Defying the popular notion that the Potomac River is a watery hothed of pernicious pollutants, an aquatic ecologist says the river has "one of the nicest crops of aquatic life of any river along the Eastern seaboard." But William T. Mason Jr. is deeply disturbed about the effects of anticipated low water flow this summer on the river's aquatic life.

Mason, who was senior adviser of Water Quality Services at the Interstate Commission on the Potomac River Basin, will become aquatic specialist in the Office of Biological Service of the Eastern Energy and Land Use Group in Harper's Ferry.

The red-haired, 38-year-old Mason says his pipe-smoking habit is more dangerous to his health than eating fish caught in the Potomac.

The Potomac, he says, is well-known for its smallmouth bass, a good, fighting sport fish. Other fresh water reliables such as sunfish and channel catfish abound in the river.

"Catfish," Mason says, "are the sturdies fish around, but you have to skin 'em - not scale 'em - the skin's too tough to eat."

Then there's the crappie (pronounced "croppie" by Mason) and German carp. Found in the canal as well as in the river, carp run as large as 13 pounds. But carp like to feed on wastes, a habit that stops many fishermen from eating them, Mason says.

An ample supple of trout can also be found in the cooler waters of the Potomac's tributaries.

A good deal of secluded land, according to Mason, still borders the river and its tributaries, and the fish and insects in these waters help repopulate other pockets of water where aquatic life is sparse.

If there's a drought this summer, however, the harmful effects of low water in the river would outweigh the beneficial ones, Mason warns.

For instance, less water will result in warmer water, which retains less oxygen than does cold water; therefore less dissolved oxygen will be available to the fish who generally dislike temperatures above 86 degrees Fahrenheit. The blue-green algae, however, thrive in such warm water, emitting dank, unpleasant odors and causing obstructions for boaters.

With low water flow, Mason says, comes a greater concentration of pollutants - organic matter, metals and other toxic substances - that may cause fish fatalities. The fish also will have difficulty overcoming man-made blockages such as dams.

In low water, the mollusc population can be easily spotted by raccoons and other predators, including people who playfully toss the molluscs out of the water. Very bad, says Mason, explaining the molluscs act as biological filters.

Less water in the river encourages more salt water incursions, driving fresh water fish and their food sources to already crowded habitats in the Little Falls area. In the Occoquan Bay locale, Mason says, salt water may lurk and these brackish conditions actually attract fish such as white perch. Fresh water fish such as bass and catfish cannot tolerate brackish water and will seek more compatible habitats. On the other hand, rockfish and bluefish welcome salt water intrusions and will penetrate even further up the river.

Mason, who says he and his family eat fish caught in the Potomac, says that dangerous bacteria exist in all untreated, natural water, not just in the Potomac. The bacteria can be ingested, not through the fish, but by hand-to-mouth contact.

Looking on the brighter side of low-water levels, reduced siltation is one beneficial effect, Mason said. Another is less disturbance to the river bottom and other aquatic habitats.

Mason, a weekend angler, said he deplores the killing of animal life for reasons other than food sources. But he likes fishing for the evening meal, citing the economy factor and the nutritious protein content in fish. Another effective protein source is insects, Mason says, but he admits that our cultural taste preclude an insect diet in the near future. Mason explaines that if the insects are pulverized, none of the hairy legs or feelers would be recognizable, thereby eliminating the cause for gastronomical queeziness.

His love of aquatic life started as a boy growing up in Louisville, Ky., close to the Ohio River. Every summer, he said, he looked forward to visiting relatives in McMinnville, Tenn., because he could fish on the river banks of the TVA reservoirs. During his undergraduate years at the University of Louisville, Mason spent summers with the Ohio River Valley Water Sanitation Commission, traveling the river to investigate effects of waste discharges on aquatic life. Later, while working on the effects of salt water intrusions in marshes, he received his masters degree at Tulane University.

Before coming to the Washington area, Mason spent eight years with the Environmental Protection Agency in Cincinnati.

Mason, who lives in Silver Spring, talks about going fishing on the weekend with his two sons, 11 and 9. One of their favorite spots is River Benk Park on the Virginia side of the Potomac, just past Great Falls Park. The view of the Maryland side is beautiful, says Mason.

"Actually, I'm a horrible fisherman," he said, adding that his wife and sons are the savvy anglers in the family.