Aren't ocean dead zones a fertiliser problem, not a meat problem?
Short answer: It's both, and they're the same problem. Most of that fertiliser and manure grows animal feed, which is why animal products dominate eutrophication.
Most of the fertiliser feeding these blooms grows animal feed, not human food, which is why Poore & Nemecek found animal products are the dominant driver of eutrophication. Trace the runoff upstream and you reach the feed trough.
Diaz & Rosenberg, Science (2008); NOAA, Gulf of Mexico
The objection
“Dead zones are caused by fertiliser running off into rivers. That’s a crop and chemical problem, nothing to do with whether I eat meat.”
The answer
The mechanism is exactly as described, so start by agreeing. A dead zone forms when nitrogen and phosphorus from fertiliser and manure wash into rivers and out to sea, feeding algal blooms that die, sink and rot, stripping the water of oxygen until fish and everything else suffocate. The Gulf of Mexico’s dead zone, fed by the Mississippi, regularly swells to thousands of square miles [2][3]. So yes, nutrient runoff is the cause.
The question is what all that fertiliser is for. The answer is mostly animal feed. A huge share of the corn and soy grown across the American Midwest, drenched in the fertiliser that ends up in the Gulf, never reaches a human plate; it’s grown to fatten livestock. Add the manure from the animals themselves, and the runoff and the meat turn out to be two ends of one pipe.
This is why Poore & Nemecek’s global analysis found that animal products are the dominant driver of eutrophication, the technical name for this oxygen-stripping pollution, far out of proportion to the calories they provide [1]. Crops aren’t innocent while meat is guilty. A large fraction of those crops simply exist to become meat.
So “it’s a fertiliser problem” is true but incomplete. Trace the fertiliser upstream and you arrive at the feed trough. Eat lower down the chain and you need far less of the fertiliser, and the dead zone shrinks. The runoff is part of the cost of the steak, just paid offshore where no one’s looking.