Amid tough criticism from scientists and conservationists, a federal task force on acid rain reported yesterday that acidification of lakes and streams in the United States "is not a new phenomenon" and that whatever damage has been done is not likely to worsen for decades, if at all.

In a report marking the mid-point of its 10-year research effort, the National Acid Precipitation Assessment Program concluded that relatively few U.S. lakes and streams have become acidified -- defined by the panel as acidic enough to kill sport fish -- and that damage to crops, forests and human health has not been demonstrated.

J. Laurence Kulp, director of the interagency task force, said the panel thinks that aquatic systems throughout most of the United States have reached a "steady state" of acidification under current pollution levels and that "a significant increase in the number of acidic lakes is unlikely to occur over the next few decades."

"Policymakers will be able to use this information for decisions on whether additional action should be undertaken to reduce the pollutants which create acidic deposition," Kulp said.

The report, the product of more than $300 million in federal research since 1980, was immediately denounced by other scientists involved in acid-rain studies as "ridiculous" and "misleading." In Canada, where the issue has become a leading irritant in relations with the United States, Environment Minister Tom McMillan called the document "awkwardly out of step . . . with prevailing scientific judgment on the subject."

Kulp said the report represents the "state of science" on acid rain. And he strongly denied accusations from conservationists that the report is "political propaganda" designed to bolster the Reagan administration's opposition to acid-rain controls.

"I resent it personally, as a professional and as a scientist," he said.

But some scientists interviewed yesterday echoed the accusation. "From what I've seen, it seems to be slanted toward making the problem appear rather minor," said Eville Gorham of the University of Minnesota, who has monitored lake acidity in the Upper Midwest. "For one thing, it rather conveniently ignores a number of lakes in Canada that have been acidified."

"I think it misrepresents the science," said James Galloway of the University of Virginia, who has spearheaded acid-rain monitoring efforts in the Blue Ridge Mountain area.

In Galloway and Gorham's view, the report understated the number of acidified lakes by using only data from a federal survey completed last year and by listing only those lakes and streams with a pH of 5.0 or lower, or about the acidity of coffee. The lower the pH, the greater the acidity. Biologists have determined that aquatic damage occurs at levels of 5.5 or even as high as 6.0

Had the report used the 6.0 cutoff preferred by some scientists, the number of acidified lakes in some areas would have appeared much higher. In the Adirondacks, for example, only 10 percent of lakes are below 5.0 but 27 percent are below 6.0.

"They say there is no stream in the Southern Blue Ridge with a pH of below 6.0," Galloway said. "We've published surveys for the Shenandoah National Park showing that a quarter to a third are not only below 6.0, but a large number are below 5.5."

Kulp said the panel made 5.0 the cutoff because that is the point at which water can no longer support sport fish, causing a quantifiable economic loss.

The panel also reported that acid rain is "unlikely" to have any effect on healthy forests, basing its conclusion in large part on experiments that "have not detected injury" to laboratory seedlings exposed to acid rain.

"That just isn't true," said Yale University scientist Gene Likens. "There are lots of published papers that show foliar damage. I'm not saying that acid rain causes forest dieback -- and nobody does as far as I know -- but to take that out of context and quote it is not right."