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El Niño And La Niña Contributed To Coral Bleaching In The Great Barrier Reef

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What is El Niño?

El Niño is a climatic event which describes an unpredictable and unusual rise in sea surface temperatures in the Eastern tropical Pacific Ocean. This unusual rise in sea surface temperatures is as a result of the El Niño’s correlation with the Southern Oscillation which causes a sudden change in air pressure over the tropical Pacific Ocean. El Niño and La Niña climatic events occur in a cycle as the ocean part of the ENSO (El Niño-Southern Oscillation). El Niño provides warmer than average temperatures however La Niña provides colder than normal sea surface temperatures.

The Negative Effects of El Niño

The correlation between the Southern Oscillation and El Niño and La Niña climatic events means that a change in air pressure in Southern Oscillation is responsible for the formation of El Niño climatic events and therefore El Niño’s are unpredictable. Normal conditions in the tropical Pacific Ocean consist of westward trade winds that blow from the equatorial areas and thus bring warm surface waters towards the western Pacific Ocean, these conditions provide favourable conditions for upwelling.

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Upwelling is a process which is extremely important to marine ecosystems as it provides phytoplankton with nutrients by elevating colder water to the upper layer of the ocean and is a major food source for many marine life species. Phytoplankton therefore use the nutrients from upwelling to photosynthesize and during this process produce up to 80% of earths oxygen supply. However, during El Niño climatic events upwelling cannot occur due to a change in direction and strength of trade winds which affects the thermocline thus colder waters are found up to 152 meters deeper rather than the normal depth of 0,5 meters from the sea surface. Due to warmer ocean surfaces as a result of El Niño climatic events upwelling cannot occur and as a result has a detrimental impact on marine ecosystems due to lack of food source for phytoplankton, which results in a decline in the population. Marine animals that feed on phytoplankton therefore die or are forced to migrate as a result of a lack of food source. (National Geographic society, 2019) Warmer sea surface temperatures not only negatively affect smaller marine life organisms such as clams due to a lack of food source, El Niño climatic events also have a negative impact on coral reefs which provide a habitat and food for larger marine life due as fish species that feed off the polyp of corals. (ScienceDaily, 2008)

Coral Bleeching

Between 2013 and 2019 nearly 80% of corals in the northern areas of the Great Barrier Reef were killed as a result of coral bleaching and heat stress (National Geographic, 2018). A rise in ocean temperatures and therefore heat stress causes coral to expel the colourful algae which provides them with nutrients and protection, this is known as coral bleaching. (ScienceDaily, 2008) Coral bleaching is a natural cycle which previously occurred every 27 years however as a result of warmer ocean temperatures, coral bleaching events presently occur more frequently due to the increased occurrence of climatic events such as the El Niño.

Between 2014 and 2017 two El Niño climatic events affected Australia and more specifically the Northern Great Barrier Reef. In 2014-15 a El Niño began to form; the El Niño did not completely develop however triggered a back-to-back, three-year-long mass coral bleaching event, one of the longest and most wide spread bleaching events ever recorded in which 70% of coral reefs worldwide were affected in terms of heat stress and coral morality. These conditions were further worsened in 2015-16 with the full development of another El Niño, this worsened the state of the already damaged coral leading to the death of 29% of shallow water corals due to high temperatures in March 2016.

Severe heat stress and coral bleaching occurred up until may 2017 however a La Niña climatic event subsided the heat stress as a result of colder than normal sea surface temperatures associated with La Niña climatic events. The cooler than average sea surface temperatures as a result of the La Niña in 2017 helped to subside coral heat stress and therefore without La Niña climatic events between 2014-2017 coral bleaching and coral decline would have been worse. (Global Coral bleaching, 2017)

La Niña climatic events subsided heat stress and food shortages during the mass coral bleaching between 2014 and 2017 as cooler ocean temperatures allowed for more surface upwelling. (National Geographic Society, 2019) This had a positive effect on marine life as it provided phytoplankton with nutrients, the healthy phytoplankton was therefore an abundant food source and marine life were able to return in areas where there were previous food shortages due to El Niño climatic events.

The El Niño climatic events between 2014-2017 had a detrimental impact on the northern Great Barrier Reef as heat stress as a result of warmer sea surface temperatures caused severe coral bleaching which caused a change in diversity and populations of marine life due to an increase in algae species such as turf and macroalgae. These species of algae flourish due to the lack of competition for from the deceased coral. The invasion of the algae therefore results in a change of fish species as original species can no longer seek protection in the composition of the coral and polyp eating fish can no longer feed as a result of a lack of food from coral bleaching. Functional fish such as herbivorous fish migrate into the area due to the abundance of food and help to ensure the algae do not grow out of control. If algae is kept under control corals are more likely to start recovering however if algae do grow out of control the coral will not recover due to its the direct competition with the abundance of algae. Herbivorous fish species are therefore very beneficial in maintaining algae populations so that bleached coral can have a chance to recover (ScienceDaily, 2008) However, the frequent warmer waters associated with El Niño climatic events deterred herbivorous fish such as Parrot fish away from areas with an abundance of algae due to coral bleaching as a result of sensitivity to the warmer waters. El Niño climatic events therefore had a negative impact on the potential recovery of bleached corals due to warm sea surface temperatures in the Great Barrier Reef. However the cool sea surfaces associated with the La Niña climatic events allowed for the potential recovery of bleached corals as herbivorous fish such as Parrot fish were able to return into the affected areas.The El Niño climatic events between 2014-2017 had a detrimental impact on the northern Great Barrier Reef as heat stress as a result of warmer sea surface temperatures caused severe coral bleaching which caused a change in diversity and populations of marine life due to an increase in algae species such as turf and macroalgae. These species of algae flourish due to the lack of competition for from the deceased coral. The invasion of the algae therefore results in a change of fish species as original species can no longer seek protection in the composition of the coral and polyp eating fish can no longer feed as a result of a lack of food from coral bleaching. Functional fish such as herbivorous fish migrate into the area due to the abundance of food and help to ensure the algae do not grow out of control. If algae is kept under control corals are more likely to start recovering however if algae do grow out of control the coral will not recover due to its the direct competition with the abundance of algae. Herbivorous fish species are therefore very beneficial in maintaining algae populations so that bleached coral can have a chance to recover(ScienceDaily, 2008) However, the frequent warmer waters associated with El Niño climatic events deterred herbivorous fish such as Parrot fish away from areas with an abundance of algae due to coral bleaching as a result of sensitivity to the warmer waters. El Niño climatic events therefore had a negative impact on the potential recovery of bleached corals due to warm sea surface temperatures in the Great Barrier Reef. However the cool sea surfaces associated with the La Niña climatic events allowed for the potential recovery of bleached corals as herbivorous fish such as Parrot fish were able to return into the affected areas.

Conclusion

El Niño climatic events have contributed greatly to coral bleaching between 2013-2019 and thus has had an incredibly negative impact on the northern Great Barrier Reef marine ecosystem. The warmer sea surface temperatures associated with the El Niño resulted in an increase in heat stress on coral which therefore resulted in coral bleaching. Coral bleaching had a detrimental impact on marine life and ecosystems as marine life that fed off of the corals and used the corals for shelter were forced to migrate in order to find new shelter and avoid starvation. El Niño climatic events also contributed to a lack of upwelling which further had a detrimental impact on marine life due to a decline in phytoplankton and therefore a further lack of food. However, La Niña climatic events have contributed positively to coral bleaching and the marine ecosystem in the northern Great Barrier Reef as cooler sea surface temperatures allowed corals affected by heat stress short intervals in which heat stress could be minimized. La Niña climatic events allowed for an increase in upwelling and therefore food shortages were temporarily minimized, cooler waters also allowed herbivorous to return to areas where algae were flourishing and therefore decrease the algae. Algae is in direct competition with corals and the decrease in algae due to herbivorous fish allowed bleached corals to potentially start recovering. Without the La Niña climatic events between the El Niño events during 2013-2019 the negative impact that the El Niño climatic events on coral bleaching and heat stress as well as the negative impacts on shelter and food for marine life may have been exponentially worse. I therefore accept my hypothesis.

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