The guys that made King Corn (slightly soft sciencey imho but good to get under your belt) have produced a follow up that uses the premise of tracing the runoff water from their (and everyone's) corn farming in Iowa down the Mississippi to the Gulf and then looks at the potential dead zones created by the fertilizers used. Worth seeing no doubt. At 30mins long this is designed for grassroots screenings in schools or community venues. More at Big River Film.
Really like the image for the publicity material as well, a similar illustration style as the King Corn stuff. Can't find who did it though... ??