Coral reefs are often located near areas of high population and little infrastructure. The lack of infrastructure often results in raw sewage, industrial pollutants and agricultural runoff entering the water column, which increases the nutrient concentration of the water dramatically, which previously would have had a very low concentration. Corals are adapted to survive in low nutrient waters (Muscatine & Porter 1977) hence high nutrient levels cause problems for the corals.
Coral and algae cover follows a gradient in water quality, coral cover decreases and algal cover increases with decreasing water quality (Fabricius et al., 2005). Increased sewage results in a decrease of water quality, as was observed in Kaneohe Bay, Hawaii by Smith et al. (1981). Kaneohe Bay was the recipient of large increases in sewage between the 1950s and 1977, when the sewage was diverted elsewhere. Here, they found, coral cover, species richness and calcification rates decreased significantly while algae biomass, especially Ulva and Hydroclathrus, increased (Figure 2). The sewage was also found to cause an increase in the green bubble algae, Dictyosphaeria cavernosa, which causes severe coral mortality.
Greater nutrient input onto the reefs is only one of the many threats to modern reefs. Increased sewage results in a general weakening of the reef’s resilience so it may become further disturbed by other threats such as climate change or the increase in sewage may prevent a reefs recovery from these events (Hughes et al., 2003).