by Fatima Ammar (2018)
Introduction
Coral reefs are amongst the most biodiverse ecosystems in the world. They are depended upon by several hundred million people in the world for food, jobs, and protection against storms and erosion (NOAA Gov, 2017).
Coral bleaching, a large scale event that causes significant damage to the reefs, has a global impact on economy, tourism, fishery, and ecology. Corals produce over 70% of the world’s oxygen supply meaning that serious endangerment of the coral reefs could be hazardous to life. The rapid decline of coral reefs has shown to cause the decline of multiple other marine species thus disturbing the natural balance within the marine ecosystem (NOOA, 2017).
Observing the biggest threats to coral reefs will help better understand the importance of preserving the reefs and how to prevent this decline. It will also show how these factors cause corals to bleach and why this happens (Vaughan, 2017).
The leading research on coral growth and bleaching utilises the latest technologies to measure photosynthesis, photoinhibition, growth, and adaptability. Pulse-Amplitude Modulated Fluorescence spectroscopy (PAM), a non-invasive method, has been used in recent years to note how zooxanthellae react to different lighting spectrums and even observe photoinhibition at a more intimate level by measuring chlorophyll fluorescence and the stress levels (non-chemical quenching) of the algae under high lighting or heating (Muller et al, 2001). These methods produce the most accurate and precise results possible.
Bleaching occurs when corals become stressed. Bleaching is when a stressed coral dispels its microalgae from its polyps. This is thought to be a response to photosynthetic damage within the symbionts. It is named bleaching as colourful specimens become ghostly white when they lose the photosynthetic pigmentation provided by the zooxanthellae. Usually this will result in the death of the coral host as it relies heavily on the endosymbiotic relationship with the dinoflagellates (Guldberg, 2011; Welle, 2017).
Bleaching inhibits coral growth and can disrupt reproduction which reduces the abundance of many species. If conditions are altered, these effects may be temporary as some zooxanthellae may return to host polyps. However, recovery requires a long period under optimal conditions which is why such efforts are almost uniquely successful in controlled lab settings alone (Guldberg, 2011; Vaughan, 2015).
Frequent and mass bleaching events are thought to cause disease outbreaks such as what is occurring within the Great Barrier Reef. Some genetic types of microalgae are thought to be more resistant to increase in temperature or lighting than others but this is yet to be studied further (GBRMPA, 2017).
Environmental Changes
Climate Change:
Research conducted by Ove Hoegh-Guldberg (1999) shows that elevated sea temperatures explain many incidents of mass bleaching and that there are other causes which cause a gradation in bleaching intensity within a colony. Limits to coral growth are much shallower in areas where sedimentation reduces the transmission of light through the water column or smothers corals. He also confirmed that photosynthetic activity of heat-stressed coral is drastically reduced.
The ideal water temperatures for coral survival are between 72-78° F, or approximately 24-28˚C. Over the past century, water temperature in the tropics has increased by 1°C.
Chemical Changes :
NOAA PMEL Carbon Program (2017) has alleged that when carbon dioxide (CO2) is absorbed by seawater. Chemical reactions occur that reduce the pH and carbonate ion concentration. This is called ocean acidification or “OA”. Around 30% of all CO2 is absorbed by seawater and the atmospheric increase in CO2 due to climate change means an increase of that amount yearly.
Calcium carbonate minerals are the building blocks for the skeletons of corals. Continued ocean acidification is causing many parts of the ocean to lose vast quantities of these minerals, which is likely to affect the ability of corals to produce and maintain their shells.
Sedimentation
While sedimentation is not usually a climate factor (hurricanes and cyclones do cause much sedimentation and also the accumulation of debris such as plastic in oceans see below), it is often caused by human activity such as urbanisation and farming which are activities known to contribute to greenhouse gas emissions and thus climate change (Martin, 2011).
Terrigenous sediment deposition on coral reefs greatly impacts survival chances as it can lead to stunted growth due to light blocking which inhibits photosynthesis. It can also cause suffocation (McClenachan et al; 2017).
Sedimentation is the major stressor to reefs in Hawaii, Guam, and the Northern Mariana Islands as well as being one of the major destructive components of various other reefs around the Americas (U.S. Commission on Ocean Policy, 2004).
Hurricanes, Tsunamis and Cyclones
Climate change is suggested to increase the frequency of hurricanes and tsunamis (USGS 2017). Coral reefs protect shores from flooding. However rising sea levels due to an increase in storm activity reduces this protective attribute significantly and fierce storms cause the mechanical destruction (breakage) of coral skeletons which leads to the spreading of disease throughout the reef (McClenachan et al, 2017; Herron et al, 2014).
In May of 2009, a 7.3-magnitude earthquake in the west Caribbean destroyed half of the Belizean Barrier Reef (second largest reef ecosystem in the world) lagoon’s corals reefs by an upheaval that displaced them into deeper waters (Martin, 2011).
Cyclones can cause a lowering of water salinity due to an increase in alkalinity, sudden changes in temperature, an increase in turbidity and sedimentation leading to erosion which all cause stress and decline of reefs (Vivien-Harmelin, 1994).
Marine debris, which is usually caused by ‘dumping’ and pollution from shipping barges and factories, can be caused by hurricanes too. Large objects can block out light which results in photo-inhibition, thus starvation and bleaching of corals. Heavy objects crush coral skeletons (NOAA Marine Debris Program, 2018).
Central Role of Symbioses in Coral Reefs
Corals have a symbiotic relationship with microscopic algae that live in their tissue. These single-celled algae are called zooxanthellae. Zooxanthellae photosynthesise to provide the coral with its pigment and translocate up to 95% of their photosynthetic production to it such as amino acids, complex carbohydrates, and sugars. Coral supplies the dinoflagellates with crucial plant nutrients such as ammonia and phosphates. Both are dependent on the other for survival.
When the symbiotic relationship becomes stressed due to ocean acidification caused by high CO2 emissions and debris or increased temperature; the algae leave the coral tissue. Without its algae, the coral loses its major food source and its colour. It is more susceptible to disease and will become bleached and vulnerable unless temperature is reduced (NOAA, 2017).
Acclimatisation and Adaptation
Reef-building corals have effectively evolved necessary adaptation and acclimatisation mechanisms over time yet this has not prepared them for the sudden and massive destruction due to the acceleration of climate change and non-natural causes which is why they are at their limits under these modern conditions.
Samoan lagoons are the unlikely home to acropora corals which are notorious for being the most sensitive to extreme temperatures. Temperatures can average at 35°C for a few hours a day. This means that they must have adapted to not only survive the heat but to thrive in it too (Mascarelli, 2014).
In the Arabian Gulf, corals can survive 36°C. Corals may adapt to tolerate these temperatures by altering their biochemical pathways. Locally, this is probably due to acclimatisation. Where the corals grow scattered across widely separated geographic areas in the Red Sea, there may be a larger component of genetic selection, especially where local tolerance to extreme conditions is involved (Veron, 2013).
Researchers are studying ways to achieve ‘human-assisted evolution’ by creating hardy, resistant corals in controlled settings to be introduced to the wild and rebuild destroyed reefs (Vollmer 2006; van Oppen, 2006).
Discussion
There is much controversy as to various anomalies discovered in data concerning coral stress and bleaching. Some argue that cyclones and hurricanes caused much reef damage during some data collation meaning climate and pollution are not wholly to blame for results. De’ath et al (2012) suggested the damage was mostly due to cyclones and unusually large swarms of crown-of-thorns starfish (Acanthaster planci), which eat reef-building corals.
Although no one is yet attempting to create genetically modified corals, some researchers are concerned that human-assisted evolution is disruptive of the natural order. “If you’re basically farming a reef, you’ve taken a natural habitat and you’ve converted it.” (Steve Vollmer, 2014).
It is evident, however, that the recent frequency and intensity of mass coral bleaching are of major concern and are directly attributable to rising atmospheric greenhouse gases. This has been shown in various researches and is a continuous trend with climate change (Hoegh-Guldberg et al, (2007).
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