Sediments smother corals, forcing them to expend much of their energy to expel these particles. At the same time, the increased turbidity reduces the light available for photosynthesis. As corals gain much of their nourishment from microscopic algae living in their tissues, sediments make corals work harder with less food supply. Oil and petrochemicals are generally buoyant, severely impacting beaches, mangroves and some shallow reefs. The use of volatile dispersants and the presence of sediments to which the oil can adhere causes these contaminants to threaten corals deeper below the surface.
Corals live best in clear waters with low nutrients.
Nutrient enrichment results in rapid algal growth which may cover and smother corals. An overgrowth of algae reduces water exchange and light needed by the corals resulting in bleaching and often death. Altered freshwater flow and watershed contamination in Florida now dumps nutrient enriched waters, replete with agricultural fertilizers and pesticides, into Florida Bay. This reduces dissolved oxygen, kills marine life and spills out onto the Florida Keys coral reefs. Under conditions of natural water flow, many sediments, land-based toxins and nutrients that might reach coral reefs are gradually filtered out by gently flowing streams and rivers, marshes and mangroves.