It's called Hypoxia.
Hypoxia occurs when the dissolved oxygen level in water is too low to sustain life.
During the last decade, up to 7,000 square miles of the Gulf of Mexico, near the mouth of the Mississippi River, become oxygen-depleted each year. As this organic matter dies, it settles to the bottom and oxygen is consumed as it decomposes. This creates a hypoxic condition. Human-caused hypoxia elsewhere in the world has had serious economic impacts on fisheries. Costs impact commercial fishing, consumers of seafood, tourism, recreational fishing, and non-users discouraged by perceived pollution.