Oxygen depleted dead zones along ocean coastlines are probably more extensive than previously imagined.

This is the conclusion of a meta-analysis published last week in the Proceedings of the National Academy of Sciences. It found that marine species including crustaceans, bivalves, fishes and gastropods die off at oxygen levels nearly twice as high as the 1983 benchmark for lethal hypoxia, which is 2mg dissolved oxygen per liter of seawater.

Ocean dead zones are caused by fertilizer pollution, usually in river run-off. The nitrogen-rich fertilizer causes algae blooms in coastal ocean waters. When these algae die and decompose, it creates a fiesta of nutrients for oxygen consuming bacteria. The ensuing massive bacterial overgrowth deprives sea creatures of oxygen.

More than 400 ocean dead zones have been discovered in the past 50 years. The total affected area is the size of Oregon. This is tiny compared with the total surface area of Earth’s oceans, but the zones are in areas critical to commercial fishing, and their number has doubled every decade since 1980.

Biologist Robert Diaz of the Virginia Institute of Marine Science has made a career of studying hypoxic zones, though he was not involved in the current study. Reacting to the study’s findings for Wired Science, he said “Everything is pointing towards a more desperate situation in all aquatic systems, freshwater and marine. People should be worried, all over the world.”