Rivers that flow into the Great Lakes are awash with tiny plastic bits, some barely visible to the human eye but big enough to infiltrate the food chain, according to the largest study of microplastics in rivers to date.

Scientists found the harmful pollutants in every one of the 107 samples taken from 29 rivers across six states, according to research published this week in the journal Environmental Science & Technology.

“It’s significant that we found how widespread the plastics were,” said hydrologist Austin Baldwin of the U.S. Geological Survey, the study’s lead author. “We found them not only in really urban watersheds but also agricultural watersheds and even forest-dominated watersheds.”

Concentrations of microplastics were greatest in urban waterways, with Michigan’s Rouge River showing the highest density: 32 particles per cubic meter. Tributaries that passed largely through more natural, forested areas showed smaller concentrations, with the lowest recorded concentration of 0.05 parts per cubic meter in samples from the St. Louis River in Wisconsin.

A growing body of research is showing that ingesting microplastics can harm the health of marine animals. In one recent study, the survival and reproduction rates of European perch dropped after the fish consumed microplastics, which can resemble the prey such species normally feed on. Another report linked microplastic pollution to lower reproduction rates in Pacific oysters.

Research led by environmental chemist Sherri Mason of the State University of New York at Fredonia, a coauthor of the new study, found microplastics in the bellies of 18 fish species in the Great Lakes, including angler favorites such as perch and brown trout.

Growing public and regulatory awareness of the issue has focused on microbeads: minuscule polymer balls added to personal care products to help scrub off dead skin cells or whiten teeth. Mason’s earlier research proved that microplastic pollution was endemic throughout the Great Lakes and helped spur a federal ban on microbeads in personal care products and cosmetics; the ban takes effect in 2017.

RELATED: The New Microbead Ban Won’t Solve the Microplastic Pollution Problem

Plastic fibers, not microbeads, made up the majority of microplastics that Baldwin and his colleagues found in the rivers. “Seventy-one percent of all the particles we found were fibers, so almost three out of four particles,” he said. “I think most people assume they’re coming from clothing. But carpet also has plastic fibers, and there are probably a lot of other sources as well that we may not even be thinking about. Clothing may not be the primary source, just an obvious one.”

The rest of the microparticles were a mix of plastic fragments, which break off from litter (bags, bottles, and other objects) as it degrades in the environment, as well as foams, films, and microbeads.

Trying to remove the particles from the water is “too big of an endeavor,” Baldwin said. “I think the best solution is to try and reduce the source. Banning microbeads was easy, but stopping litter from breaking down and washing into the stream is a lot harder.”

“People are not going to stop wearing their fleece, so who’s in a position to improve this?” he added. “Is it up to the textile manufacturers…to make textiles that don’t shed as much? Or is it on washing machine makers to build a machine that does a better job of filtering these fibers? Is it on the wastewater treatment plant to filter them out?”