Water flowing from agricultural, domestic and industrial areas run off into the rivers and then to the sea. The run off water carries dissolved fertilizers and nitrates. Flourishing of macro and unicellular algae can change the balance of organic matter in the sea. Organic matter accumulates in surface water and the seabed. An accumulation takes place in the silt of high concentrations of organic matter, low oxygen levels and a high concentration of H2S. As a result, many types of invertebrates and fish become smothered and disappear.