Human threats to coral reefs and coral damage

In 2001, it was estimated that 58% of the world’s coral reefs were threatened by human behaviour (Spalding et al., 2001). While there is debate over whether coral bleaching is a naturally occurring event or largely caused by humans, there are many causes of coral destruction that are undoubtedly anthropogenic.

Firstly, overfishing is regarded as one of the main anthropogenic threats to reefs. This has a direct impact on the ecosystem as the balance of producers, consumers and predators is shifted, but the indirect consequences of this are also significant. As many reef fish are herbivores, feeding on algae, removal of these consumers leads to high rates of algal growth on the surface of corals. Unlike the algae living within the coral polyps, these algae are larger, fleshy algae and are incapable of forming a symbiosis with the coral. Instead, they can reproduce at rapid rates and completely cover large surface areas of corals at a very high rate. This can either suffocate the corals, because of the increased levels of algal respiration leading to decreased levels of oxygen, or can block sunlight from reaching the intracellular zooxanthellae (Roberts, 1995). The latter means the zooxanthellae cannot photosynthesise, leading to their death, which can also lead to coral death as the polyps will no longer be receiving nutrients from the zooxanthellae. In many countries, fishing within coral reefs has become such a large source of income that it is being carried out at unsustainable levels. In order to try and allow fishing to proceed at sustainable levels, regulations need to be put in place to prevent overfishing.

Human threats to coral reefs and coral damage (continued)

Dynamite or blast fishing is a method of fishing used extensively in South East Asia that is highly damaging to coral reefs (Spalding et al., 2001). It involves using dynamite or other explosives to create an explosion underwater, to stun or kill fish and bring them to the surface where they can be collected. Often a secondary explosion is carried out in order to kill predators that have been attracted to the sounds and the smells of the first explosion. This method of fishing is devastating to the ecosystem because it instantly destroys the corals and the limestone matrix, the habitat for countless organisms, as well as directly killing any plants or animals in close proximity. Evidence shows that most reefs are somehow affected by fishing, as a study of 315 reefs in 31 countries showed lower than expected levels of ‘indicator’ organisms, even in reefs that had not been affected by factors such as pollution and sedimentation (Hodgson, 1999).

Human threats to coral reefs and coral damage (continued)

Nutrient enrichment is another threat to coral reefs. Because corals require relatively low nutrient levels in the water to maintain a balanced system within the ecosystem, nutrient enrichment disturbs this balance and can have detrimental effects on the ecosystem. Nutrient enrichment is usually a result of either agricultural runoff, or human waste. In both examples, nutrient rich waste enters rivers from factories or farms inland, and flows downstream and enters the oceans. Due to rapidly increasing human populations in communities near to the coast, especially in less developed countries where reefs are most common, this problem is becoming increasingly significant with respect to coral damage. An excess of nutrients in the water encourages algae to grow over the corals. They utilise phosphorus and nitrogen in particular. Phosphorus is used to manufacture DNA and ATP, and nitrogen is used primarily for amino acid synthesis. The consequences of this lead to high rates of algal growth, and therefore zooxanthellae death or coral suffocation. In the Amazon rainforest, for example, due to deforestation, runoff has increased by around 30%, while nitrogen and phosphorus levels in the Mississippi River were around ten times higher in 1997 than they were in the 1960s (Birkeland,1997). This has detrimentally affected coastal reefs in South America. However, some evidence suggests that nutrient enrichment may not be as damaging as other anthropogenic impacts, and may only act at a local level. Nutrient enrichment occurs most often in coastal regions with low water flow, such as in bays, whereas the majority of coral reefs are located in more exposed areas with high water flow (Szmant, 2002).