This dead zone is, in large measure, the result of US agriculture and ethanol subsidies, which encourage expanded and more intensive production than would otherwise exist.
In the case of the Mississippi outflow I'm with you to an extent. There's no doubt in my mind that agriculture runoff is the biggest contributor. Unfortunately removing agriculture subsidies won't fix the problem.
A number of people in the Chesapeake basin point the finger at chicken and pig farms in the region due to their controversial use of manure lagoons [1]. To the best of my knowledge these industries aren't heavily subsidized like corn and soy.
Then there's the whole issue of baselining. What's normal for the Mississippi river outflow? We've been observing the impacts to the fisheries there and the Chesapeake for hundreds of years, but we've only been measuring DO for a few decades and DO is far from the only thing that impacts fisheries. Further, the Mississippi dumps a ton of relatively still water that's full of tannins directly into the relatively still and warm gulf. I'm nowhere near qualified to say this conclusively, but I'd bet there have been yearly cycles of hypoxia there for millennia. Does this multiply or dampen our impacts?
This is a crazy complex multi-headed monster that looks completely different in every region it inhabits. For every one of these massive hypoxic regions there's a ton of other smaller ones. Next time you're bored take a look at the waterways in your area on Google maps, especially where small local canals or drainage ditches empty into larger bodies of water. Then go look at the topography of the area and try to back-of-the-envelope account for what all is pouring out of there. Then go do that for a completely different region. The diversity if nutrient inputs is insane.
The only reliable long-term way I can see to fix the problem is to get people to understand the impacts of their actions, and to associate those impacts with the things that they buy, consume, and produce. When you flush the toilet your poop goes somewhere. The same is true with your lawn clippings, farm waste, fertilizers, antibiotics, birth control, etc.
