WASHINGTON -- Maryland scientists think the long, heavy rains that fell from February to June are the main reason the state's rivers were spared a major outbreak of Pfiesteria piscicida this year.
The rains set off a cascade of changes that turned the Pocomoke River from a perfect breeding ground for the toxic microbe in 1997 into a place where Pfiesteria merely put in an appearance this year, Maryland Department of Natural Resources ecologist Robert Magnien said yesterday.
"These are the first ideas we have," Magnien said at a gathering of scientists and policy-makers at Georgetown University. "This is hot off the computers. We have some additional work to do."
The state's findings raise the hope that scientists could soon predict Pfiesteria's behavior by looking at the combination of natural and man-made conditions in local waterways.
In 1997, 13 people were diagnosed with memory loss, confusion and other mental problems after they were exposed to Pfiesteria piscicida as it was killing fish.
The state closed a 7-mile stretch of the Pocomoke River for more than six weeks, and imposed shorter closures on two Lower Eastern Shore waterways. The incidents triggered a panic over the safety of Maryland seafood, new laws aimed at reducing farm runoff that is suspected of setting the stage for toxic outbreaks, and an outpouring of research.
This year no known fish kills were traced to Pfiesteria. The toxic microbe was found on the scene of two low-level outbreaks, in which a few fish developed the bloody sores that are considered a hallmark of Pfiesteria. No waterways were closed, and state officials reported no confirmed cases of Pfiesteria-related illness.
State scientists feel that last year, conditions were "exactly right or exactly wrong, depending on how you look at it," for a toxic outbreak, said DNR biologist David Goshorn.
In 1997, high levels of nutrients, especially phosphorus, fueled a persistent algae bloom in the Pocomoke just above Shelltown in Somerset County.
Soils in the Pocomoke are saturated with phosphorus, and the high levels have been attributed to chicken manure spread on nearby farm fields.
Scientists think 1997's warm weather and low rainfall triggered a natural chemical reaction that drew phosphorus out of ditches and streams, Magnien said. The nutrients accumulated near Shelltown, where the river becomes wide and shallow. They fueled algae blooms that provided food for menhaden -- and for Pfiesteria, the experts say.
The algae blooms also used up much of the oxygen in the water, creating a swath of oxygen-poor water across the river's mouth for most of July and August.
Scientists say the schools of menhaden that migrate upriver were blocked by the low oxygen levels and congregated near Shelltown instead. The oily fish secrete a chemical signal that seems to trigger Pfiesteria attacks, "and the dense concentration of menhaden in a slow-moving portion of the river was ideal for Pfiesteria to transform to its toxic state," the ecologists wrote.
This year, rainfall sloshed through the river at more than 10 times the rate it did in 1997, and temperatures were a little cooler. The result: Nutrient concentrations weren't as high, the algae bloom was briefer and more spread, and oxygen levels were generally higher. DNR researchers also found few menhaden at the river's mouth.
Scientists initially thought that fewer menhaden were in the bay this year, and that was the reason Pfiesteria didn't appear. Other studies have found signs that young menhaden are becoming scarce, lending credence to that theory. But now, Magnien said, DNR biologists aren't sure whether whether the small fish were simply more scattered.
If the next few years confirm the biologists' theory, "we might be in a position to predict outbreaks," Magnien said. "We might not be able to prevent them, Mother Nature being what it is. But we could take steps to protect the public health."
In other findings, the Maryland medical team that found the link between Pfiesteria exposure and human illness added details to their 1997 research, making it clearer that those health effects are real.
Three University of Maryland Medical School psychological researchers found the Pfiesteria outbreak did not fit the pattern for illnesses caused by mass hysteria.
The researchers also found that seven sick watermen were emotionally fit, and that psychological factors did not explain their symptoms. Finally, they found that four people who developed skin lesions after being exposed to Pfiesteria suffered the most extreme memory problems.
All patients who suffered memory problems scored at normal levels on standardized tests six months later, team members said.
Pub Date: 12/11/98