Every summer for the last three decades, researchers have cruised the northern Gulf of Mexico during July to study the extent of hypoxia, or low oxygen levels. This summer, they found the largest area ever on record: 22,720 square kilometers, about the size of New Jersey.

This year's "dead zone," where oxygen levels are so low they threaten fish and other small aquatic life, is about 50 percent larger than normal. The average size of the dead zone over the last 31 years has been 14,037 square kilometers, according to Nancy Rabalais, a researcher at Louisiana State University who has long studied the issue. The dead zone was likely even larger than what the scientists found, but there was insufficient time on board the ship to measure its entire extent.

Based upon the hypoxia report released this week, this year's large dead zone was driven primarily by high nitrogen loads from the Mississippi River, due to heavy use of fertilizers in the midwestern United States. In some locations conditions were especially extreme. "A notable feature of this year’s distribution of low oxygen is the mostly continuous band of extremely low oxygen concentrations alongshore at the nearshore edge of the zone," the report states. "Values there were very often less than 0.5 milligrams per liter and close to 0 milligrams per liter."

The definition of hypoxia is 2 milligrams of dissolved oxygen per liter, or lower. Heavy river flows cause this problem when excess nutrients in the water create large algae blooms, which act in concert with plankton to create more food for fish, which are then consumed by other marine life. However, when the blooms get too large, they consume all of the oxygen in the water, creating low oxygen levels. This can kill fish unable to swim away.

This is what is happening every summer in the northern Gulf of Mexico. Annual measurements of the hypoxic area's size have allowed scientists to find a direct causal link between the dead zone and Mississippi River flows. And now, other researchers have begun to document the economic effect of hypoxia on Gulf fisheries, such as higher prices for shrimp when dead zones reach their maximum extent.