Thursday, February 8, 2018

Those are Some Gravelly Mussels - Aquatic Invasive Species-

     They're in our lakes, they're in our pipes, they're on our boats. They're in every major watershed in the US except the Columbia, which is ours.


     Mussels are bivalve mollusks. They have two shell halves that protect the mussel's soft body. Mussels can attach to many surfaces, including the outsides of boats, which enables them to travel between bodies of water. Such mobility is the reason for careful boat-cleaning requirements. Female zebra mussels can produce 500,000 eggs per year, with mature mussels sometimes reaching a million eggs per year, resulting in rapid reproduction and establishment in an ecosystem where there was no prior mussel population. This kind of population growth can threaten the ability of native species to survive. Originally from Eurasia and now thriving across North America, mussels provide the perfect example of an aquatic invasive species for WEN's Aquatic Invasive Species (AIS) curriculum, called Columbia Headwaters Education Kit 4.

     Heidi Sedivy leads AIS lessons once a week at Willard Alternative High School, where small class sizes enable intimate engagement with the imminent threat and application of science to relevant local issues. On January 30th I joined Heidi and Bailey, AIS Assistant, on my first classroom program.

    Bailey began by asking the students to think about how mussel invasion might affect human activities. As mussels settle down in the pipe that will become their home, they reduce the water flow rate exponentially. Given their proclivity for clogging up pipes, mussels could prevent the efficient transport of water for agricultural irrigation or hydrating cattle. This increases the difficulty of raising crops and animals, which would increase prices for consumers and decrease profits for companies.

     With human implications in mind, the class set up a lab to examine the physical effects of mussel populations on water flow. Heidi and Bailey demonstrated how to use the equipment. A translucent funnel (aka the top cut off of a plastic soda bottle) sat on top of a clear tube, about an inch in diameter. At the bottom was a filter before the rubber output. This tube simulates a pipe. White gravel would serve as our mussels (though they reminded me of the barnacles that grow on submerged posts and rocks in the Puget Sound).

   Split into two groups, the students poured 2 liters of water through the funnel and timed how long it took to travel through the tube and empty out into a bucket below. After a test-run of an empty tube, they added 5cm of gravel and timed the water again.

     "Mm, look at that dirty water," one girl remarked on the dirt washing off the tiny rocks.
     "You're cleaning our rocks for us!" Bailey replied.

     Even at this lowest increment of "mussels" the water took significantly longer to fully filter through.

     The students noticed that the tubes had slightly different rates, even without the rocks slowing the water.

     "Maybe one team is pouring faster," someone suggested.

     After several rounds of adding "mussels" and pouring water through, the data was ready for analysis. The students noticed that the water did filter through more slowly the more gravel they added to the tube. However, the times from each team did not quite match up...

     "I think those rates are really weird," one student commented.

     Indeed, the average time for 20 cm of gravel was .4 seconds faster than for 15 cm! How could this be?

     "The tube might be haunted," someone else said. While this counts as a hypothesis, we could not test it, so the conversation moved on. What else could cause variation in the experiment, or how could real mussel pipe-clogging differ from the model?

     "I think the rate would fluctuate because the mussels stick to the sides [of pipes] and not just the bottom." This could allow many more mussels to congregate before problematic water flow indicates their presence.

     The class wrapped up with some brief art: creating diagrams, or visual models, of the experiment. This way, when they created graphs the next week they would have a handy reference of what they did.

Mussel experiment diagram

-Cassie Sevigny
AmeriCorps Team Member
Media Coordinator

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