Fishermen seeking bottom dwellers such as crab and summer flounder can rule out dropping a line in much of Narragansett Bay.

Teams of researchers measuring levels of dissolved oxygen in the Bay discovered Thursday that fish and crabs cannot currently live on the Bay bottom in all 53 places they tested from Providence down to just north of Jamestown because there was not enough oxygen.

In every place that they tested, scientists found that the water below 4 meters -- about 12 feet -- had less then 3 milliliters of dissolved oxygen per liter of water, a condition known as hypoxia.

All water below 12 feet was hypoxic, meaning that it did not have enough dissolved oxygen to support fish and crabs. Clams, particularly the hardy quahog, can withstand these types of conditions for a month or more, but bottom fish must move out of that entire region or die.

The state's director of Environmental Management, W. Michael Sullivan, said that global warming was one of many factors contributing to the Bay's depleted oxygen levels. Sullivan said the annual average temperature of the Bay had increased by 3 degrees in 50 years, which, combined with nitrogen from sewage and a cut in tidal flow due to sedimentation, had stimulated the growth of algae and plankton. When that plant growth dies and decays, it consumes oxygen.

Thursday's sampling found oxygen levels above four meters were generally healthful, so fishermen will still see striped bass and blue fish chasing bait near the surface.

Chris Deacutis, chief scientist for the Narragansett Bay Estuary Program, said, "There are probably pockets where it's not all that bad" along the bottom, but teams from the Estuary Program and from Brown University did not find any of those pockets in Thursday's sampling.

"If you were down around Jamestown, I still might try [bottom fishing], and the Quonset area, where there's some pretty good current, I might still try the bottom," Deacutis said. "Your fishing luck may be a lot lower in the upper third to upper half of the Bay to . . . down around Quonset. I wouldn't say Jamestown because Jamestown does have some pretty good water in the ship's passage.

"The upper Bay and the Providence River definitely have pretty bad water on the bottom, and Greenwich Bay does, too."

Deacutis said Greenwich Bay had "practically no oxygen" beyond 10 feet. Readings there were as bad or worse than the "dead zone" of the Gulf of Mexico, where oxygen levels are typically less than 2.

Although it may sound dramatic to say that much of Narragansett Bay's bottom cannot support fish, the current outbreak of hypoxia is not unusual. For at least the past few summers, a wide swath of the Bay bottom has become barren of fish.

"It's not unexpected," Deacutis said of Thursday's readings.

Tests taken July 6 also showed widespread hypoxia on the Bay bottom, although not as extensive as the current outbreak. And in August 2003, when schools of fish washed up dead in Greenwich Bay, scientists think the hypoxic area extended all the way to the Jamestown Bridge.

Scientists say these factors contribute to hypoxic conditions:

"Nutrient loading" from sewage flowing into the Bay through treatment plants, septic systems, and from lawn fertilizers. Nitrogen from these sources stimulates plankton and algae growth; when these organisms die, they decompose and suck up oxygen.

Scott Nixon, an oceanographer at the University of Rhode Island, cautions against blaming all of the Bay's low oxygen cycles on pollution. He has noted that Rhode Islanders have been dumping sewage into the Bay for more than a century without causing fish kills of the kind seen in 2003.

High water temperatures. Warm water stimulates plankton and algae growth. It also holds less dissolved oxygen than cold water. Sullivan, the DEM director, said global warming has driven average annual Bay temperatures 3 degrees above their 1950s levels. During the seven-day heat wave that broke yesterday, Bay temperatures jumped 5 degrees, to the high 70s. At midweek, John Torgan of Save The Bay recorded a temperature of 81 degrees in the shallows off Gaspee Point, where dead baby clams washed ashore ankle deep in places.

Calm winds. Wind infuses surface waters with oxygen and churns the water, mixing oxygen into the depths. The 2003 fish kill was preceded by several days of relatively calm weather.

Hard rains. Rain causes hypoxia by washing nutrients into the Bay, and by "stratifying" the water into surface waters of freshwater buoyed by bottom waters of denser saltwater. When water stratifies this way, it is more difficult for winds to infuse oxygen through mixing.

"Neap" tides. There are two types of tides, spring and neap. Spring tides have nothing to do with the season of the year; they take their name from the German verb "springen," meaning to move quickly. Spring tides occur during full and new moons, when the tidal pull is strongest, creating a greater variation between high and low tides. Spring tides result in greater mixing of the Bay waters, which brings oxygen to the lower layers.

Neap tides are the time of smallest variation between high and low tides, occurring every two weeks at the half moon. The 2003 fish kills took place during a neap tide.

When researchers tested the Bay on Thursday, it was during a time of neap tide. Warren Prell, Doherty professor of oceanography at Brown, said the timing was deliberate so researchers could see how extensive the hypoxia was. Teams will also test the Bay next Thursday during a spring tide.

Sullivan agreed with Nixon's argument that nutrients aren't solely responsible for recent outbreaks of hypoxia in Narragansett Bay. "That's one piece of it," Sullivan said. "But it's a complex biological and geological system. If the Bay stays cool, plants don't grow as much. But you have got to drive your car less to have an impact on global warming."

The DEM can't control global climate change, but it can reduce nutrients flowing into the Bay. The state recently took steps to do that through an agreement with the Narragansett Bay Commission, which runs the state's two largest sewage-treatment plants -- Fields Point and Bucklin Point. Together those plants treat an average of 62 million gallons of sewage a day.

The commission recently agreed to install $100 million worth of equipment to cut its summertime nitrogen discharges to less than 8 parts per million by 2008, a 50-percent reduction mandated by state law.

Sullivan termed this a "historic" agreement that should help the Bay's waters.

However, Sullivan said, Massachusetts has more sewage-treatment plants dumping into Bay waters than Rhode Island has, and unless that state also mandates nitrogen reductions, Rhode Island's efforts may be in vain.

"Why should Rhode Island continue to live with the flushing of Massachusetts toilets?" Sullivan said.

gcarbone@projo.com / (401) 277-7434