Independent Scuba

Threats to coral reefs

Tropical storms are the perhaps largest source of massive reef mortality. Hurricanes may mechanically destroy large areas of a reef, which is a major problem since most reefs are located in areas frequently traversed by typhoons or hurricanes.

Crown of thorns – Acanthaster planci

The sea star Acanthaster planci can, when the population increases, be another major source of mass morality. The sea star managed to devastate 90% of the coral reefs along 38 km of the Guam shoreline in 2,5 years. Little is known about the reason for the outbreak of these population explosions. Suggestions are that humans have removed an important predator of severe tropical storms opening space for juvenile settlement or adult aggregation.

These outbreaks are not new however. Acanthaster skeletal remains in sediment and local folklore of certain islands indicates earlier outbreaks as well, but it is impossible to tell whether they have increased or not. It is estimated that the recovery time for a reef devastated by Acanthaster is 7-40 years.

Crown of thorns on coral reef

El Niño

El Niño is a natural, recurring natural phenomenon manifested as a massive influx of warm and nutrient-poor water in usually cold and nutrient-rich waters. This is the effect of many variables, among them atmospheric pressure, winds, rainfall patterns, ocean currents etc. Thus, the effect varies greatly between occurrences. The 1982-83 El Niño has been called the worst during the 20th century and resulted in ocean temperatures 2-4C above normal for several months, which caused massive coral mortality.

Coral bleaching

Coral bleaching is the name for the phenomenon when corals expel the zooxanthellae from their tissues. The expulsion results in the corals getting white, which explains the name of the phenomenon. If the zooxanthellae are absent for longer periods, the coral will die. It is believed that stress induced by water temperatures higher than 30C is the main cause.

Coral bleaching

Human activities

Human activities affect coral reef to a large extent, mainly negative. Common such activities are dredging, pollution and over fishing. Over fishing may cause outbreaks of Acanthaster, which may devastate entire reefs. Dynamite and poison fishing in the Philippines is another major source of massive coral destruction. 

Dynamite fishing

The final section is about the Lophelia reefs in Scandinavia.