A rare combination of conditions -- man-made and natural -- could be to blame for the outbreak of one or more toxic microorganisms in the lower Pocomoke River, a state task force said yesterday.
"Some of these things are falling into place, suggesting what's going on," said Robert E. Magnien, head of a task force of state officials responding to the outbreak.
Those factors include record rains last year, the Pocomoke's large and intensively farmed drainage basin and high pollution levels in the tributary.
The Pocomoke may also be vulnerable because of its unusually narrow banks and 12-foot depth in the upper part of the river -- as well as the darkness of its waters, partly due to sediment runoff, partly due to tannin from the cypress swamps that drain into the river.
Because the river's waters are murky, tiny plants that would normally eat nutrients can't grow. That means more nutrients downstream. That means outbreaks of toxic organisms such as Pfiesteria piscicida and two newly discovered cousins.
"This is new, this is preliminary, this is a theory or hypothesis," Magnien told scientists gathered in Annapolis to assess the state's response to the Pocomoke's ills. But it's the best explanation he's found, he said, of why the Pocomoke has been stricken while other Chesapeake rivers have not.
If the theory is true, he said, "it's less likely" that the microorganisms will kill fish or sicken people elsewhere.
"But I certainly wouldn't suggest it couldn't happen," he said.
State officials said yesterday that the recent fish kill on the Pocomoke, the second of the summer, appears to be over.
However, a seven-mile stretch of the river remains closed as a health precaution. Maryland and Virginia officials banned people from the river last week after fish began dying and a medical team linked human health problems to contact with Pfiesteria-laden waters.
The task force's theory suggests that heavy rains could trigger another episode of fish lesions or kills.
And if there is another kill, toxins in the water could again produce temporary blisters, shortness of breath and memory problems among people who come in contact with the water.
For people who eat fish and shellfish from the bay, the news seems better. Scientists and public officials appeared to agree: Healthy fish are safe; only sick ones should be avoided.
"If you don't see fish in distress, then you don't have a problem," said JoAnn Burkholder, a North Carolina aquatic ecologist who has been advising the state on coping with the toxic organisms. She spoke at yesterday's meeting.
These single-celled predators, which use a cocktail of toxins to stun and strip the flesh from their prey, have been found in other rivers in the bay watershed and may have been there for millenniums.
They are thought to live quietly as plant-eating microbes in the absence of just the right conditions.
"It's around, but normally not causing too much trouble," said Burkholder, who led the team that identified the first of these organisms, Pfiesteria piscicida.
That microscopic critter has been linked to the slaughter of millions of fish in the Neuse and other North Carolina rivers.
Burkholder said she has discovered two other Pfiesteria-like organisms, in addition to Pfiesteria itself, in Pocomoke water samples.
Magnien said Pfiesteria outbreaks in the Pocomoke appear to follow a series of steps.
First, leaves, tannin and other organic materials have been pouring into the northern reaches of the river during heavy rains over the past year. Bacteria feed on the material, also consuming most of the water's dissolved oxygen.
The Pocomoke's dissolved oxygen levels this summer have been extremely low -- so low, Magnien said, that "I would consider it a unique situation in the bay."
The Pocomoke's water is also loaded with inorganic "nutrients" such as nitrogen and phosphorus from chicken farms and other sources. In other rivers, this would trigger algae blooms.
But the upper Pocomoke is too narrow, too deep and too clouded by sediment and organic debris for sunlight to reach below the surface.
And algae, like other plants, need sunlight to live.
About two miles south of Shelltown, the river abruptly broadens and turns very shallow. As the nutrients are exposed to the sun, algal blooms erupt. This attracts schools of fish, especially algae-eating menhaden.
The lack of oxygen also blocks fish from swimming upriver.
"This may be the equivalent of putting a dam for some of these fish above Shelltown," Magnien said.
Thousands of fish cram into a small area. Chemicals in their excrement build up in the water.
Normally, Pfiesteria and similar organisms graze on algae.
But scientists suspect that in the presence of high nutrient levels and big schools of fish, the organisms change their shape and their diet -- turning from bottom-dwelling, rocklike cysts into free-swimming predators.
Dan Terlizzi, a water quality specialist with the University of Maryland Sea Grant Program, called it "a great hypothesis."
"It's based on a lot of sound knowledge and theory," said Richard Lacouture of the Philadelphia Academy of Natural Sciences laboratory in Calvert County. "I think it's a very useful and fairly accurate model."
State Department of the Environment officials have monitored streams feeding the Pocomoke, river sediments and the fish.
None have been found to harbor unusual levels of pesticides or other poisons that could provide an alternate explanation for the fish kills.
Pub Date: 9/03/97