Causes of Coral Reef Bleaching

Coral reefs are found in shallow tropical waters along the shores of islands and continents on a substrate composed of calcium carbonate produced from living and dead corals. Corals, of the phylum cnidaria, are actually small animals related to sea anemones, and live in large colonies that we, as humans, most associate with reefs. Many other invertebrates, vertebrates, and plants live in close association to the corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high productivity and biodiversity, such that they are referred to as ‘the Tropical Rainforests of the Oceans’. Out of all these associations corals experience, they share a close symbiotic relationship with a photosynthetic unicellular alga called zooxanthellae, in which the coral provides protection to the algae and the zooxanthellae provide nutrients and oxygen to the coral. Zooxanthellae are the cause of the many diverse and vibrant colors of tropical reef coral. Without the algae, corals cannot survive.

Coral bleaching occurs when the densities of zooxanthellae in the coral colony decline and/or the concentration of photosynthetic pigments within the zooxanthellae fall to far below normal levels. Most reef-building corals normally contain around 1-5 x 10⁶ zooxanthellae cm^-2 of live surface tissue and 2-10 pg of chlorophyll per zooxanthella. When corals bleach, they commonly lose 60-90% of their zooxanthellae and each zooxanthella may lose 50-80% of its photosynthetic pigments. The pale appearance of bleached corals is due to the cnidarian’s calcareous skeleton showing through the translucent tissues (that are nearly devoid of pigmented zooxanthellae). If the stressor causing the bleaching is not too severe and if it decreases in time, the affected corals usually regain their symbiotic algae within several weeks or a few months. If zooxanthellae loss is prolonged, i.e. if the stress continues and depleted zooxanthellae populations do not recover, the coral host eventually dies.

There are several stressors scientists hypothesize cause coral bleaching. Temperature is a huge factor, and is especially relevant now due to climate change and the warming of the world’s oceans. Coral species live within a relatively narrow temperature margin, and anomalously low and high sea temperatures can induce coral bleaching. Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes and seasonal cold-air outbreaks. Bleaching is much more frequently reported from elevated seawater temperature. A small positive anomaly of 1-2 degrees Celsius for 5-10 weeks during the summer season will usually induce bleaching. In truth, any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (such as carbon dioxide or ammonium) will lead to bleaching. Triggers such as ocean acidification; changes in salinity; bacterial infections; and the addition of toxic chemicals such as herbicides, oil, and, found in a recent study, sunscreen, are all major causes of the phenomenon known as coral bleaching.