Scientists ask: Where have all the dolphins gone?

You're talking about the dead zones, which is a common occurance, man makes it worse of course, but the dead zones have been there and will always be there long after we're gone. Floods and other natural occurances put nutrients into rivers and streams via run-off which is then taken to the ocean via rivers. Thats why the dead zone in the gulf is so huge(7,000 square miles) but its also run off from the farm lands up and down the mississippi river. When farmers use nitrogen to fertilize their fields,the rain washes it out nearby streams and so on. This year the dead zone was not as bad as expected, it actually improved, but IMO its because of the draught this summer in the southeast.

Dead zones occur all over, the chesapeake bay water shed is another big one. Where all those rivers and streams that make up the watershed wash out into chesapeake bay carrying all of the chemicals and pesticides people use on their lawns. Thus destroying a lot of life in the chesapeake bay.

Here is a map of all the dead zones on the globe(notice the northeaster US).

 

Here is the link to where that image come from... Save the Sea