The watchdog who protects the Northern Virginia suburbs from the ravages of a highway construction project walked through Arlington mud last week, lamenting certain doomed trees and saying that highways basically are ugly.

"Ugly," said John E. Schaeffer Jr., 26, a fisheries biologist who last worked in Maryland conducting field studies on the habitat of Muhlenberg's turtle. "Ugly," he repeated, "but necessary."

Walking on red mud that in 1982 will lie beneath Interstate 66, the four-lane highway cutting from the Beltway to the Potomac River, Schaeffer kept checking for holes in siltation fences that are supposed to keep the red mud out of the Potomac.

Earlier that morning, Schaeffer found a square hole sliced in a nylon siltation fence, a common act of vandalism on this project. Opponents of the highway, which is being built despite 21 years of citizen attempts to stop it, have lobbed paint-filled balloons at construction equipment, tarred tractor seats and refused to climb out of trees that had to be chopped down.

While the still loyal opposition has failed to stop I-66, it succeeded in having an environmental watchdog hired to make sure the ugliness would be kept at an absolute minimum.

Although some groups still opposed to I-66 doubt if Schaeffer has any real influence on highway construction, a nationally known ecologist, Eugene Odom, said that Schaeffer "gives the citizens someone they can talk to other than the bulldozer man."

Odom, who heads the Institute of Ecology at the University of Georgia, recommended last year that an environmentalist be hired to make sure that the Virginia highway officials "follow through on all those nice things (such as making I-66 look like the George Washington Parkway) that they have promised."

Schaeffer, a solemn-looking University of Maryland graduate with oversize brown-rim glasses and a neatlyclipped beard, was hired because he exhibited a unique capacity for listening to people's complaints, according to his boss.

That boss, Keith Argow, an environmentalist affiliated with an organization called Trout Unlimited out of Blacksburg, Va., said yesterday that, yes, it is a good question why an outfit with trout in its name would be monitoring the construction of an urban highway that doesn't go near any trout.

"I'm known for having worked on construction projects," said Argow, who worked for the state of Alaska as a consultant on the building of the oil pipeline. Argow also used to live on Kirkwood Road in Arlington, near where I-66 is being built.

Argow said he could have hired an engineer to look after I-66, but that he could not find anybody with a greater ability to listen to the neighborhood than Schaeffer.

Schaeffer, who has worked since Nov. 16 out of a little green trailer near the construction site on Glebe Road, said his job is to make sure that contractors do not kill any more trees than they have to, do not run their tractors over siltation fences and do not take any shortcuts that would increase noise when the highway is completed.

The biologist, who dismisses his lack of engineering knowledge by saying that "it is hard to hire somebody who knows about everything," keeps meticulous written records of his dealings with contractors and complaining citizens.

To save one 50-year-old white oak, Shaeffer spent well over eight working hours, contacting engineers on the job and in Richmond.

Sometimes, as in this report written on Dec. 9, Schaeffer's efforts are foiled by a fickle public:

"On Monday I looked at two large tulip poplars and a holly tree located on Louis Newsome's (a highway contractor) job.Mr. Newsome will try to save the tulip poplars, but the holly tree is definitely going. The Joneses, whose former property the holly tree is on, expressed an interest in saving the tree and Mr Newsome agreed to... transplant the tree to their yard. However, it now appears the Joneses no longer want the tree, in which case it will be cut."

Similar chronicles of Schaeffer's tree-saving exploits are on file in Arlington public libraries.

Schaffer said he enjoys talking to people who live beside the proposed highway and is somewhat disappointed that so few citizens (five, as of last week) have come to his green trailer to talk to him.

The contracl under which Schaeffer will monitor construction and landscaping of I-66, runs out when the road is completed. Schaeffer said he is hoping that then, as a proved employe of Trout Unlimited, he will be able to do what he wants -- look after trout.