Sentence:  By the time the Mississippi enters the gulf, its current has been transformed into a conduit for chemical nutrients, and this enriched current stimulates massive blooms of phytoplanktons. Consequently, it forms a <WSD>dead zone</WSD> the size of Massachusetts (7,900 square  miles or 20,461 square kilometers) every summer, which has existed since the 1970s and supports almost no life beyond phytoplankton and bacteria. 
Meaning of the word marked with <WSD> tags:  An area of the ocean where oxygen levels are too low to support life, especially as a result of pollution.