- 1 Tropical Cyclones
- 1.1 What is a tropical cyclone?
- 1.2 What Kind of Damage Does It Do to Coral Reefs?
- 1.3 Recovery
- 1.4 Impacts on the Great Barrier Reef
- 1.5 Impacts on Caribbean Reefs
- 1.6 Notes
- 1.7 References
What is a tropical cyclone?
Tropical cyclones are intense cells of low pressure accompanied by circular winds, thunderstorms, and heavy rain. They contain a central eye. The steep pressure gradient between the storm and surrounding air creates damaging storm surges of 2-5 m above the predicted level. The storm surge is located on the left of the storm in the southern hemisphere because winds spin in the clockwise direction and to the right in the northern hemisphere because winds circulate in the counterclockwise direction. The pathway of a cyclone is unpredictable, and it can make instantaneous switches. Cyclones can create waves up to 8-10 m depending on its wind strength.  A tropical cyclone starts out as a tropical depression, a low pressure system with no eye that contains unorganized storms and winds less than 34 knots, and becomes a tropical storm as it gains strength. It finally graduates to become a cyclone when winds reach a speed of 34 m/s. Depending on the location of the storm, a cyclone has different names: in the North Atlantic or North-Eastern Pacific Oceans, it is referred to as a hurricane; in the Western Pacific region, it is called a typhoon; and in the Southern Hemisphere and Indian Ocean, it is known as a cyclone. Tropical cyclones can devastate coastal communities, infrastructure, and ecosystems. In coral reefs, tropical cyclones are a primary, physical damaging force. Cyclones can affect coral cover, species diversity, and reef productivity. Strong cyclones have the ability to shape the coral reef by affecting the benthic reef communities and underlying reef structure. Fortunately, coral reefs have a certain physical resilience against damaging cyclones. They can recover relatively quickly if there are not many outside stressors. 
What Kind of Damage Does It Do to Coral Reefs?
Reef damage varies with the intensity, distance, and location of a cyclone. With the right conditions, a cyclone can be catastrophic to a coral community and its inhabitants.
Effects on Water
Cyclones can increase sedimentation, alter local salinity, and cause short-term changes in sea level. High winds and torrential rainfall increase turbidity and result in more sedimentation on coral reefs. This sediment can negatively affect the growth of coral and even kill it by decreasing the permeability of light to coral, which is necessary for photosynthetic processes. Sedimentation can also decrease shelter availability for other organisms residing on coral reefs. Torrential rainfall can lower salinity levels. Corals can bleach from the resulting change of the water's pH. Finally, sea levels can also decrease with the low pressure generated by tropical cyclones. This can leave some corals exposed. Alternatively, when sea levels rise, some corals may not have enough sunlight, which is essential for the coral to absorb nutrients and energy.
Broken coral, sediment, and other organisms are displaced due to stronger ocean currents and result in some areas of the reef buried in sediment while others are left bare. Intense tropical cyclones with high storm surges can uplift these materials and push them up to the shore into features known as storm ridges. These are detrimental to atoll reefs because storm ridges have the potential to block off lagoons and induce deterioration of water quality. Stronger storm surges can also break off pieces of larger coral colonies. Reef fracturing depends on the location, shape, and strength of the coral itself: branching ReefInhabitants#Coral|corals are often more affected when compared to colonies of brain or boulder coral, since brain and boulder coral form more stable, massive colonies and create the foundational layer of coral reefs. 
Effects on Biodiversity
Cyclones affect species richness, distribution, and behavior. Most tropical cyclones do not directly kill reef fish. Reef fish are usually killed through the disruptions of their surroundings as discussed above. However, these mortalities are not selective, which means any number of species can be affected. This can be catastrophic if keystone and other important species are wiped out. Cyclones influence juvenile fish the most in shallow waters. When the density and distribution of these juvenile fish is disrupted, it negatively impacts the future species richness of the coral reef. The destruction of a coral reef is usually followed by a change in trophic levels of the fish community. Other reef species such as algae, sponges, and echinoderms are reduced in density, especially in shallow waters. Algae and sponges can be uprooted and displaced onshore due to strong winds and storm surges. 
Coral reefs were built to withstand the physical damage of natural disasters like tropical cyclones. However, when reefs are weakened by outside stressors such as changing pH, salinity, and nutrient levels, coral reefs can take a long period of time to recover. Damaged coral reefs can take 5 to 40 years to recover, depending on the extent of the damage and how healthy the reef was originally. Favorable conditions for growth also have to be present for the coral reef to recover. Salinity and sea levels need to return back to normal. Ideally, the reef should have connections to other healthy reefs, so that organisms and larvae can travel to reseed the damaged community. Without an outside source of larvae, a coral reef will take much longer to regrow. The regrowth of coral fragments is also necessary to repopulate the reefs. The regeneration of partially damaged coral colonies help reestablish old colonies of coral. The presence of human stressors should be minimal. Coral reefs that are unaffected by outside stressors have been shown to recover from tropical cyclones more quickly. 
Impacts on the Great Barrier Reef
Case Study: Cyclone Ingrid
Tropical Cyclone Ingrid hit the Great Barrier Reef in March of 2005. Ingrid ranged between a category 3 and category 5 cyclone with winds up to 250 km/s; however, the actual size of the cyclone was small: the core of the cyclone was only 10-15 km diameter. The winds created waves of up to 15 meters(m) in the open ocean and around 5m in the Great Barrier Reef. Unfortunately, Ingrid hit three different states in Australia. This is the first time in history that a tropical cyclone hit this many states in Australia. The intensity of the damage was caused by the high winds in such a small area. While offshore reefs showed damage even at great depth, the inshore reefs took the brunt of the blow. Areas with less than 25 m/s winds suffered minimal damage, while areas with greater than 33 m/s winds encountered cataclysmic damage. In the worst affected areas, hard coral cover decreased 800%. (((NOT POSSIBLE))) The biodiversity decreased 250%, and the amount of coral recruits decreased by about 30%.
Case study: Cyclone Yasi
Tropical Cyclone Yasi made landfall in Queensland, Australia in February of 2011. Yasi reached a category 5 cyclone with strong, damaging winds of 285 km/s. Reefs near the eye of the storm suffered the most damage, and the reefs south of the eye suffered more than the reefs to the north of the eye. Specifically, the 61 reefs that were around the peak winds were damaged the most. Out of these, 58 reefs suffered destruction to the living communities, and 47 of the reefs suffered from structural damage. Shelf position also affected the extent of damage to the coral: outer reefs experienced more devastation than mid-shelf reefs. Some coral gardens around the Queensland area were reduced to rubble. Many coral heads, known as "bommies" in the area, were found lying on the ocean bed far away from where they were supposed to be. Because of these reasons, the damage was patchy, so some neighboring coral gardens remaining intact. These nearby corals can help facilitate the regrowth of the damaged coral gardens. Cyclone Yasi's physical destruction ranged from minor tissue injuries on the edges of some coral colonies to complete removal of all sessile organisms. Fragile, fast-growing corals such as branching and plating corals endured the most damage, while more durable, slow-growing coral such as encrusting and brain coral suffered relatively less damage. Yasi's damage to the coral gardens have already caused the coral trout to disappear from the area. It is possible that Yasi may have caused more damage than any other storm since early 1900s.> The extent of the damage could take anywhere between 10 to 20 years to restore good coral cover; however, some coral could take longer than that to fully recover. Months before Yasi, several floods swept through the same area, carrying sediment and pesticides into the ocean. This was compounded by the high tides caused by the cyclone. These pesticides and sediments could be potentially damaging to already fragile coral, causing latent death. Fortunately for people in the area, Queensland is not a popular reef site; therefore, there was not a severe impact to the tourism industry.
Impacts on Caribbean Reefs
Case study: Hurricane Mitch
Hurricane Mitch struck the Western Caribbean and the Gulf of Mexico in 1998 with 180 mph winds and killed almost 20,000 people, the highest death toll from a Hurricane since the 18th century. Initially it was feared that the powerful storm caused a lot of damage to the reefs of the island of Roatan in Honduras. This assumption was based on the fact that the hurricane had torn apart the sunken freighter El Aquila which had been sunk a decade before as a recreational dive site. However this assumption proved to be wrong, the reef was relatively intact. Roatan's steep reef walls weathered Mitch's well, however branching corals were not so lucky. The waves swept away the harmful kinds of algae covering the reef. Corals in the process of bleaching were aided by the upwelling of cooler bottom water. Robert S. Young a geology professor at Western Carolina University said "the reef was swept clean of sediment, the water quality had improved, and a lot of the deleterious algae that had been growing on it was torn away, on the whole, Mitch was far more beneficial than it was harmful."  Phillip Stevens the owner of a local dive shop echoed this saying “the hurricane actually saved the reef, we'd thought we were going to lose half of our corals, but instead we lost 10 or 15 percent.”  Typically moderate hurricanes can be beneficial to a reef by removing sedimentation and algae "It's very much like treefalls in a forest -- they create a small opening for short-lived, fast-growing species to coexist with more dominant ones," says Joseph H. Connell, professor emeritus of biology at the University of California at Santa Barbara.  Coral reefs have evolved with hurricanes as a constant factor in their existence but with human pressures the upshot of a hurricane cleaning and pruning a reef is undone by human development and impacts. Jennifer Keck, a coral-reef scientist at the Roatan Institute for Marine Sciences found live coral cover had fallen from 29 percent of the reef's surface to 20 percent after Mitch.  Coral reef growth has remained stagnant on Roatan since the hurricane; meaning that in the event of another severe hurricane the coral cover count will fall again and likely not regrow. Caribbean corals face a variety of man made stressors and it is believed that these stressors impair a reef’s ability to repair itself after a traumatic event such as a hurricane. Robert B. Halley, a research geologist at the United States Geological Survey's Center for Coastal and Marine Geology, in St. Petersburg, Fla said "in the '60s and '70s, you could damage corals and they had an incredible ability to heal, but something has changed throughout the Caribbean in the past decade. Instead of healing themselves after an injury, now more often than not, they will get some disease or infection and die."  The reefs of Cayos Cochinos an island chain 12 miles off the coast of Honduras and 18 miles from Roatan came down with coral diseases such as bleaching, black band, and white pox one year after Mitch. The culprit was pollution in the water that reached the Caribbean Sea. "You hear anecdotal stories about whole warehouses full of fertilizer and pesticides being swept down the rivers, so it's clear there was a lot of nasty water sitting over the coast after that," said Mr. Halley.  This is a tale of two reefs; Roatan benefited from the Mitch while Cayos Cochinos was negatively impacted. Simple geography is the explanation; Cayos Cochinos is much closer to the mainland and was subject to much poorer water quality and thicker sedimentation after Mitch.
Case Study: Hurricane Hugo (1989)
St. Croix, U.S. Virgin Islands: Hurricane Hugo was the most severe hurricane to impact the US Virgin Islands in over 60 years with sustained winds reaching 160 mph over a 12 hour period. The eye of Hugo passed directly over St. Croix, making the island a desirable site to study and understand the impact of such high intensity Hurricanes on coral reefs generally. As a result of the storm, reef damage was highly variable and localized with some areas being razed and others experiencing only minor loss on the magnitude of approximately a 10% reduction as compared to pre-storm coral coverage.  The study noted that minorly damaged coral locales were able to return to pre-hurricane coral cover rates within two years, while regions that experienced major loss were unlikely to muster sustained regrowth to pre-hurricane levels. The authors show that coral which sustained significant damage did not recover in the 12 months following the storm.  This limited time horizon only allows for an extrapolation of the long-term impacts, pointing to a limitation of the study. It was also found that the hurricane damage to the reefs had a significant spatial variability, which affected recovery rates as remaining coral populations attempted to seed injured population and spur recovery growth.  The amount of substrate present for use for coral growth increased dramatically. While increasing the potential for recolonization, this did not necessarily precipitate a population bounce back in locales where a dramatic loss of live coral was sustained. 
St. John, U.S. Virgin Islands: At St. John, another of the U.S. Virgin islands impacted by Hugo, the damage to reefs was also shown to have a large spatial variability, creating patchy loss of coral, specifically of the reef building coral M. annularis. In significantly damaged locales, the measured coral loss approached 30%.  This particular study was vital to our understanding of the impacts of hurricanes because of the authors conclusive predictions about which factors play a role in determining amount of damaged sustained during hurricanes. These include: reef depth, composition of pre-storm bottom communities, and the orientation of the shoreline and shoreline reefs. The study found that reefs at a depth of 7 meters or more were unlikely to be majorly affected. With regard to the composition of bottom-communities, the authors observed what would be intuitively expected. That is branching corals, such as elkhorn, were substantially more susceptible to damage than sturdier species such as boulder coral. With regard to coral orientation, regions that have signifiant sheltering, either from an above water land mass or underwater structure, survived with less damage at a higher frequency than those corals without some structural protection. The study also found that some regions that experienced loss were recolonized by different types of coral than were present before loss sustained during hurricane. The amount of recolonization was dependent on the magnitude of destruction, with areas that were razed experiencing only minor recolonization in the year following the Hugo. Noted as specifically occurring in St. John was the potential for flushing - the clearance of debris and sentiment. A Study by Hubbard et. al concluded that this flushing could possibly lead to healthier reefs in the long-term.. However, the authors did not hypothesize where coral reef health improved specifically as a result of the flushing caused by Hugo.
Case Study: Hurricane Allen (1989)
Jamaica: Allen reached Jamaica with 110 km/h sustained winds and 12 m waves. The amount of damage sustained was dependent on reef location, depth, and topography. Reefs on the northern shore of Jamaica, specifically those found in Discovery Bay suffered significant damage as a result of poor topographic shielding from the storms intensity.  With regards to the effects of depth, coral that was found in less than 8 m of water was found to have significant but highly variable damage. Similarly, in depths of 8 m to 12 m, loss of coral was less severe yet still highly variable. In depths of 12 m or more, no loss of coral was observed.  The authors conclude that depth is the most important factor in predicting susceptibility of coral populations to high intensity weather events. 
Barbados: Allen maintained a similar intensity as it passed over Barbados. The coral reef damage assessment was based on data collected in a pre-hurricane 1974 survey with data collected in the days following the storm taken over the same geography. Reefs monitored on the western shore of the island saw a global increase in the amount of substrate present as measured 8 months post storm. Shannon-Weaver diversity indices of the corals dropped from 1.61 to 1.26. The changes in diversity are best explained by the intermediate disturbance hypothesis.  This hypothesis suggests that the frequency of intense weather should neither be too frequent nor too rare in order for maximal species diversity. With regards to the topographical changes of the underwater reef environment, transport of debris and sediment was observed both onshore and offshore, yet the beneficial effects of flushing were not noted in the survey taken 8 months following the storm.  No data indicated that high magnitude changes in underwater topography were not observed despite reef depth. This affords consistency with previous observations of the large variability of reef damage observed with high intensity weather events.
Case Study: David and Frederic
Between August 29th and September 6 1979 almost 30 inches of rain fell on the Island of St. Croix from Hurricanes David and Frederick. David was the stronger of the two storms and passed 200km south of the Virgin Islands while Frederick passed 74km north of St. Croix. Runoff from the excessive rainfall was less severe than it could have been because St. Croix is a low island with no permanent rivers. 
A week after the Hurricanes crews placed tags on 100 broken Acropora palmata branches. Nine months after the Hurricanes significant regrowth of corals were recorded. 
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- Effects of Hurricanes David and Frederic (1979) on Shallow Acropora Palmata Reef Communities: St. Croix, U.S. Virgin Islands
- Effects of Hurricanes David and Frederic (1979) on Shallow Acropora Palmata Reef Communities: St. Croix, U.S. Virgin Islands
Blythell, J.C., Blythell, M., Gladfelter, E. H. “Initial resulst of long-term coral reef monitoring program: impact of Hurrican Hugo at Buck Island reef national Monument, St. Croix U.S. Virgin islands.” Journal of Experimental Marine Biology and Ecology Volume 172, Issues 1–2, 11 November 1993, Pages 171–183