Gulf of Mexico’s oxygen-starved ‘dead zone’ approaches record size

Scientists are predicting a near-record Gulf of Mexico “dead zone” where the water holds too little oxygen to sustain marine life.

“A major factor contributing to the large dead zone this year is the abnormally high amount of spring rainfall in many parts of the Mississippi River watershed,” the National Oceanic and Atmospheric Administration said in a news release Monday. That led to record amounts of water carrying large amounts of fertilizer and other nutrients downriver, it said.

The nutrients feed algae, which die and then decompose on the sea floor, using up oxygen from the bottom up in an area along the coasts of Louisiana and Texas.

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The low-oxygen, or hypoxic, area is likely to cover about 20,200 square kilometres — roughly the size of Slovenia or all the land in Massachusetts, NOAA said. That’s also roughly the area of all the fresh water in British Columbia. A Louisiana-based team has estimated the dead zone will be 22,560 square kilometres.

It will be measured during an annual July cruise by Nancy Rabalais of the Louisiana Universities Marine Consortium.

The record set in 2017 is 22,700 square kilometres.

Scientists had said earlier that widespread flooding made a large dead zone likely this year.

A task force of federal, tribal and state agencies from 12 of the 31 states that make up the Mississippi River watershed set a goal nearly two decades ago of reducing the dead zone from an average of about 15,000 square kilometres to an average of 4,900.

“While this year’s zone will be larger than usual because of the flooding, the long-term trend is still not changing,” University of Michigan aquatic ecologist Don Scavia, professor emeritus at the School for Environment and Sustainability, said in a news release. “The bottom line is that we will never reach the dead zone reduction target of 1,900 square miles (4921 square kilometres) until more serious actions are taken to reduce the loss of Midwest fertilizers into the Mississippi River system.”

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Rabalais has been measuring the hypoxic zone since 1985.

Storms before last year’s mapping cruise reduced that hypoxic zone to about 7,040 square kilometres, about 40 per cent the average size that had been predicted, and among the smallest recorded.