C. Halas. 1993. Coral reef decline: anthropogenic nutrients and the degradation of Western Atlantic and Caribbean coral reefs. National Geographic Research and Exploration. Vol. 9 (3): 358-378.
Holden, Constance. 1998. Reef trouble. Science. Vol. 282: 871.
Hughes, T.P. 1994. Catastrophes, phase shifts, and large-scale degradation of Caribbean coral reef. Science. Vol. 265: 1547-15550.
McManus, J.W., L.A.B. Menez, K.N. Kesner-Reyes, S.G. Vergara, and M.C. Ablan. 2000. Coral reef fishing and coral-algal phase shifts: implications for global reef status. ICES Journal of Marine Science. Vol. 57: 572-578.
Medio, D., R.F.G. Ormond, and M. Pearson. 1997. Effect of briefing on rates of damage to
corals by scuba divers. Biological Conservation Vol. 79: 91-95.
Pennisi, E. 1998. New threat seen from carbon dioxide. Science. Vol.. 279: 989-990.
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Ryan, John C. 2001. Indonesia's coral reefs on the line. World Watch. Vol. 14(3): 12-19.
Tickell, O. 1999. Coral Grief. Geographical, 71 (8): 67-70.
Website References
Bermuda Biological Station for Research and the College of Exploration (BBSR-TCEB). 2001. Bermuda's coral reefs: basic coral biology. Available from Internet, http://www.coexploration.org/bbsr/coral/ html/body_basic_coral_biology.html.
Madl, Pierre, and Maricela Yip. 1999. Marine Laboratory General Resume of Coral Diseases. Available on the Internet, http://www.sbg.ac.at/ipk/avstudio/pierofun/aquaba/disease1.htm.
Normile, Dennis. 2000. Coral reefs feel the heat. Science Online. Available on the Internet, http://sciencenow.sciencemag.org/cgi/content/full/2000/1025/3.
Sea World Education Department. 2001. Corals and coral reefs: reef ecosystem. Available on the Internet, http://www.seaworld.org/coral_reefs/ecocr.html.
United States Geological Survey (USGS). 2000. Coral Mortality and African Dust. Available on the Internet, http://catbert.er.usgs.gov/african_dust/.
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