A giant underwater “dead zone” in the Chesapeake Bay is growing at an alarming rate because of unusually high nutrient pollution levels this year, according to Virginia and Maryland officials. They said the expanding area of oxygen-starved water is on track to become the bay’s largest ever.

This year’s Chesapeake Bay dead zone covers a third of the bay, stretching from the Baltimore Harbor to the bay’s mid-channel region in the Potomac River, about 83 miles, when it was last measured in late June. It has since expanded beyond the Potomac into Virginia, officials said.

Especially heavy flows of tainted water from the Susquehanna River brought as much nutrient pollution into the bay by May as normally comes in an entire average year, a Maryland Department of Natural Resources researcher said.

As a result, “in Maryland we saw the worst June” ever for nutrient pollution, said Bruce Michael, director of the DNR’s resource assessment service.

That’s bad news for biologists who monitor the bay and horrible news for oysters and fish. Dead zones suck out oxygen from deep waters and kill any marine life that can’t get out of the way.

Nutrient pollution from chemicals such as fertilizers provide a feast for bay algae, which bloom and die in a rapid cycle. They decompose into a black glop that sucks oxygen out of deeper waters.

Oysters and other shellfish are doomed in dead zones. Fish and crabs can skitter to surface waters where there’s more oxygen, but some don’t make it, Michael said.

Donald Boesch, president of the Center for Environmental Science at the University of Maryland and an expert on dead zones, said this year’s water flow will rank at least among the five largest, a result of heavier-than-normal rains and snow melt mixed with high amounts of nitrogen, phosphorus and sediment.

Dead zones are a yearly occurrence caused by pollution in water that runs off cities and farms. They form in summer and usually dissipate in fall, sucking oxygen from deep waters and leaving dead oysters, clams, fish and crabs in their wake.