Just off the coast of Louisiana and Texas where the Mississippi River empties, the ocean is dying. The cyclical event known as the dead zone occurs every year, but scientists predict that this year could be one of the largest in recorded history.
Annual spring rains wash the nutrients used in fertilizers and sewage into the Mississippi. That fresh water, less dense than ocean water, sits on top of the ocean, preventing oxygen from mixing through the water column. Eventually, those freshwater nutrients can spur a burst of algal growth, which consumes oxygen as the plants decompose.
The resulting patch of low-oxygen waters leads to a condition called hypoxia, where animals in the area suffocate and die. Scientists estimate that this year the dead zone in the Gulf of Mexico will spread for just over or just under 8,000 square miles across the continental shelf situated off the coast.
"When the oxygen is below two parts per million, any shrimp, crabs, and fish that can swim away, will swim away," says Louisiana State University ocean ecologist Nancy Rabalais. "The animals in the sediment [that can't swim away] can be close to annihilated."
Animals like shrimp will often search for more oxygen in shallower waters closer to the shore. Shrimp subjected to hypoxic waters are smaller, their growth stunted by pollution. One study published in 2017 noted how the dead zone affects Gulf Coast shrimpers by driving down the price of shrimp and reducing profit for local businesses.
Dead zones are not unique to the Gulf of Mexico. The Gulf is estimated to be the world's second largest. The world's largest dead zone is the Baltic Sea where low oxygen devastated fisheries, and most marine animals can no longer survive there.
Off the West Coast of the United States, California and Oregon crab and oyster industries have reported profit losses since the early 21st century, saying the annual wave of low oxygen ocean water has destroyed many of the animals they normally fish from the sediment.
Rabalais says she's not surprised that this year's dead zone will be particularly large. Much of the Midwest saw unprecedented rainfall this spring, leading to a large increase in the amount of runoff washing into the sea. Many farmers were so affected by the intense rains that they were unable to plant crops like corn and soybean, meaning all the nitrogen and phosphorus-rich fertilizer they had spread washed into the Mississippi. Scientists are predicting that a warming climate could lead to more extreme rainfall in the region and ultimately make it more difficult to control fertilizer runoff.
"The best way to solve the issue is to limit the nutrients at their source," says Rabalais. "Once they're in the river, there's no good way to reduce them."
