For scientists reviewing the Pocomoke problem, the situation is a classic whodunit.
Was it nutrients from farms? Or was it naturally occurring nutrients? Was it toxic runoff? Acid water? Was it something else? Or a conspiracy, with several suspects acting together?
Collecting and examining evidence that leads to a verdict is going to take time, they say.
"It will take laboratory work, experimentation and detective work," said Donald Boesch, president of the University of Maryland's Center for Environmental Science, who chairs a technical advisory committee that is reviewing the state's Pocomoke investigation.
The most likely culprit is the alga Pfiesteria piscicida-and possibly a closely related species found in the Pocomoke-which has been associated with a series of fish kills involving up to a billion fish in North Carolina.
But the fact that Pfiesteria is in the Bay watershed is hardly news. Previous studies had identified the organism in the Patuxent River and in Jenkins Creek, a tributary to the Choptank River.
Because of those findings, a number of scientists believe the organism occurs naturally in the Bay, though until this summer it has not been linked to fish kills in the Chesapeake or its tributaries.
So the real mystery is if Pfiesteria-or a closely related species-is behind the killings, what caused it to turn deadly now? And why in the Pocomoke? And will it strike again?
"There are more questions than answers, really," said Rob Magnien, a scientist with the Maryland Department of Natural Resources who has been heading the state's investigation.
The prime suspect-and subject of widespread speculation-is excess nutrients in the water. The Pocomoke watershed has 170,000 acres of farmland and a large poultry industry, all of which generate nutrients. And nutrients are well-known for stimulating algae blooms.
Indeed, many scientists interviewed agree that further nutrient reduction in the Pocomoke is a worthwhile endeavor. Said one: "You can't go wrong by trying to cut down on the nonpoint source nutrient load entering an aquatic system."
But many caution that the Pfiesteria case may be far more complex, and that simply reducing nutrients does not guarantee an end to the problem. "I certainly think there's a lot of reasons to control those nutrients, but I'm increasingly concerned that we're rushing to judgment," Boesch said.
One problem with the nutrient scenario is that Pfiesteria is heterotrophic. That is, it eats other organisms.
As a result, the classic picture of more nutrients resulting in more algae doesn't necessarily apply. "If there is a nutrient stimulation effect, it has to be mediated through some other organisms in the food chain," Boesch said.
So it is possible that nutrients could stimulate blooms of other algae which, in turn, provide food for Pfiesteria cells, but these linkages are not clearly understood.
The Pocomoke problem began last fall when watermen started reporting large numbers of fish with lesions. That makes many believe that the investigative trail begins earlier that year, when heavy rains caused unusually high flows through the river.
"It seems like the '96 big flush of the Bay must have been connected somehow to the fact that a few months later we start seeing a problem with fish lesions in the Pocomoke," said Richard Lacouture, of the Academy of Natural Sciences' Environmental Research Center on the Patuxent River.
Those flows caused a slight increase in nutrient levels in the river and flushed a large amount of organic material out of the cypress swamps in its watershed. The high flows lowered the salinity of the downstream water, and the cypress swamp flush contributed to an increase in the water's acidity.
Those factors alone could have stressed the fish, and even caused lesions. In any event, the added stress could well have made fish more susceptible to attacks from Pfiesteria, bacteria, fungi or other organisms that occur naturally in the water.
In all likelihood, Lacouture noted, the high flows also flushed toxics such as agricultural pesticides into the water, and they may also be a factor.
Besides affecting the fish, scientists say various compounds could promote the growth of one type of algae over another which, in turn, may have stimulated Pfiesteria growth.
There may even have been more steps in the process. Boesch noted that research in other areas has shown that the presence of various metals reduced the population of viruses which, in turn, altered the makeup of the algae population.
Another potential explanation is that the form of nutrients people generally worry about-inorganic nutrients from fertilizers and the like-may not be primarily responsible. Instead, organic nutrients- many of which are naturally occurring -may be playing an role.
"We don't normally think of those as being compounds that would be pollutants," said Pat Glibert, a scientist at the University of Maryland's Horn Point Laboratory.
The Pocomoke cypress swamps are a rich source of organic nutrients. And while inorganic nutrients were only somewhat elevated in the Pocomoke last year, the spike in organic nutrients was much higher.
Scientists caution that there is no research yet on the Pocomoke that links organic nutrients to Pfiesteria. But brown tide algae blooms off Long Island-caused by another type of alga, which can be heterotrophic-are at least partly due to high levels of organic nutrients found there.
Glibert, who has been studying the Long Island situation, said bloom conditions there seem to be established not merely by the presence of organic or inorganic nutrients, but the relative amounts of one type of nutrient to the other.
"I don't have any direct evidence that in the case of Pfiesteria, it is important," Glibert said. "All I can say is it's certainly worth looking at based on what we know about some of these types of organisms that show harmful algal bloom characteristics."
Because of all the complexities, scientists say that it will take time-perhaps years-before they have enough information to convict any individual factor of group of suspects.
"We're going to have to live with this organism for a while before we know how it works and how it can potentially be controlled," Lacouture said. "I have a tremendous amount of respect for this organism. I think it releases its toxins a lot easier than it releases its secrets."
Unlocking those secrets will not only help close the books on the Pocomoke case, but will also give water quality managers the information they need to help keep the villain from striking again.
"These details are more important than just for esoteric scientific reasons," Boesch said, "because it could point us in the direction of things we can manage, and manage with the most effect."