This January I worked with the Marine Biology class at Truckee High School to pilot a project that studied the threat of invasive Quagga Mussels in Donner Lake. The class sidestepped into some freshwater experiments as an introduction to a segment on calcifying organisms and ocean acidification.
The background we gave to the students:
- Quagga Mussels are an invasive species that is nearly impossible to get rid of and devastating to beaches and aquatic ecosystems.
Quagga Mussel Shells on a Beach in Michigan source: northerswag.com
- Quagga Mussels, like many mollusks, make their shells out of calcium taken from the water.
- The Lake Tahoe/Donner Lake area has long been classified as at low risk of invasion by Quagga Mussels because the water are naturally depleted in Calcium. Here is a map from the Ecological Society of America that shows a nationwide calcium based risk assessment.
Calcium Based Risk Assessment Map from the Ecological Society of America
- Recent research in Lake Tahoe has suggested that calcium levels in the lake are much closer to the minimum threshold needed for mussels to survive than previously believed. (You can find the article here https://peerj.com/articles/1276/)
- Finally that in water low in calcium, concrete can decalcify leaching calcium into the water.
The Research Question:
Could leaching from concrete structures in Donner Lake meaningfully increase the risk of Quagga Mussel establishment?
Given this was the first time the class had attempted to answer this question and that minimal research was available on rates of concrete decalcification the class decided to start by testing the rates concrete decalcification in Donner Lake. Before Christmas Break we collected water from Donner Lake and added cured concrete to it. Students took 500ml of lake water and added 200g of cured concrete. The students also decided to add another layer with different trials for the grain size of concrete. They crushed the 200g of concrete into large, medium, or small pieces for different trials to see how grain size affected leaching. Each trial had 3-4 replicates.
After Christmas Break the students came back to class and tested the calcium in the water mixed with concrete as well as Donner Lake water we had set aside as a control. We measured the Calcium in the water using a basic titration water hardness kit. (Teacher’s note: While water hardness is a measure of Calcium and Magnesium in the water, the teacher and I had previously tested a method that uses Sodium Hydroxide to precipitate out the Magnesium, but not the Calcium in the water. After testing the Hardness before and after precipitating out the Magnesium we found that there was minimal Magnesium present and it was reasonably accurate to use Hardness as a measure of Calcium.)
The Results: The lake water that had been treated with concrete had on average 3 times higher calcium levels than the control.
|Treatment||Average ppm Calcium|
|Large concrete grains||80-120 ppm|
|Medium concrete grains||100-140 ppm|
|Small concrete grains||60-100 ppm|
The range in the results reflect the resolution of the water hardness test. N=3 for the Control, Large, and Small trials, N=4 for the Medium trial.
While students recognized the potential for concrete to increase the concentration of calcium in Donner Lake they were split on the biological significance of this experiment. Skeptical students pointed out that adding 200 grams of concrete to 500mL of water is the same as adding 5 billion kilograms of concrete to Donner Lake. Other students recognized that while their experiment was not realistic on a lake wide scale, the parameters of this experiment could be similar to localized conditions around boat ramps. About 1/3rd of the students said that if the town of Truckee were to build a new boat ramp they would recommend using a material other than concrete, an equal number thought the town would fine to create a new boat ramp out of concrete, and the final third said they were unsure.
In wrapping up this experiment I brought up Isaac Newton’s quote “If I have seen further, it is by standing on the shoulders of giants” and how science is built upon people using and improving on each others’ research. With that in mind I asked the students to provide next years class with suggestions or ideas for experiments that could help answer this question further. The class wrote down a summary of their project and suggestions for next years’ class. Here are some of the future experiments they suggested:
- Try testing the water near and far from the existing concrete boat ramp to look for evidence of localized conditions in the lake.
- Use a water hardness test with higher resolution, ideally < 10ppm.
- Try the leaching experiment with smaller amounts of concrete that might more accurately represent what could happen to Donner Lake.
- This experiment was done in the winter, future experiments should examine if the season could affect the results.
- Try testing different types of concrete products that could potentially end up in the lake.
Both the teacher and I were quite happy with how the first attempt at this experiment went and are very excited to see where next years’ class picks up from the research these students conducted. If you are interested with in conducting a similar experiment in your own classroom and have questions on any of the methods we used feel free to contact firstname.lastname@example.org.