CHICAGO, June 7 (Xinhua) -- U.S. researchers forecast that the hypoxic zone or "dead zone" this summer in the Gulf of Mexico will be approximately 5,780 square miles, about the size of the state of Connecticut.

The 2018 forecast, released by the National Oceanic and Atmospheric Administration (NOAA) Thursday, is similar to the 33-year average dead zone of 5,460 square miles, but is smaller than the 8,776-square-mile dead zone reported in 2017, the largest measured since mapping began in 1985.

Nevertheless, the forecast remains three times larger than the long-term target set by the Interagency Mississippi River and Gulf of Mexico Hypoxia Task Force, a group charged with reducing the size of the dead zone.

The Gulf's hypoxic zone is caused by excess nutrient pollution, primarily from human activities in the watershed, such as urbanization and agriculture.

The excess nutrients stimulate an overgrowth of algae, which then sinks and decomposes in the water. The resulting low oxygen levels near the bottom are insufficient to support most marine life.

Studies have shown a multitude of other impacts within watersheds where nutrient concentrations are high, including high nitrates in groundwater, higher drinking and wastewater treatment costs, and wasted fertilizer applications.

NOAA issues a dead zone forecast each year. The forecast is based on nitrogen runoff and river discharge data from the U.S. Geological Survey.