• Sedimentation
  Sedimentation from coastal development and unwise land-use policies cause soil erosion inducing coral stress and blocking light necessary for coral growth.
   
• Run-Off
  Run-off from cropland and animal feed lots introduce excessive amounts of fertilizers and untreated sewage to reef environments. These nutrients slow growth rates, reduce light and water flow to coral surfaces, and induce coral bleaching (a stress response causing corals to turn white and eventually die).
   
• Global Warming
  Global warming trends due to an increase in greenhouse gases and ozone depletion warm ocean temperatures, stressing coral reef ecosystems, often causing corals to bleach and die.
   
• Destructive Fishing Practices
  Destructive fishing practices, such as the use of reef killing poisons like cyanide, explosives, and fishing devices reduce coral reef ecosystems to lifeless rubble every day.
   
• Pollution
  Pollution from oil, petroleum products, untreated sewage and marine debris often poison and injure coral reef life.
   
• Lost or Discarded Fishing Nets
  Lost or discarded fishing nets (ghost nets) entangle thousands of fish and mammals and suffocate reefs.
   
• Coastal Development
  Poorly conceived coastal development destroys vital ecosystems such as mangrove forests and sea grass beds which  serve as nursery grounds for many reef inhabitants.

Source : Project Aware

Coral colonies are made up of hundreds or even thousands of genetically identical individuals called polyps. These polyps have microscopic algae called zooxanthellae living within their tissues. The zooxanthellae work like an internal symbiotic vegetable garden, carrying out photosynthesis and providing nutrients which help reef-building corals create reef structures. When a coral bleaches, it loses its zooxanthellae, and will die within a matter of weeks unless the zooxanthellae can be replaced. The term bleaching is used because the dazzling colors of living corals are due to the colors of zooxanthellae in coral tissue, and when zooxanthellae are lost, corals appear white, or “bleached.”

Coral bleaching is not well understood by scientists. Many different hypotheses exist as to the cause behind coral bleaching, but the strongest evidence points to unusually warm sea surface temperatures as being the main factor (Glynn 1993). Coral bleaching events worldwide have been attributed to sea surface temperatures (SSTs) rising and staying as little as 1°C higher than the usual average monthly maximum SST during the hottest months of the year (Goreau and Hayes 1994). In Jamaica, significant coral bleaching and death occur when SSTs remain at 29.3°C or higher for one month (Hoegh-Guldberg 1999). Therefore, in the Caribbean and Florida Keys, when SSTs rise and stay above this thermal threshold, coral bleaching is likely to occur.

Corals polyps are beautiful living animals that are enjoyed by millions of snorkelers and divers world wide, but they are also of vital importance the whole coral reef ecosystem that they live upon. The corals themselves are the main building blocks of coral reefs, building these massive structures bit by bit through the construction of their skeletons. If corals cannot survive and multiply, reefs will be lost through natural physical and biological erosion as waves, predators, and other factors affect corals.

The death of corals and the resulting disappearance of reefs would result in the loss of an uncountable number of marine invertebrates and fishes that rely on the physical structure of the reef for survival. Coral reefs are home to hundreds of thousands of species, including many not yet known to science. The elimination of coral reefs would also result in great losses of income and resources from tourism, fishing, and the discovery of new medicines (Carte 1996). The reefs of the Florida Keys alone account for 1.6 billion dollars of revenue annually (Birkeland 1997). Coral reefs also protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion and flooding.

Until the 1980’s, the only coral bleaching event recorded was due to flooding from Hurricane Flora that resulted in a large drop in salinity that bleached and killed many corals in Jamaica (Goreau 1964). Mass coral bleaching was first recognized on the Pacific coast of Panama following the 1982-83 El Nino event (Glynn 1984). The warm SSTs associated with the El Nino event were identified as the cause of death of over 99% of corals and the complete loss of reef structures in the Galapagos Islands and the death of over 50% of corals in Panama (Glynn and D’Croz 1990; Glynn 1993). The sensitivity of corals to small temperature changes then became a major concern of researchers as predictions of global warming and stronger, farther-reaching El Nino events came to light. The 1997-98 El Nino event is the strongest on record to date, resulting in unprecedented coral bleaching and death across the globe (Wilkinson et al. 1999). SSTs are expected to continue to increase worldwide (Hoegh-Guldberg 1999) and El Nino events are expected to increase in frequency, strength and duration, endangering the future survival of coral reefs.

It has been shown that stress caused by pollution, solar radiation, changing salinity, and bacteria can also result in coral bleaching. The Coral Reef Early Warning System (CREWS) implemented by the National Oceanic and Atmospheric Administration (NOAA) considers various physical factors and issues ‘bleaching alerts’ when coral bleaching appears likely. The CREWS software evaluates the intensity of solar radiation, wind speed and sea surface temperatures (SSTs) when issuing coral bleaching alerts.

Wind speed and surface temperature must be considered together because low wind speeds allow greater amounts of solar energy to penetrate the smoother water surface, thus raising the temperature. Also, when corals are exposed to very high sunlight combined with low wind speed, the zooxanthellae may become photodamaged, making a bleaching response more likely. Using CREWS buoy data near coral reefs helps give scientists a heads up so that they can adequately monitor and study these events. During the 1997-98 El Nino event, the effectiveness of this system was shown as bleaching was predicted and occurred on Sombrero Reef in the Florida Keys (Hendee et al. 2001). It is currently impossible for us to prevent bleaching of corals, but through the use of the CREWS buoy data we can at least predict and learn more about this destructive process.

Source: NOAA

1. Dive carefully to protect fragile aquatic ecosystems.
   Many aquatic organisms are delicate and can be harmed by the bump of a camera, the swipe of a in or even the gentle touch of a hand. Some aquatic organisms like corals grow very slowly and breaking even a small piece can destroy decades of growth. By being careful you can prevent long-term damage to magnificent dive sites.
   
    
2. Be aware of your body and equipment placement when diving.
   Keep your gauges and alternate air source secured so they don’t drag over the reef or other vital habitat. Control your buoyancy, taking care not to touch fragile organisms with your body or equipment. You can do your part and prevent injury to aquatic life every time you dive.
   
    
3. Keep your dive skills sharp through continuing education.
   Before heading to open water seek bottom time with a certified professional in a pool or other environment that won’t be damaged. You can also refresh your skills and knowledge with a Scuba Review, Advanced Open Water Diver course or Project AWARE Specialty course such as Peak Performance Buoyancy.
   
    
4. Consider how your interactions affect aquatic life.
   Avoid touching, handling, feeding or riding on aquatic life. These actions may stress the animal, interrupt feeding and mating behavior or provoke aggressive behavior in normally nonaggressive species.
   
    
5. Understand and respect underwater life.
   Playing with animals or using them as food for other species can leave a trail of destruction, disrupt local ecosystems and rob other divers of their experiences with these creatures. Consider enrolling in a Underwater Naturalist, AWARE Fish Identification or Coral Reef Conservation Specialty course to better understand sustainable interactions.
   
    
6. Be an ecotourist.
   Make informed decisions when selecting a destination and choose Project AWARE Environmental Operators or other facilities dedicated to sustainable business practices. Obey all local laws and regulations and understand your effect on the environment. Don’t collect souvenirs like corals or shells. Instead, take underwater photos and follow Project AWARE’s 10 Tips for Underwater Photographers.
   
    
7. Respect underwater cultural heritage.
   Divers are privileged to access dive sites that are part of our cultural heritage and maritime history. Wrecks can also serve as important habitats for fish and other aquatic life. Help preserve these sites for future generations by obeying local laws, diving responsibly and treating wrecks with respect.
   
    
8. Report environmental disturbances or destruction.
   As a diver, you’re in a unique position to monitor the health of local waters. If you notice unusual depletion of aquatic life, injury to aquatic animals or strange substances in the water, report these observations to responsible authorities in your area.
   
    
9. Be a role model for other divers and non-divers when interacting with the environment.
   As a diver, you see the underwater results of carelessness and neglect. Set a good example in your own interactions so that others can learn from you.
   
    
10. Get involved in local environmental activities and issues.
    You can greatly affect your corner of the planet. There are plenty of opportunities to support healthy aquatic environments including Project AWARE conservation and data collection activities like local beach and underwater cleanups and CoralWatch monitoring, supporting environmental legislative issues, attending public hearings on local water resources, conserving water or making responsible seafood choices.

Source: Project Aware