Scientists say a so-called dead zone in the Gulf of Mexico -- where the water contains so little oxygen, if any, that bottom fish and other creatures can't survive -- has grown about as big as the state of Connecticut.

At about 5,000 square miles, or 13,000 sq kilometers, the zone is the second largest such area on the globe, though it's smaller than it has been in previous years -- when at least twice, say researchers, it grew as large as the state of Massachusetts, at about 8,200 square miles, or 21,000 square km.

Gene Turner, a researcher at Louisiana State University's Coastal Ecology Institute, explained in a report by Reuters that the annual phenomenon is primarily due to excess nutrient runoff from farms along the Mississippi River, which empties into the Gulf.

The nutrients feed the growth of algae, which, in turn, consume oxygen as they work their way toward to the bottom of the Gulf.

"It's a poster child for how we are using and abusing our natural resources," Turner told Reuters.

The Gulf dead zone, which measured as much as 5,052 square miles this summer, is second in size only to a zone in the Baltic Sea, around Finland, Turner said.

The total number of dead zones worldwide currently stands at more than 550 and has been growing for decades, according to a report by Turner and fellow researcher Nancy Rabalais from the Louisiana Universities Marine Consortium.

"The elongated Gulf zone typically hugs the Louisiana coastline from the Mississippi River Delta to the state's border with Texas, and some years extending offshore of Texas and Mississippi, Dr Rabalais said," Reuters reports.

The latest research points to the growth in farmed land along the Mississippi River in the 1960s as the biggest source of increasing pollution.

By the 1970s, oxygen levels in parts of the Gulf fell below the needs of bottom-dwelling fish; the zone has been generally growing ever since.

Floods, droughts, storms and other factors affect the volume of nutrients flowing into the Gulf, accounting for yearly fluctuations, Turner said, although "it seems to have leveled out in size, but it could get worse" again, depending on changes in pollution levels, Rabalais said.

A 2001 federal effort with river states to reduce nutrient runoff has had no substantial success, the researchers reported.