Coral reefs are the most biologically diverse ecosystems on earth. They cover a fraction of a percent of the ocean floor but shelter almost 25 percent of all marine life. Additionally, the world relies on reefs as a source of food due to their abundance; they even serve as attractions in the tourism industry. However, coral reefs have recently suffered losses in size and diversity through bleaching.
Most corals are part of a symbiotic relationship with algae from the genus Symbiodinium, collectively known as zooxanthellae. These single-celled algae living within coral tissue give reefs their renowned colors. More importantly, they are necessary for coral growth and proliferation. Coral polyps and zooxanthellae interact mutualistically: coral protects the algae within its tissue, and in return, the algae conduct photosynthesis which serves as a food source for coral. Corals thrive in nutrient poor areas because this relationship allows them to subsist while feeding minimally from the surrounding water. Despite its pervasiveness, the relationship between coral and algae is extremely delicate and only occurs under specific conditions. In recent years, changes in ocean conditions have caused disruptions to the symbiosis between zooxanthellae and corals. These disruptions manifest in the form of large scale bleaching events. Corals bleach when they are stressed to the point that they are unable to retain zooxanthellae; they discharge the algae into the surrounding water, leaving their skeletons and tissues drained of color. In this state, coral tissue is alive but lacks an internal food source and must obtain nourishment from its environment. Bleached corals are much more susceptible to disease and almost certain to die, even in nutrient rich waters. The harm of bleaching does not stop at coral death: reefs serve as the habitat for millions of species of fish, crustaceans, mammals, and turtles. Their destruction causes the displacement and death of marine life that relies on reefs to survive.
Coral bleaching is caused by various environmental changes, all of which are the product of humans. The most prominent of these is rising ocean temperatures. Corals are so sensitive that sudden increases in temperature as little as 1-2º celsius prolonged over weeks can cause enough stress to bleach them. Heat–induced bleaching occurred globally between 2014 and 2017 during an unusually warm period. Reefs in the South Pacific and Indian Oceans suffered the greatest reductions in coral populations. In 2016, northern shallow water sections of the Great Barrier Reef lost 30 percent of their corals in less than one year according to NOAA. Another direct cause of bleaching is water pollution in the form of runoff stemming from onshore human activity. Pesticides, dust and debris from mining, and waste from urban areas subject corals to unnatural amounts of stress and cause them to rid themselves of zooxanthellae. Even fertilizers, which are necessary for agriculture and used in yards and gardens all across the world, lead to coral bleaching. Despite being beneficial to plants, fertilizers change the composition of seawater when brought into the ocean by rain. According to a study presented by the Harbor Branch Oceanographic Institute, by increasing the concentration of nitrogen in seawater, fertilizers inhibit corals’ access to other nutrients such as phosphorus, of which corals become starved. An additional source of nitrogen runoff is water discharged from sewage plants that has not been treated completely. Researchers from the same study tracked a portion of reef in Looe Key Preservation Area that saw a decline in coral populations from 33 percent coverage in 1984 to 6 percent in 2008 and is now nearly eradicated. The study found a strong correlation between periods of increased rainfall (meaning increased runoff) and coral bleaching in the Florida Keys. Though commonly believed to be solely a product of climate change, the specific causes of coral bleaching have more recently been identified.
Combating coral bleaching is very difficult because its roots are numerous, widespread, and integrated into society. Hence, there is no obvious answer to this problem. Though there are some conceivable changes that can be made, such as the implementation of safer, more effective sewage treatment plants and control of runoff water, the threat of increasing temperatures still looms over coral reefs. However, the situation is far from hopeless: corals, with the help of humans can recover from bleaching. The Scripps Institution of Oceanography witnessed such an occurrence as they tracked the recovery of the Palmyra Atoll reef after a bleaching event in 2015. During this event, high temperatures caused 90 percent of corals to bleach; however, only about 10 percent of those died. To observe the recovery, Scripps developed a method of 3D mapping the reef using thousands of photographs. In addition, they monitored the growth of a certain species of algae that allows new corals to settle: crustose coralline. The observation of the successful recovery of the Palmyra Atoll reef using this new method helps oceanographers learn how to assist the recovery of reefs following future bleaching events.