Reducing nutrients likely to lessen risk of pfiesteria
A panel of scientific experts that has been reviewing the Bay's pfiesteria problem since this summer has concluded that reducing nutrients - a longtime goal of the Bay states - should reduce the risk of future pfiesteria outbreaks.
While Baywide nutrient concentrations have declined in recent years, data reviewed by the panel showed that nutrient concentrations had increased in the downstream portions of rivers on Maryland's lower Eastern Shore and in Virginia's Rappahannock River - areas where fish lesions were noted this year.
On top of that, the panel stated, record precipitation in 1996 and early 1997 delivered larger than normal amounts of nutrients to lower Eastern Shore rivers.
"You get an impression from the data that we're not making as much progress on the Eastern shore tributaries as we're making on the Western Shore," said Donald Boesch, president of the University System of Maryland's Center for Environmental Science, who chaired the panel.
Boesch described the areas that had pfiesteria outbreaks in the Bay this year as "obviously enriched sites."
The scientific panel presented its consensus findings to a special blue-ribbon committee established by Maryland Gov. Parris Glendening, and headed by former Maryland Gov. Harry Hughes, which was to make recommendations by Nov. 1 for state responses to the pfiesteria problem.
Pfiesteria piscicida is an unusual, single-cell microbe with more than two dozen life stages, most of which are nontoxic. But in the presence of large numbers of fish, the microbe can turn into a toxic form that preys on fish.
Pfiesteria, and several pfiesteria-like organisms, have been associated with lesions found in fish in the Rappahannock and several lower Eastern Shore tributaries. During August, about 20,000 fish died in pfiesteria-related fish kills on the Pocomoke River. Pfiesteria has also been associated with human illnesses, including lesions and short-term memory loss, but the scientific panel said it was the presence of fish - not humans - that causes the microbe to transform into its toxic form. However, humans who come into contact with the organism during its toxic phase may also suffer ill effects, the panel said.
In laboratory cultures, the growth of non-toxic stages of pfiesteria is stimulated by the addition of nutrients, the panel reported. In addition, nutrients also stimulate the growth of high concentrations of algae on which the pfiesteria feed.
While nutrients help set up the conditions for the nontoxic stages of pfiesteria to grow, it is the presence of fish - not nutrients - which cause the microbe to transform into toxic life stages, the panel said. Nor are high levels of nutrients necessary to make that transformation.
The panel reported that "even dramatic decreases" in nutrients would not completely eliminate the potential of future fish kills because pfiesteria is believed to be native to the Bay and will "use toxics to attack fish when presented with the opportunity," the panel stated in its report.
But the panel said that "it is probable" that reducing nutrients will decrease both the frequency and intensity of pfiesteria outbreaks. The panel did not conclude whether nitrogen or phosphorus was more important in stimulating the growth of pfiesteria-like organisms, but suggested that may vary from place to place based on a variety of factors.
Because of the large amount of phosphorus-laden chicken manure produced by the lower Eastern Shore's poultry industry, many have suggested that nutrient control strategies should be switched from focusing on nitrogen to phosphorus.
But Boesch cautioned against such a "rush to judgment." While he said additional focus on phosphorus may be warranted, reductions in both nutrients are important.
"It's a mistake now to swing dramatically in the other direction and just focus on phosphorus," he said. "In fact, the water quality trends don't show any real improvement in the Pocomoke River of nitrogen or phosphorus concentrations. The stronger upward trend is in nitrogen."
Phosphorus tends to spur algae blooms in fresh water, while nitrogen is primarily responsible for algae growth in salt water. Baywide, phosphorus concentrations have dropped significantly over the years, largely as the result of phosphate detergent bans passed by the Bay states in the 1980s, and upgrades at sewage treatment plants. Nitrogen levels have remained steady or declined slightly.
Areas of consensus by the scientific panel
- Nutrient concentrations in Eastern Shore tidal rivers are high relative to other rivers with similar salinity.
- Nutrient concentrations in lower portions of these tidal rivers and in the Rappahannock river have increased over the last 12 years.
- Record precipitation and runoff during 1996 and early 1997 resulted in increased nutrients to Lower Eastern Shore rivers.
- In laboratory cultures, the growth of nontoxic stages of Pfiesteria piscicida can be stimulated by the addition of nutrients.
- Nutrients stimulate the growth of algae and other microbes on which pfiesteria-like dinoflagellates can grow.
- Pfiesteria-like dinoflagellates are abundant in areas with high concentrations of algae and microbes.
- At this point, it cannot be determined which nutrient (nitrogen or phosphorus) is more important in stimulating the growth of pfiesteria-like dinoflagellates.
- High nutrient concentrations are not required for pfiesteria-like dinoflagellates to transform into toxic stages.
- Populations of pfiesteria-like dinoflagellates may also be controlled by predators and pathogens, but this has not yet been demonstrated.
- It is improbable that toxic contaminants, such as pesticides and trace metals, are primarily responsible for outbreaks of pfiesteria-like dinoflagellates.
- Donald F. Boech, University of Maryland's Center for Environmental Science (UMCES)
- Walter M. Boynton, Chesapeake Biological Laboratory, UMCES
- JoAnn M. Burkholder, North Carolina State University
- D. Wayne Coats, Smithsonian Environmental Research Center
- Hugh W. Ducklow, Virginia Institute of Marine Science, College of William and Mary
- Thomas R. Fisher, Horn Point Laboratory, UMCES
- Patricia M. Glibert, Horn Point Laboratory, UMCES
- Thomas C. Malone, Horn Point Laboratory, UMCES
- Kevin G. Sellner, Academy of Natural Sciences
- Diane K. Stoecker, Horn Point Laboratory, UMCES
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