Human Threats to Coral Reefs and Coral Damage (continued)

Sedimentation can also occur, which can have similar effects on corals to algae preventing sunlight from reaching the corals. There are a number of causes of sedimentation, such as soil erosion, dredging, deforestation and construction. Sediment as a by-product from activities such as these can suspend in the water and block light reaching zooxanthellae. Unfortunately, the majority of sediment is entering the oceans in developing countries with high coral reef densities (Birkeland, 1997), therefore the sedimentation is having much more of an effect on the ecosystem than it would otherwise have in regions with less coral reefs. The countries producing the highest levels of sedimentation include those in South East Asia and in the Western Pacific. In fact, almost 100% of all the sediment entering the oceans originates from these areas (Birkeland, 1997). Sediment can also form a thin film on the upper surface of coral, which can be removed by direct movement of the polyps, via secretion of mucus from the mucus glands within the polyps, or tissue swelling (Weber et al., 2006). However, these solutions use up resources, and therefore if these defensive mechanisms are sustained due to settling sediment over long periods the polyps may become damaged, and can die in extreme cases. Damage to the coral, and its recovery, is directly related to the sediment type as well as the quantity (Weber et al., 2006). High nutrient sediment has been found to lead to greater stress levels in corals than lower nutrient sediment. In Puerto Rico, reefs subjected to high levels of sediment influx had significant loss of coral species in comparison to other locations. Most of the species affected usually reside in other geographical locations in deeper water, from 25-30m, but due largely to the lack of light caused by the sediment these coral species cannot survive, leading to reefs occurring only in shallower waters (Acevedo et al., 1989).