Earth Observatory: The size and number of marine dead zones -- areas where the deep water is so low in dissolved oxygen that sea creatures can't survive -- have grown exponentially in the past half-century. Red circles on this map show the location and size of many of our planet's dead zones. Black dots show where dead zones have been observed, but their size is unknown.
It's no coincidence that dead zones occur downriver of places where human population density is high (darkest brown). Some of the fertilizer we apply to crops is washed into streams and rivers. Fertilizer-laden runoff triggers explosive planktonic algae growth in coastal areas. The algae die and rain down into deep waters, where their remains are like fertilizer for microbes. The microbes decompose the organic matter, using up the oxygen. Mass killing of fish and other sea life often results.
