What is the leading cause of coral bleaching? post briefly explains about what is coral bleaching, what causes coral bleaching.
What is coral bleaching
Coral Bleaching is the ecological damage/degradation of coral reefs as a result
of natural and man-made factors. In recent times, coral bleaching has increased
with alarming rapidity. This increase has been largely because of “anthropogenic
impacts like overexploitation, overfishing, increased sedimentation and nutrient
overloading in water”.
Before and after coral bleaching, Image Source: https://www.insider.com/
There are natural disturbances which cause damage to coral reefs. Such disturbances are violent storms, flooding, high and low temperature extremes, El Nino Southern Oscillation (ENSO) events etc.
Coral reef bleaching is a common stress response of corals to many of the various disturbances. Bleaching occurs when the densities of zooxanthellae reduce. It also occurs when the concentration of photosynthetic pigments within the zooxanthellae fall. It is held that when corals bleach they commonly lose 60-90% of their zooxanthellae and each zooxanthellae may lose 50-80% of its photosynthetic pigments. However, if the disturbing factor is not severe and its impact reduces over time, then the affected corals usually regain their symbiotic algae over a period of time. However, if zooxanthellae loss is prolonged over a period of time in which degradation of zooxanthellae is beyond recovery, the coral host eventually dies.
What causes coral bleaching
Magnified levels of sea-surface temperatures because of climate change related factors as well as warm El-Nino events are the main reasons behind coral bleaching. Coral species live within a relatively narrow temperature range. They cannot thrive in extremely high or extremely low sea temperatures. Corals are not present on the west coast of tropical temperate continents because of the cold ambience created by cold currents.
Extreme temperature conditions induce coral bleaching. Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes (ElNino) and seasonal cold-air outbreaks. Coral bleaching has occurred mostly during the summer seasons or near the end of summer season with elevated temperature scenario. For Example, the bleached corals in the Gulf of Mexico are the result of increased water temperatures.
Ocean Acidification Process
As a result of intense global warming, there has been increased presence of carbon in oceans catalysing elevated levels of ocean acidification. While the rising temperatures have increased the frequency and intensity of bleaching, acidification has decreased calcifying ability of corals. It is also postulated that “by 2085 ocean conditions will be acidic enough for corals around the globe to begin to dissolve” as per scientific predictions.
Coral bleaching also occurs during times of low wind velocity, clear skies, calm seas and low turbidity. These conditions favour localised heating and high ultraviolet (UV) radiation. UV radiation readily penetrates clear sea waters. The corals actually contain UV-absorbing compounds which can block potentially damaging UV radiation. But rising temperatures mean reduction in the concentration of these UV absorbing compounds in corals.
Dilution of fresh water
Quick dilution of reef waters from storm-generated precipitation and runoff also cause coral reef bleaching.
Increased inorganic Nutrients
An enhanced level of ambient elemental nutrient concentrations like ammonia and nitrate actually increases zooxanthellae densities many times. It causes secondary adverse effects such as lowering of coral resistance and greater susceptibility to diseases. When corals are exposed to high concentrations of chemical contaminants like copper, herbicides and oil, coral bleaching happens.
Coral reefs are also threatened by invasive species like the lionfish. The lionfish is native to Indo-Pacific waters having a rapidly growing population in waters of the Atlantic Ocean. It is reported that the lionfish could potentially impact coral reef ecosystems. This could happen through consumption of and competition with native coral reef animals.
Even activities like deforestation threaten coral reefs. Deforestation quickens the pace of soil erosion which increases the load of sentiments into seas adversely affecting coral reefs.
Thus, the major causes of coral bleaching are temperature changes and ocean acidification (both causes have gained prominence because of global warming). However, coral bleaching can also occur because of diseases, pollution from urban or agricultural run-off, changes in the salinity of seawater, sedimentation from undersea activities like dredging etc.