Photosynthesis requires sunlight, and the depth and intensity of sunlight penetration is reduced by suspended sediments. Silt can also settle out on corals and interfere with feeding and recolonization. Corals can survive occasional short-term siltation events. When stressed they produce mucus, which helps them shed the fine-grained sediments that have settled upon them. However, repeated or chronic silt plumes or a single large event will kill coral.
Nutrient loading is also harmful to coral reefs.
Excess nutrient levels in coastal waters can lead to algal blooms , which compete with coral colonies for space and light and disrupt the coral reef ecosystem.