The annual Gulf of Mexico "dead zone" – a region of oxygen-depleted water off the Louisiana and Texas coasts that's harmful to sea life – will be the second-largest on record this summer, scientists announced Monday.

This year's zone should be about 8,717 square miles, an area roughly the size of New Hampshire, according to researchers at Louisiana State University. The average Gulf dead zone is about 5,309 square miles; the record is 8,776 square miles set in 2017.

A dead zone occurs at the bottom of a body of water when there isn't enough oxygen in the water to support marine life. Also known as hypoxia, it's created by nutrient runoff, mostly from over-application of fertilizer on agricultural fields during the spring.

Nutrients such as nitrogen flow from North America's corn belt through streams and rivers before ending up in the Gulf. Heavy rains fueled near-record flooding along the Mississippi River throughout the spring.

The low oxygen conditions in the Gulf's most productive waters stress organisms and may even cause their death, threatening living resources, including fish, shrimp and crabs caught there.

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A separate forecast from federal scientists at the National Oceanic and Atmospheric Administration also predicted an unusually large dead zone, approximately 7,829 square miles, which is about the size of Massachusetts. NOAA cited "historic and sustained river flows" as a cause of the large dead zone.

Last month, NOAA said discharge in the Mississippi and Atchafalaya Rivers was about 67% above the long-term average.

The Gulf of Mexico dead zone is one of the largest in the world, according to NOAA.

"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 said in a statement. "The bottom line is that we will never reach the dead zone reduction target of 1,900 square miles until more serious actions are taken to reduce the loss of Midwest fertilizers into the Mississippi River system."

Annual forecasts and measurements of the Gulf dead zone began in 1985. There is also an annual dead zone in the Chesapeake Bay.

More:Gulf of Mexico 'dead zone' will persist for decades