Slime on river rocks is a forensics tool for environmental scientists

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Rock slime: What is it good for?

We’ve all been there. You’re having a nice day playing in the water at your local lake or river, and all of the sudden you slip on a slimy rock! That slick, brownish goop squelches between your toes, and next thing you know, you’re falling backwards into the water.

While most of us may not appreciate the slime that grows on river rocks, Dr. William Hobbs and his colleagues have found this slime useful for investigating the sources of toxic chemicals in the water. This rock slime, known as biofilm, helps scientists measure where toxic chemicals are the most concentrated along a river. Their recently published paper* in the journal Environmental Science and Technology shares more of the slimy details.

Sleuthing out the slime

"Biofilm is a broader term that refers to both the living communities [like algae] and the dead things that build up on rocks over time,” said Hobbs.

As biofilm accumulates on river rocks, it slowly absorbs or takes up chemicals from the water. Hobbs and his team scrape the biofilm off rocks in order to test for various chemicals. They may also install plastic sheets on the bed of the stream, where biofilm can grow and be sampled. Either sampling approach allows them to study the chemicals that are present in a particular area.

GIF file showing various biofilm waving to an underwater current. It's brown and stringy, with foggy water.

A dense biofilm mat moving with the current.

Hobbs says that some chemical mixtures have a “fingerprint.” This fingerprint helps his team identify the source of a contaminant. For example, a chemical mixture with a specific composition may get into the environment. When the team finds the same chemical composition in the biofilm of a nearby river, they may have identified a source!
So far, this source tracking tool has been used to identify the following chemicals in metals.several rivers throughout Washington:
  • Polychlorinated biphenyls (PCBs) — legacy industrial chemicals.
  • Polybrominated diphenyl ethers (PBDEs) — flame retardant chemicals in household products.
  • Metals (copper, zinc, lead, arsenic, and cadmium).

The slime is greener on the other side

Overhead view of an inest with a long tail, round greenish brown body and short head, three legs on each side.

Mayfly nymphs like this one are sensitive to changes in water quality.

This robust monitoring tool is also useful because biofilms are at the base of the food web in river ecosystems. That means most creatures in the river either consume biofilm or prey on the critters that eat the biofilm. The team’s work shows that toxics in biofilm can affect the organisms that consume it.

For example, the research team found that certain metals in the biofilm can affect the types of insects that live in a particular environment. “We use the presence and absence of aquatic insects [as a way to measure] the health of the stream.” If certain insects are missing, there may be a chemical in the water and biofilm deterring them from living there.

In pursuit of cleaner slime

This environmental forensics work marks the beginning of a long process. “It’s the first step in work to remediate or reduce the impacts of toxics,” says Hobbs. His team works in areas that are often classified as “impaired” by Clean Water Act standards. “The hope is that if we can identify the sources of these toxics and implement effective source control, we can remove their classification as impaired.”

A hand holds a small knife that scraped a brown biofilm from a river rock.

Biofilm on a rock in the Wenatchee River, Washington. 

Until then, expect to see Hobbs and his team out at local rivers and streams scraping the slime off of rocks … all in the name of science! 

If you would like to read the full paper, please contact the author William Hobbs.