Dead zones are a global water pollution challenge – but with sustained effort they can come back to life

Scientists have identified a dead zone as large as Florida in the Gulf of Oman, which connects the Arabian Sea to the Persian Gulf. Around the world there are more than 400 current dead zones in oceans and lakes, where water contains so little oxygen that aquatic life can’t survive.

Dead zones form when aquatic organisms consume dissolved oxygen faster than it can be supplied. This typically happens when warmer water sits on top of colder water, or freshwater sits on top of saltier water - for example, where a river meets the sea. In either case the water on top is less dense and floats. The layers don’t mix much, so very little oxygen from the atmosphere reaches the lower layers.

https://www.sfchronicle.com/news/article/Dead-zones-are-a-global-water-pollution-challenge-12887295.php