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).