As the surging waters of the Mississippi pass downstream, they leave behind flooded towns and inundated lives and carry forward a brew of farm chemicals and waste that this year — given record flooding — is expected to result in the largest dead zone ever in the Gulf of Mexico.

Dead zones have been occurring in the gulf since the 1970s, and studies show that the main culprits are nitrogen and phosphorous from crop fertilizers and animal manure in river runoff. They settle in at the mouth of the gulf and fertilize algae, which prospers and eventually starves other living things of oxygen.

Government studies have traced a majority of those chemicals in the runoff to nine farming states, and yet today, decades after the dead zones began forming, there is still little political common ground on how to abate this perennial problem. Scientists who study dead zones predict that the affected area will increase significantly this year, breaking records for size and damage.