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Human Impact On Coral Reefs In The Last 30 Years
Coral reefs are the homes that provide shelter for approximately one quarter of all known marine species as well as being the most diverse ecosystems. Coral reefs are vulnerable to even the slightest environmental changes. A review of the literature of the last 30 years shows that the frequency of human activity has intensified to a new level where human actions have extended to the coral reefs on a global scale. Humans are actively damaging the coral reef ecosystems on our earth. Coral reefs are overfished, poisoned by chemical pollution, smothered by sediment, and choked by macroalgae growing on nutrient rich sewage and fertilizer runoff. In the meantime, global warming and ocean acidification is a greater contribution to coral reef mortality. The main driving force behind the coral crisis is the continuing increase in human population. These will give a burden to environment, and eventually will impact on marine life, especially the coral reefs. Through the review of the past papers, I found that from the least and latest topic of ocean acidification to the largest one (sedimentation and nutrient enrichment), scientists are getting concerned about the problem relate to carbon dioxide which results in climate change and ocean acidification globally, especially in the recent years. Lots of coral reef scientists have risen to this key issue.
“Coral reefs are evaluated to cover 284,300 square kilometers, with the Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) accounting for 91.9% of the total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs only account for 7.6% of world total”. (Spalding, M., Ravilious, C., and Green, E. (2001). World Atlas of Coral Reefs. Berkeley, CA: University of California Press and UNEP/WCMC)
It is reported that Australia, Indonesia, Philippines, Papua New Guinea, Fiji contain almost over half of the world's reefs. It is now generally acknowledged that coral reefs are among the most threatened global ecosystems, and among the most vital (Costanza et al. 1997; Bryant et al. 1998; Boesch et al. 2000; Reaser et al. 2000; Wilkinson 2000). However, one of the upcoming crises is a major threat of death of corals and large scale damage of coral reefs in all over the world. This crisis is partly a result of the frequency of human activities which are having a negative effect on marine ecosystems, especially on coral reef ecosystems.
A study from 1998 showed that coral reefs around the world were affected by human activities (Lauretta B. et al. 1998, Fig. 2). The study showed the relative proportion of reefs at risk in five regions, with Southeast Asia having the highest area at risk. This phenomenon probably is related to the overfishing in this area which leads to coral reefs being at the highest risk compared to the rest of the world. It is known that the current situation is directly or indirectly consistent with human activities. However, the question remains that to what extent has human activity changed the coral reef ecosystems? So in this report, I would use ISI web database to search articles related to coral reefs crisis. Basically, I will try to answer this question. To do this I will discuss the various threats to coral reef, including overfishing and fish live trade issue, pollution-including terrestrial runoff, chemical compounds and sedimentation, and macroalgae overgrowth which is causing by nutrient enrichment under the seawater, and global warming and ocean acidification which contribute to coral disease and coral mortality.
Impacts Of Overfishing
It is sure about that overfishing in general is a damaging problem to many coral reefs around the world. In this field, it is shown that scientists did research in the topic of overfishing and fish trade was increasing since 1990. (Figure 3)
In recent years, as a result of human interference on coral reefs, coral mortality is higher than ever before. When this occurs, the algae immediately take this opportunity for growth, which results in seedlings of coral not being able to attach to the reef. This type of event highlights the important role of herbivorous fish on the restoration of coral reefs. But if herbivorous coral reef fish are captured in large numbers, ethnic groups are difficult to recover. In this situation, macroalgae continues to overgrow, and inhibits the growth of corals.
For example, The Caribbean region has been hit particularly hard, with 40% of absolute coral cover lost since the late 1970s due to overfishing and macroalgae overgrowth (Gardner et al. 2003). So some authors argue that overfishing is an important reason why reef corals have declined in recent years. (Aronson et al. 2006)
However, overfishing has also been due to the improvement of global fishing technology, and a variety of advanced equipment is used in the exploitation of fishing. A large number of fish becomes extinct annually. As a result of advanced fishing technology, the world's annual catch of fish is continuing to rise. Indeed, on one hand, increased fishing will provide sufficient global supply of fish products. However, at the same time, the increase in fisheries production will reduce the global average price of fish in the global market, and this decline will lead to the reduction of fishing profits, which in turn will drive an increase in catches again, ultimately leading to a vicious circle in fisheries. On the other hand, large scale fishing can lead to the sharp decline in fish population. Without predators, algae can overgrow and suppress coral recruitment. Even though, some scientists argue that these algae are not directly killing corals. In fact, they are competing with corals in the space and intake of light. So without light, corals will die eventually due to corals can not get nutrients to survive. (Figure 4)
A major threat besides the overexploitation of fishery resources is undoubtedly the strong increase in coastal development, and discharge of untreated sewage into the near-shore waters, resulting in enormous amounts of nutrients spreading into the sea and coastal zones (Burke et al., 2002; Wilkinson, 2002; Brown et al., 2006; UNEP, 2006). For example, around 60% of the wastewater discharged into the Caspian Sea is untreated, in Latin America and the Caribbean the figure is close to 80%, and in large parts of Africa and the Indo-Pacific the proportion is as high as 80-90% (UNEP, 2006). Agricultural run-off to the ocean, Nitrogen exports to the marine environment is projected to increase at least 14% globally by 2030 (UNEP, 2006).
During the past 30 years, more and more scientists were on to the problems of sedimentation and nutrient enrichment and chemical pollution and oil spills. It is shown that a big number of topics on pollution and nutrient enrichment and so on. (Figure 5 and 6)
Basically, the scientists wanted to understand how did sedimentation and nutrient enrichment and chemical pollution and oil spills impact on the corals? There are various mechanisms by which these factors can harm coral reefs. First of all, suspended sediment makes water turbid which allows less sunlight to penetrate the water. With less sunlight, zooxanthellae which live within the coral's tissue can not photosynthesize to produce organic nutrients that support the corals to thrive. In addition, it is known that zooxanthellae are sensitive to chemical changes (Parker et al. 2008). In industrial wastewater and farming runoff, there are always chemical compounds which can make zooxanthellae toxic to corals as well as in the case of oil spills. So to protect themselves, corals have to eject the zooxanthellae (Parker et al. 2008). After a period in this situation, corals will die. Furthermore, farming near the coast results in large quantities of sediment and soil going into sea and onto coral reefs. This dirt, silt or sand can make water muddy, smothering the corals (Rogers 1990). In addition, the use of fertilizers for farming is another problem resulting in increased nutrient flow into the ocean. The discharge human sewage leads to the nutrient enrichment in certain areas, especially in the estuary, which may result in rapid growth of algae which as mentioned before they will outcompete corals, cutting off the supply of light as well.
In this area, it is shown that more and more conservationists were interesting in ocean acidification in recent years (Figure 7). It seems a very new field that scientists have only focused on just 7 years ago. It might be scientists aware of the issue of acidification rising, which is now noticing that acid level rising makes an impact on coral reefs.
Ocean acidification is related to the carbon dioxide being released in to the atmosphere and reacting with seawater. The concentration of carbon dioxide in the Earth's atmosphere now exceeds 380 ppm, which is more than 80 ppm above the maximum values of the past 740,000 years (Petit et al. 1999; EPICA community members, Nature. 2004). Carbon dioxide reacts with water to form a weak acid H2CO3, which results in acid levels increasing in the oceans of the world. Ocean acidification is a current key problem that all marine animals have to face, especially coral reefs. As previous mentioned that corals get organic nutrients from a mutualistic symbiont - zooxanthellae to grow. These algae are sensitive to acid rising in the seawater. If the environmental conditions are changing, corals will eject algae and turn to white, a phenomenon known as coral bleaching.
With respect to global warming, corals are sensitive to the sudden temperature changing as well, which means the change of temperature is a factor causes their photosynthetic process to break down. When this happens, zooxanthellae become toxic to corals. Then, if they want to protect themselves, the corals have to expel the zooxanthellae. And they will get starved and become bleaching. As is figure 8 shown that another very important threat affecting coral reefs is global warming. The rise in temperature can result coral bleaching.
It is usual to see coral bleaching just temperature a little higher than hottest summer temperature (Parker et al. 2008). Large-scale bleaching events seem to be becoming increasingly common (Wellington et al. 2001). If temperature continues to increase, in the future, this means that coral bleaching events may increase their frequency an extent of severity. Even though populations adapt and corals do survive, this pressure increases their susceptibility to disease and reduces their ability to reproduce.
From 11393 articles found in the literature search on the topics of coral reefs, one eighth of papers are related to the impact of human activities on coral reefs (Figure 9 and Figure 10). Of course, there should be more factors impacting on coral reefs and have contributed to their mortality. Indeed, the coral reef crisis in all over the world is getting worse and worse, even in the future the coral reef would face further threats. But this does not mean we have no ideas to save them, the figure 11 also shows that marine biologists were concerned about the coral reef crisis more than before, larger possibly because more and more scientists were aware of the importance of coral reefs to the entire marine ecosystems. Protecting coral reef ecosystems probably is an effective pathway to get most species recovered.
During the past 30 years, marine ecosystems took place some changes. The interference of human activities directly or indirectly gives more and more burden for our earth. The damaged environment makes some species become threatened species or endangered species, even critical endangered species. Even though, coral reefs are not becoming extinct, they are facing high risk which leads them to die. Indeed, the world lost a large scale of coral reefs which all due to the humans since early 20th century. From the review of the literature of the past 3 decades, scientists are focusing now more on climate changes, including global warming and ocean acidification, which results in corals death. Compared to the previous large number and key issues, like fishing and pollution problems, scientists are beginning to find more new factors that influence the coral reefs. From this trend, we can see that there should be more work to do to find a solution to marine scientists. With the population continuing to rise, it is difficult to imagine what will happen in the near future. For the humans, perhaps, they have to think about what they can and should do to reduce the dead of corals and the damage of coral reefs. To find a solution to solve the coral reef issues, humans have to give more actively motivations and passions to change the current situations.