More of our class explores Lighthouse Cave, San Salvador, Bahamas.
It has been reported in countless journals and scientific publications that coral reefs are in a state of worldwide decline (1,4,5,8,11,14). Many of these sources take into account anthropogenic and natural causation in the deterioration of the reefs. It is likely that a combination of factors is contributing to reef decline. Reefs are naturally resilient to disturbance forces like those of a tropical storm; however, mounting anthropogenic inputs are changing reef recovery dynamics. In fact, coral reefs are geologically robust and have persisted through major climatic shifts in the past. They are however, sensitive to small environmental perturbations over the short-term. A tropical storm is one of these short-term perturbations. This is the concern among conservation planners, marine ecologist, and reef lovers over the world. When taken as a whole, reef ecosystems all over the globe are a center of biodiversity. Biodiversity is one of the prime factors that ecosystems have to aid in recovery after a disturbance. Loss of biodiversity can lead to loss of entire ecosystems. In many parts of the world, reef systems are losing biodiversity at an alarming rate.
Coral reefs in decline have lost a large portion of their biodiversity. Without a diversified community structure, recovery from disturbance events becomes a longer, more difficult, and sometimes impossible task. Current estimates note that 10 percent of all coral reefs are degraded beyond recovery. Thirty percent are in critical condition and may die within 10 to 20 years. Experts predict that if current pressures are allowed to continue unabated, 60 percent of the worldŐs coral reefs may die completely by 2050 (14). This being the case, it is important to study coral reef recovery dynamics to understand the ecological processes at work.
I. Disturbance Events
The way that coral reefs respond to chronic changes in climate/environment is important for numerous reasons. Monitoring reef fragility can serve as an indicator of such ecological anomalies as climate change or as a test of marine fitness. An emerging geological field, paleotempestology seeks to just that. Paleotempestologist use coral core samples to determine periods of increased storm activity in the geologic record. Scientists search for chemical clues about the temperature and salinity of the water. Corals grow faster in warmer waters, the slowly branch out as do trees, leaving growth bands. These bands can reveal evidence of El Nino/La Nina events in the past. This research can then serve as an indirect chart of hurricane activity in the geologic past. This is not a measurement of specific hurricanes, but rather a measure of hurricane frequency. Other scientists are examining sedimentation dropped from storm surges in near shore lakes. Scientists have studied a coarse layer of sediment on the bottom of Shelby Lake in Alabama to determine that a series of category storms struck the region about six hundred years ago. Utilizing yet another source for geologic evidence of hurricane activity, researchers in the Bahamas are looking to cave stalagmites as keepers of storm records. The rain that falls in the cloudbanks surrounding a hurricane has a special chemical signature that leaves its mark when hurricane rain flushes straight through the water table and into underground cave deposits. By analyzing and dating sections of stalagmites, scientists can tally the number of hurricanes that have swept through a particular region. All of these new techniques can serve to help us understand long-term geologic/climate fluctuations, however, there are limitations to these techniques. The data can on yield a limited amount of information. Harvard geochemist, Dan Shrag, says of the research, ŇitŐs a space-time problem . . . even if you could reconstruct all the storms in Louisiana over a certain time period, thatŐs only a small part of the coast, that doesnŐt yield any global storm frequency dataÓ (14). If the data that these researchers are proved to be scientifically verifiable and relevant, then their work could reveal direct evidence of what effect past climate change events have had on hurricane frequency.
As noted before, there are two prime categories under which disturbances occur. The first of which is natural disturbances. Natural disturbance events that affect coral reefs include, tropical storms, outbreaks of coral predators, disease, extended periods of elevated or low water temperatures, and extremely low tides (2,3,4,8,9). The focus of this research is the effect that tropical storms have on reef ecosystems, however, one cannot view reef disturbance and recovery from only one perspective. Disturbance events often compound themselves (4). Although these events disturb the reefs and may kill a significant amount of coral, they are part of a natural cycle that reefs experience on a temporal and spatial scale; furthermore, the reef ecosystem may benefit from the disturbance in other ways. The destruction caused by a hurricane, for example, opens space for reef organisms that had been excluded by larger and longer-lived corals. Hurricanes also flush out accumulated sediment within the reef and create more substrate for organisms to settle and grow on. A healthy reef ecosystem will eventually recover from natural disturbance events. However, when these natural disturbances occur to a reef system that has been impacted by human activities, the reef system may have a reduced or even no capacity to rebound. A natural disturbance acting synergistically with accumulated human impacts may result in destruction that is not reversed in the same time frame it naturally would occur. A recent World Resources Institute report estimates that nearly 60 percent of the world's reefs are threatened by increasing human activity (14). The expanding human population and its activities may impact coral reef health in a number of ways. Development, urbanization, and agriculture lead to increases in freshwater runoff, polluted runoff, sedimentation, and nutrient inputs. Growing industry and automobile usage cause an increase in emissions contributing to the green house effect and chemical deposition from air to water. Commercial and private vessel traffic means the possibility of fuel leaks or spills, vessel groundings, and anchor damage. Harvest of reef resources is also taking a toll on the health of coral reef ecosystems. Over-fishing on reefs leads to an unbalanced ecosystem, allowing more competitive or less desirable organisms to become dominant. Fishing methods such as the use of explosives and poisons severely harm reefs and reef organisms. Harvest of coral skeleton for souvenirs depletes healthy corals or substrate where coral larvae might have settled. Increased tourism in areas of coral reef habitat contributes to increased pressure from scuba diving, recreational fishing, and vessel traffic. All of these anthropogenic effects can compound the permanent damage that is caused when a tropical storm strikes over a reef. The impact that human inputs have on reef systems varies throughout the world, however, it is evident that many reefs are currently in decline.
II. San Salvador Hurricane History
While we are situated in San Salvador, I intend to document (where possible) any damaged coral at our dive sites. I would like to correlate this data with San SalvadorŐs recent hurricane history. San Salvador is among the top fifteen (ranked 10th) most hurricane prone areas in the Caribbean and Gulf of Mexico since 1871. The island has been struck 46 times in the last 100 years (this does not mean it was hit in 46 out of 132 years, but rather 46 times total). According to one source, the island is struck on average every 2.86 years (12). This frequency is certainly demonstrative of a long-term and chronic disturbance regime to the reef ecosystems around the island. However, as noted before, these reefs have developed adaptations to such events and can even benefit from them.
San Salvador is a small, island with only two major resorts and a limited population of about 1000 people. To my knowledge, the island lacks much of the industrial/polluting elements that you would find on a larger island, or closer to the US. I would surmise, then, that indirect anthropogenic effects would affect the coral communities much less than direct disturbances. Thus, it may be true that these coral communities are much healthier than their counterparts closer to the mainland. Such questions will be explored as we are in the region, with results to be published at a later date.
San Salvador was either hit or brushed by a hurricane twice in 1996 (Bertha and Lili), and twice in 1999 (Floyd and Dennis). Prior to these events, the island was last struck in 1981 (Katrina). These recent strikes provide an opportunity to directly observe reef recovery. This is a high frequency of hurricane occurrence in comparison with the previous forty years. From 1965 to 1996, only Hurricane Katrina directly affected the island. The lull in hurricane activity perhaps allowed for higher than normal growth rates for the coral ecosystem, and thus produced healthier systems.
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7. Garver, John. http://zircon.geology.union.edu/carb/hurricane/damage
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12. National Oceanic and Atmospheric Administration: www.nhc.noaa.gov
13. Perry, C.T. Storm Induced Coral Rubble Deposition: Pleistocene Records of Natural Reef Disturbance and Community Response. Coral Reefs. 20: 2001. pp. 171-183.
14. Reef Relief: www.reefrelief.org
15. Walker, Lawrence R. et al. An intro to Hurricanes in the Caribbean. Biotropica. V. 23. Issue 4. Dec. 1991. pp. 313-316.
16. Woodley, J.D. et al. Hurricane AllenŐs Impact on Coral Reefs. Science. New Series. V. 214. Issue 4522. Nov. 1981. pp. 749-755.
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