A definition by Nixon (1995) states eutrophication to be ‘An increase in the rate of supply of organic matter to an ecosystem.’ Coral reefs are in a delicate balance in a very nutrient poor environment so an apparent new source of nutrients can disrupt this balance and cause problems. Eutrophication can be caused by many factors; an increase in nutrients, a decrease in turbidity of the water or a decline in the number of grazers (Nixon 1995). All these reasons cause an algal bloom above the level for what is ‘normal’ for a reef in balance. These levels are what the corals have adapted to withstand (Muscatin & Porter 1977)
The input of nutrients can be due to surface run-off from agricultural land, sewage pipelines from poor infrastructure within a city or fish farming. Fish farming (Figure 3) has emerged as an activity by humans that can increase the amount of sediment on a fringing reef and therefore promote eutrophication. Fish farming is usually done within pens that are suspended above the seabed. A study by Villanueva et al. (2005) found that the waste matter from a fish farm in Bilinao, the Philippines, prevented the growth of reef building corals (Seriatopora caliendrum) and even killed the juvenile corals which were placed there. The build up of sediment and algae were the cause as they decreased the amount of light and space available. The investigation found higher levels of ammonia and phosphate as well as decreased water transparency and oxygen levels at the site of the farm.
Conversely Bongiorni et al. (2005) investigated fish farms but found that an Acropora species increased in weight and volume more so than the reference site which was in an area of no extra nutrient input. These two ideas, although contradicting, could provide insight into the resilience and adaptations of different species of coral in different areas. With this knowledge, where to place the fish farms in the future could greatly improve the state of coral reefs within that area by avoiding damage to the corals and even in some extreme cases improving them.