Researchers Look for Sinks of Microplastics Pollution in Lake Tahoe

The most comprehensive characterization of microplastics in the Lake Tahoe Basin to date

Microplastic pollution is seemingly ubiquitous, and few know this as well as staff research associate with the UC Davis Tahoe Environmental Research Center Katie Senft. Senft, in collaboration with the Nevada Division of Environmental Protection, is looking for sinks of microplastics in Lake Tahoe.

“A sink is a place where the plastics will collect once they enter Lake Tahoe. Based on the work U.C. Davis has done, as well as research done by the Desert Research Institute looking at stormwater samples, we know that plastics are in the lake,” said Senft by phone. “The big question now is, where are they going once they get in there.”

Senft is looking at five different areas of the lake. The first is surface water.

“A lot of plastics are buoyant and they float. So monthly we’ve been dragging a net through the northern part of the lake and collecting surface waters, which we analyze back at the lab for plastics,” Senft said. “And that’s kind of the classic … when you think of ocean plastic research, the Manta Trawl is what they’re towing behind their boats, and that’s what we’re using on the lake.”

Manta Trawl – A manta trawl is towed on the surface behind a UC Davis research vessel to collect buoyant particles sitting on the surface. Some of the particles captured are likely microplastics – photo: Brant Allen

Lake Tahoe is nearly 1,700 feet deep. The second area of inquiry is the vertical distribution of microplastics.

“As plastics get colonized with biofilms or algal growth that can change their density and make them a little heavier, they’ll settle eventually. So we’re looking at water at six different depths from top to bottom.”

Senft will also study deep-water sediments.

“There’s a lot of deep areas in Lake Tahoe, and we went out and collected sediments from about 400 meters deep in the water. We want to see if the plastics are settling down there and getting locked up and away from fish and other wildlife.”

The fourth area of scrutiny is municipal waters. Researchers will collect water from two different water treatment plants in the Tahoe basin on a quarterly basis. 

For the current study, the fifth and final area of microplastics inquiry will be two species of nonnative wildlife. 

“We’ll look at Kokanee salmon guts as well as Asian clams,” Senft said. 

The kokanee salmon is the landlocked relative of the ocean-going sockeye salmon. According to the U.S. Forest Service, biologists introduced the fish to Lake Tahoe in 1944. 

The Asian clam is an invasive species. Each rapidly-multiplying clam filters algae from as many as 5 gallons of water per day for food. Studying the presence of microplastics in bottom-dwelling clams and top of the food chain salmon will likely offer a variety of insights, the upshot is, microplastics are bad for aquatic wildlife.

“Sometimes microplastics cause digestion problems, and they can get so full of microplastics there’s no room for their usual food and they can actually starve to death,” Senft said.

Manta Trawl Sampling Cup – Researcher Katie Senft holds a sampling cup with particles collected from Lake Tahoe’s surface using a manta trawl. The samples contain many organic materials such as pine needles and insects, so additional processing will be done in the lab to remove them before microplastics can be isolated for analysis – photo: Brant Allen

Microplastics pollution moves up the food chain with a variety of negative outcomes. The tiny particles serve as an excellent pathway or “raft,” for chemical toxins and pathogenic bacteria, according to Senft.

“Some of the dangers of that, aside from them just consuming them and causing digestion issues, is when those toxins attach to the microplastics you can have what we call bioaccumulation of toxins up the food web,” Senft said. “So you might have a little tiny zooplankton swimming around in the lake, and they might have eaten a piece of the plastic, but then you have the fish that comes by and eats a thousand of the zooplankton, and now that fish has a thousand pieces of microplastics in them. As you move up the food chain, you keep that bioaccumulation of those negative things going.”

Petri Dish – Suspected microplastics are removed from manta trawl samples in the lab and mounted on petri dishes for validation using a Raman microscope, which confirms polymer type, giving UC Davis researchers greater insight into potential sources – photo: Steven Sesma

As alarming as the results can be, an accurate characterization of the presence of microplastics leads to an understanding of its sources, an important first step to being able to solve the problem. And while cleanup is not yet feasible, once identified, the sources can be curbed. Senft said that washing clothing made from synthetic materials is the largest contributor to microplastics pollution, but society also has a serious glitter problem.

“Microplastics, one of the real challenging things about them is, once they get into the environment, they are incredibly difficult, if not impossible, to remove with current technologies.

“So every time glitter goes down the drain at your house, if it’s not getting captured by the wastewater treatment plant, if their filtration system isn’t really top notch, that could be expelled into rivers. And once it’s in there, as I said, there is no known way to get it out.

“Glitter is used in everything from cosmetics to inflatable beach toys to put sparkles in them to the craft projects we often do with our children at home. And glitter is what’s known as a primary microplastic,” Senft said. 

There are two types of microplastics, according to Senft. A microplastic is considered a piece of plastic five millimeters or smaller, about the size of a grain of rice. Primary microplastics, like glitter, are small to begin with and notoriously difficult to remove from the environment. They don’t need to be broken down any more to be considered a microplastic. 

Secondary microplastics start off as larger items, so think of water bottles or plastic bags. Ultraviolet light, wind, or waves can fragment these larger items to microplastic size.

Seemingly, wherever researchers look for microplastics, they find them. 

“As we all know there is plastics everywhere in our everyday lives. Since 1950, there’s been over eight billion metric tonnes of plastic produced. And over half of that has been produced in the last 13 years. So there’s plastic everywhere we look,” Senft said.

As a researcher who works to accurately characterize the problem in one of the nation’s more scenically attractive places, is she optimistic?

“I find a lot of optimism in it actually,” Senft said. “One thing I want to say is that plastics, they’re not evil. The way it’s turned down a bad path is our appetite for single-use plastics. That’s very avoidable. I carry a water bottle everywhere I go, and that probably saves hundreds of pounds of plastic from going into the landfill every year, so one of the great things that everyone can do is try to eliminate single-use plastics from your lives.”

Top photo caption and credit: Manta Trawl – A manta trawl is towed on the surface behind a UC Davis research vessel to collect buoyant particles sitting on the surface. Some of the particles captured are likely microplastics – photo: Brant Allen

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