SOUTH-EAST of New Orleans, where the Mississippi empties into the Gulf of Mexico, the North American land mass does not end so much as gently give up. Land subsides to welts of green poking up through the water, and the river grows wider and flatter until it meets the ocean, where a solid line divides the Mississippi's brown water from the gulf's blue.
On its long journey south the water has scooped up nutrients such as nitrogen and phosphorus, mainly from the fields of the Midwest. So much so that agriculture's gift to the gulf is a 'dead zone'. The excess nutrients cause algae to bloom, consuming all the available oxygen in the sea, making it hostile to other forms of marine life. Creatures that can swim away, such as shrimp and fish, do so; those that cannot, die. In the four decades since the dead zone was discovered it has grown steadily. Today it covers 6,700 square miles, an area larger than Connecticut.
This ecological disaster area imperils the region's commercial and recreational fisheries, worth around $2.8 billion a year. One study suggests yearly shrimp-fishery losses of nearly 13%. The dead zone drives shrimp farther out to sea, making it costlier and more time-consuming to catch them. It also makes them smaller.