Dead-zone

Any body of water which has become devoid of oxygen, and typically eutrophied, so that it can no longer support a balanced eco system and everything in it dies: except perhaps an undesirable monoculture, like stinging jelly-fish. There is a 22,000 km dead zone in the Gulf of Mexico (and many more around the planet) resulting from the nutrients poured into it by the Mississippi River, which gathers up polluted water right from its headwaters. How do we restore dead-zones? One floating island at a time!