Microscopic fibers and pieces of plastic are accumulating at the bottom of the Mississippi River in the metro area, posing a rising threat to fish and other wildlife and reflecting changes in urban life along its banks.

A new report on the health of the Mississippi has found that tiny fragments from artificial materials like clothing, plastic bags, tires, carpeting and plastic bottles are found in high concentrations in the river’s sediment, especially downstream of a wastewater treatment plant.

The river is in many ways healthier than ever, according to the report released Wednesday by Friends of the Mississippi River, but such microplastics are a new form of pollution. They are the focus of intense studies from the Great Lakes to the oceans, but this is the first time they’ve been documented in Minnesota’s greatest river.

While microbeads — the tiny spheres of plastic in cosmetics and soaps — are being phased out by manufacturers after a backlash from consumers and regulators, they were a fraction of what was found in the sediment of the Mississippi.

The vast majority of the microplastics were fibers from clothing and other synthetic materials, said Trevor Russell, program director of Friends of the Mississippi, which compiled the report along with the Mississippi River National Park and Recreation Area.

“When I heard that, I thought, ‘Oh no, it’s the fleece we all love to wear,’ ” said Faye Sleeper, director of the University of Minnesota’s Water Resources Center. “It’s such a great use for recycled plastic.”

Russell said dust-sized pieces of artificial fibers that sink to the river’s bottom present just as much an environmental risk as microbeads or other plastics. “We may have put our attention on the wrong thing,” he said.

Plastic and other materials break down into pieces a tenth of a millimeter across or smaller, but they don’t biodegrade like natural materials. They absorb toxic chemicals like PCBs and are eaten by fish, mussels and even phytoplankton.

“There have been documented effects on reproduction, growth, hatching rates and liver toxicity,” said Austin Baldwin, a hydrologist with the U.S. Geological Survey, which conducted the sediment study. “They can get to a size where they can pass through gut walls and cell membranes, into the circulatory system and cause damage.”

Baldwin said his research in the Mississippi is preliminary, but he found that concentrations of microplastics were more than twice as high downstream of the Metropolitan Council’s wastewater treatment plant than upstream. It doesn’t prove they’re coming from washing machines and industry, he said, but studies elsewhere have found that such water-treatment plants may be a pipeline for the synthetics that are now ubiquitous in American consumer life.

But microplastics are so small, and dilute so thoroughly in the wastewater stream, that removing them at a treatment plant would be extremely difficult and enormously expensive, said Larry Rogacki, assistant manager for environmental services at the Metropolitan Council.

The more effective solution would be to design fabrics and products so they are biodegradable or don’t fragment, he said.

Progress, too

At the same time, the report found many signs that the river, a designated national park along the 72 miles from Dayton to Prescott, is improving — especially above its confluence with the Minnesota River at Fort Snelling.

Marking that progress is important, said John Linc Stine, commissioner of the Minnesota Pollution Control Agency. But the comprehensive view of the river, which provides drinking water to more than a million people, is also a reminder that vigilance is key.

“When we take our eye off the goals, we lose important ground,” Stine said.

In recent decades, federal and state regulations, together with millions of dollars invested to separate sewage and stormwater pipes, have paid off in reducing human and industrial waste in the river, said Sleeper.

“The river has improved significantly over the years,” said Sleeper. “You can swim in it — but I wouldn’t do it after a big storm.”

Walleye and small mouth bass have not only come back but are flourishing. Some stretches of the river are prime spots for anglers. The river corridor has 55 eagles’ nests — so many that younger birds are now being forced into smaller, secondary trees.

But, they too, show the effects of life in an urban area. Many species of fish carry health advisories on consumption because they are contaminated with mercury and other toxins. Researchers who have been tracking the eagles for years found that eaglets along the corridor carry very high levels of lead, possibly from fishing weights and ammunition, and — downstream of the treatment plant — toxic chemicals from flame retardants used in clothing and other products.

The water is still polluted with phosphorus, a nutrient from agriculture, urban runoff and wastewater treatment plants that causes ugly and sometimes toxic algae blooms. But overall phosphorus levels have declined by 35 percent since 1976, largely from improved technologies in wastewater treatment.

Nitrates, another nutrient that is a health risk in drinking water, have soared 44 percent since 1976, primarily from the Minnesota River, which drains a large agricultural portion of western Minnesota before flowing into the Mississippi near Fort Snelling. But the peak concentrations have declined since 2010, and now range from 2.5 to 3 parts per million, less than half of what’s considered a risk for infants and pregnant women.

Salt from roads and water softeners, however, has soared by 81 percent since 1985, reaching concentrations of nearly 40 parts per million in the river, and much, much higher in some of its urban tributaries. While that is still well below the levels considered dangerous for fish and freshwater aquatic ecosystems, the rate of increase is alarming, said Whitney Clark, executive director of Friends of the Mississippi River.

Unlike other pollutants that move downstream, most of the salt that flows into the river from the metro area stays put, increasing annually.

“It’s a permanent pollutant,” Clark said.