The bad water shifts like fog under the surface.

Some days, there's no sign of it in this shallow part of the bay off St. Mary's County, where Buddy Evans puts out his pots to catch blue crabs. Other days, he pulls up the pots and finds all the crabs dead inside.

"It's just like a fog. It rolls in, it rolls out," said Evans, a 37-year-old waterman from Smith Island.

What Evans and others call "bad water" has been robbed by algae of its dissolved oxygen, which fish and crabs need to breathe. Scientists blame man-made pollutants -- animal manure, suburban lawn-care products and treated sewage -- that act like underwater fertilizer for the algae.

For 20 years, this kind of pollution has been recognized as the most pervasive of the Chesapeake's many troubles. But years of effort to curb it has made little change: Last summer, when heat and high rainfall made the algae bloom heavily, about 40 percent of bay water lacked adequate oxygen, according to the Environmental Protection Agency's Chesapeake Bay Program.

The season for bad water has come again. And in one recent week, a tour of the bay and its tributaries revealed a tide of frustration.

Aug. 2: University of Maryland

Robert J. Diaz, a professor at the Virginia Institute of Marine Science, was talking to a room of scientists gathered for a dissolved-oxygen conference. His presentation included one slide that summed up the day's message: "When you can't breathe, nothing else matters."

"Humans have not screwed up any other measurement" of the Chesapeake's health, Diaz told the group, "as much as oxygen."

The process of oxygen depletion starts in the bay's tributaries, including the Potomac, Severn and Susquehanna rivers, the scientists said. Rain washes animal manure and lawn fertilizer into creeks and rivers, and sewage plants dump treated waste. Even air pollution, released by power plants hundreds of miles away, can land on the water.

Through those kinds of pollution, the water absorbs large amounts of nitrogen and phosphorus. These nutrients feed algae blooms that choke off light to underwater grasses. And when the algae die, they decompose in a way that consumes large amounts of dissolved oxygen.

The result is bad water. How much of it depends on rainfall; in a dry year such as 2002, less runoff occurs, and the algae decline. The bad water is more common in the deep channels of the middle bay, scientists said, where it doesn't mix with oxygen from the air. But winds or water currents can move it into shallower areas where crabs and fish are plentiful.

Because the bad water doesn't stay in one solid mass, scientists at this gathering said they don't like the term "dead zone" -- used by the private Chesapeake Bay Foundation to describe the low-oxygen areas.

In addition, they said, bad water isn't always dead: In some areas, there is enough oxygen for more resilient species such as crabs and oysters to survive.

"It's sort of like you and I going up to the top of Mount Everest," said Rich Batiuk of the Chesapeake Bay Program. "We can survive up there. But darn, it's hard to breathe."

This summer, the most recent survey estimated that about 35 percent of the Chesapeake's water had oxygen levels low enough to be stressful to some bay life.

Seeking to improve the situation, states in the bay watershed -- including Virginia, Maryland and Pennsylvania -- set goals in the late 1980s for reducing pollution to the bay by 2000. They missed them.

They then set new goals for 2010. But even now, the states have not finished their blueprints for how it will be done.

"At the end of 2010, we're going to have to answer some questions," said Donald F. Boesch, president of the University of Maryland Center for Environmental Science.

Aug. 3: Potomac River

Fishing guide Steve Chaconas has hundreds of lures designed to tempt largemouth bass. He also has about 30 or 45 minutes' worth of good jokes, for entertaining clients when the fish aren't biting.

This year, he's needed both of them -- lures and jokes.

"The areas where I used to fish, there's no grass there, so there's no easy fishing," he said.

The problem is blue-green algae, which have infested the river this year in the highest concentrations in 20 years, according to the Maryland Department of Natural Resources.

Unlike the algae that trouble the saltier Chesapeake Bay, these freshwater algae tend to float on the surface of the Potomac, forming a scum that resembles bright-green paint.

"It just looks like a golf course out there, it's so green," Ken Penrod, another longtime Potomac bass guide, said in a telephone interview.

The algae can be toxic. Any concentration of algae exceeding 10,000 cells per milliliter can be harmful to some aquatic life, scientists said. This year, the Potomac has had concentrations as high as 80 million cells per milliliter.

Earlier this summer, Maryland officials warned against drinking or swimming in algae-filled water, and the river town of Colonial Beach, Va., had to shut its beaches when a large bloom appeared there.

In the 1980s, a series of large algae blooms on the Potomac were blamed in part on runoff from the Blue Plains sewage plant in the District.

But Blue Plains was upgraded with state-of-the-art technology, so it could not have played a major role in causing this year's blooms, said Bruce Michael, a scientist with the Maryland Department of Natural Resources.

"We really can't say what that source is," Michael said.

As for Chaconas, he said he's burning more gas trying to find new fishing spots -- and also stretching his volume of jokes. There's a new one about the northern snakehead and President Bush asking for help with a "fish of mass destruction."

Kidding aside, Chaconas said he believes the algae might be worse.

"A lot of fishermen are more concerned with this than they are with snakeheads," he said, "because this has an immediate effect."

Aug. 4: Severn River

The fish-finder told the story.

John Page Williams, a naturalist for the Chesapeake Bay Foundation, parked his boat upstream from Annapolis and watched the squiggles that represented fish on his small, black-and-white screen.

"Almost everything's above 30 feet here," Williams said, meaning most of the fish were less than 30 feet deep. They were avoiding the bottom 20 feet of water.

As Williams lowered a dissolved-oxygen meter over the side, it was easy to see why. At 30 feet, the level of dissolved oxygen dropped below three milligrams per liter. Most fish can't survive with less than that.

" '74, '64, '54, there would have been oxygen all the way down there" to the bottom, he said.

Among the river's casualties: the yellow perch. Once a common fish here, its population has declined, and scientists believe that its young are endangered by areas of low oxygen.

The Severn doesn't have much pollution from sewage treatment or farmland. Instead, its nutrients seep in from the bay or wash down from the picturesque neighborhoods that line its banks.

Frederick L. Kelly, a lawyer who is known as the Severn "Riverkeeper," has tried to get his neighbors to build "living shorelines" -- man-made marshes that absorb fertilizer and other nutrients before they wash into open water.

But he knows there's much work ahead: He can still look out from the deck of his boat and see fields of green, well-fertilized grass.

"Look at this nice, cultured lawn," Kelly said, picking one opulent house with an American flag out front. "That's what we've got to stop."

Aug. 5: Smith Island

From the deck of Buddy Evans's boat, the "dead zone" seemed very much alive. The crabs that fell out of his pots were not only living -- they were so feisty that they tussled with each other in their baskets for hours afterward.

"It's not something you see every day," Evans said of the bad water. "It's not like it's this big kill-off all the time."

But he said he sees it about once a week: The bad water rolls over a pot, and the crabs can't escape. If they're still alive when he pulls them out, they're so weak they can't flip themselves off of their backs.

Fish, which generally have a lower tolerance than crabs for bad water, are often in worse shape.

"You shake the pot," Evans said. "And the fish just disintegrates."

And on this day, though Evans found no problems, his father, Elmer Evans, saw red water full of algae a few miles to the south.

"You don't catch any crabs in it. . . . It ruins it," he said, in the distinctive Smith Island accent that turns "ruins" into "rinns."

In the past year, problems with bad water have inspired some of Maryland's watermen to consider something very out of step with their fiercely independent history. Watermen's association officials have said they are talking with a D.C. lawyer about filing a class-action lawsuit against polluters.

For his part, Elmer Evans blames the bad water on people continually moving into the area, bringing more sewage runoff.

"That's what Chesapeake Bay is now," he said, "a big septic tank."

Aug. 6: Solomons Island

The customers don't look happy when they come back to Bunky's Charter Boats after a day on the water. They ask Ryan Payne, behind the counter, whether they should try a different kind of lure.

Payne tells them no. Wouldn't help.

"We know that it's not what they're fishing," Payne said, surrounded by racks of brightly colored lures. "But they don't."

The fishing hasn't been good here for a few weeks, since the water turned red with algae, Payne said.

In the bay's charter fishing business, it's a common lament. In some places, captains have said the fishing is fine. But other captains have said they're spending extra time and fuel chasing the fish.

"I can roll into a spot where yesterday there was plenty of fish there . . . and it's like the Dead Sea," said Richie Gaines, president of the Chesapeake Guides Association. He said these dead spots can range from 100 yards to five miles in diameter.

Bruno Vasta, a charter captain out of Solomons, said he has to travel 10 to 12 miles across the bay to find good water. His workdays have stretched from eight hours to 10 or 11, he said, because it takes customers longer to catch their limit.

"We can't continue to go as we are going," Vasta said in a telephone interview. "You just can't let it go down the drain."

"You shake the pot. And the fish just disintegrates," waterman Buddy Evans, 37, of Smith Island says of fishing in the Chesapeake Bay's zones of oxygen-depleted "bad water."A map based on data from last month shows bay oxygen levels, with "dead zones" illustrated in red. Graphic on Page B2.Frederick L. Kelly, known as the Severn "Riverkeeper," has tried to get neighbors to build "living shorelines," which are man-made marshes that absorb fertilizer and other nutrients before they wash into open water."That's what Chesapeake Bay is now, a big septic tank," says waterman Elmer Evans of Smith Island, working in his crab shanty.