While stories of a fish-killing, human-sickening phytoplankton that lives in the Bay have dominated news about the Chesapeake lately, the fact that such organisms live here is hardly news at all.

Over the years, scientists have identified plenty of questionable characters floating around the Bay.

There are several members of the family Dinophysis, which are known to contaminate shellfish in other parts of the country.

There are members of the family Pseudo-nitzschia, which can contaminate shellfish and, in other parts of the world, have caused short-term amnesia in humans.

There is Prorocentrum minimum, which has been linked to fish and oyster deaths in the laboratory.

In the past decade, at least 12 phytoplankton species that have shown toxic effects either in the laboratory or in other coastal waters have been identified in the Bay. Until recent fish kills and human sicknesses associated with Pfiesteria piscicida, none had been associated with toxic effects in the Chesapeake.

Why not?

No one is certain. But Harold Marshall, a professor at Virginia's Old Dominion University, suggests that the species found in the Bay could be slightly different strains of those found elsewhere, and may not produce toxics. Or, he said, the Bay may not present the conditions needed for them to form dense, toxic blooms.

"Just because you have these organisms doesn't mean you are going to have a bloom event, or any kind of related toxic event," Marshall cautioned. "We're not raising a warning flag here that we're inundated with toxic species."

In fact, he described many of the species as "cosmopolitan" because they are found in estuaries throughout the world. "In some situations, they have produced toxic events, but it is not the common occurrence for these to do so," Marshall said. "It's a puzzling situation, isn't it?"

It is a puzzle that is getting increased attention, both in the Bay and around the world as many scientists- including Marshall-have suggested that the number of "harmful algae blooms" is on the rise in coastal areas, possibly as a result of increased pollution, particularly nutrients.

And there is ample reason for concern. In places, these blooms can cause millions of dollars in economic damage by blanketing beaches with dead fish and sending tourists somewhere else. A "red tide" off Florida last year not only killed fish, but also more than 150 endangered manatees.

Some forms emit toxic aerosols as they decompose, causing respiratory problems for people downwind. Others contaminate shellfish, and can cause symptoms from stomach illnesses to neurological disorders to death when people eat them.

Yet of more than 4,000 species of phytoplankton known worldwide, only about 60 are associated with toxic effects. Most of those are harmful only when they bloom to densities so great they discolor the water, hence the terms "brown," "red" or "mahogany" tides.

The story behind these blooms is complex, and scientists are working to try to unravel the intricacies. Not all blooms that color the water are toxic, for example. At the same time, even blooms that don't produce toxins can result in fish kills. When more algae is produced than can be consumed by predators, the excess will ultimately die and sink to the bottom and decompose in a process that can deplete the water of oxygen, sometimes killing fish, shellfish or other species in the area. They can produce other harmful effects too, such as blocking sunlight to underwater grasses that provide important habitat for crabs, clams, waterfowl, juvenile fish and other species. Without enough sunlight, the grass will die.

Except for problems caused by pfiesteria, those problems-rather than the toxic problems that have plagued other areas-have been the most tangible effects of the Bay's blooms. But, said Donald Boesch, chair of the University of Maryland's Center for Environmental Studies, who recently chaired a scientific panel studying harmful algae blooms, "We shouldn't be falsely confident about that. Those sorts of things could occur."

Algae blooms are nothing new. One of the Biblical plagues, in which the water is turned to blood, is thought to be a reference to red tides. Florida Indians told Spanish explorers of massive fish kills when the water turned red.

In recent years, though, there has been growing consensus that the number of algae blooms in general-and toxic blooms in particular-has grown, though many scientists acknowledge that the evidence is not conclusive.

There may be more algae blooms reported simply because more scientists are looking for them, and their impacts. In the Bay, for example, the discovery of pfiesteria, or pfiesteria-like organisms has caused scientists to re-examine past fish kills that had been attributed to other factors for evidence that they may actually be linked to the toxic organism.

On the other hand, red tides off Florida's West Coast seem to be lasting longer, and harmful algae that has contaminated shellfish in Maine for years has spread south into Massachusetts water.

And in recent years, "brown tide" blooms have been reported off the coasts of Texas and Long Island, where they were never before seen. The Texas event is now 7 years old. "It's brown water continuously," said Kevin Sellner, a scientist from the Academy of Natural Sciences Estuarine Research Laboratory on the Patuxent River.

Sellner is now heading a multiyear research project known as ECOHAB-Ecology and Oceanography of Harmful Algal Blooms-which is supported by the National Oceanic and Atmospheric Administration, National Science Foundation, the EPA and the Office of Naval Research.

The effort is aimed at understanding better the biology of potentially harmful phytoplankton species, as well as the conditions-from pollution to natural physical characteristics-that cause those species to bloom to harmful levels.

If scientists can better understand the blooms, Sellner said, they may be able to recommend steps to prevent them, or at least predict when they are likely to occur so managers can take steps to lessen their impact. It's also possible that techniques may be developed to treat blooms at sea, either with chemicals, natural predators or other methods.

In general, scientists see harmful algae blooms as a symptom of eutrophication-the overloading of waterways by nutrients, largely as the result of human activities-which is considered the main water quality problem for the Bay and other coastal waters. Simply put, more nutrients means more phytoplankton.

While that may explain many blooms, scientists caution that it does not explain them all.

"It's not clear that these are worsening consistent with human activity," Boesch said. For example, he noted, Florida red tides begin far offshore, not in the nutrient-rich coastal waters. Still, Boesch said, the tides seem to grow as the move toward shore.

Sellner noted that one species found in Europe doesn't even have to form deadly blooms-it produces toxic effects when only 100 to 200 cells per liter are present. "You wouldn't even see that," Sellner said. "In fact, routine cell counting using a microscope and a normal cell-counting techniques would barely distinguish that."

Other human factors may also be at play. Cargo vessels often draw in large amounts of water as "ballast" for stabilization while at sea.That water is drained when the ship reaches the destination port, along with any fish, algae or other organisms that were sucked in by the pumps. Ballast water transport has been linked to some toxic blooms in Australia.

Climate change-particularly changing temperature and rainfall patterns-could also play a role in the spread of harmful blooms, Boesch said. For example, the shellfish poisoning by the species Alexandrium has gradually been spreading south from Maine, and one potential explanation, Boesch said, is climate.

The Chesapeake Bay contains more than 700 species of phytoplankton, many of which can cause water-discoloring blooms when the wrong species is matched with just the right conditions-light, salinity, temperature, nutrients, flows and other factors.

"Each one of the of these organisms has a certain range of ideal conditions that favor its growth," Marshall said. "They are not all blooming at the same time."

Because so many factors are in play, there is no guarantee that a species-even a toxic one-will behave here as it does elsewhere, even if it reaches bloom proportions, Marshall said. Even pfiesteria has not-so far at least-had anywhere near the impact in the Bay as it has had in North Carolina, where it has been blamed for fish kills involving more than a billion fish over the past decade. The number of fish deaths blamed on pfiesteria in the Chesapeake, by contrast, is less than 20,000.

Just the same, Marshall and others say vigilance is called for in tracking potentially harmful species. A monitoring effort coordinated by Bay Program has been following the species makeup and distribution of the Chesapeake's phytoplankton population since 1985.

When blooms occur, Marshall said, they warrant closer inspection. And there certainly are species worth watching.

One common species in the Bay, Prorocentrum minimum, blooms every year. Although it has not been associated with kills in the Chesapeake, laboratory work has linked very high concentrations to the deaths of juvenile oysters.

"The implication is, if bloom levels were sufficiently experienced in a cove or an environment where juvenile oysters were either recently set, or had been growing for just a little while, mortalities might occur," Sellner said. "And who would look for Prorocentrum? Because it's always there, everyone always assumes Prorocentrum is nontoxic."

Likewise, another common species, Cochlodinium heterolobatum, has been shown in laboratory work to inhibit calcium growth in oysters which, in turn, prevents them from growing shells, so they die. It has caused large blooms in the Bay, but so far, scientists have not seen direct ill-effects in the water.

The best protective action to take against the blooms, scientists say, is nutrient control efforts. While no single factor will trigger an algae bloom, nutrients are the one factor required for many species that humans can affect. "Efforts in that direction would be desirable," Marshall said. "If they can reduce the amount coming in, this should have a beneficial effect."

Nutrient control will reduce phytoplankton in general, which he and others agree is a good idea. Blooms can dramatically alter the makeup of the Bay's phytoplankton population, which forms the base of its food web. That can have implications for other species. Off Long Island, for example, the brown tide has been blamed for the demise of the bay scallop industry because it shifted the phytoplankton population to species that could not sustain the bottom dwellers.

To some extent, recent fish kills linked to pfiesteria or closely related organisms are a reminder of just how much remains unknown about the world's coastal areas. pfiesteria and other harmful algae blooms, Boesch said, may be giving people a warning.

"It's a message that these coastal systems do have their limits in assimilating our wastes," he said. "And when they start to get out of them, they not only do things like deplete oxygen and things we knew about, but they can do some strange things that we haven't thought about yet."