Tuesday, August 14, 2018

What Goes into an Experiment? - Sentinel Floating Islands


     I joined Sylvia at Sentinel High School again as her two classes learned about experimental design. They had been given an article the previous session detailing an experiment about the efficacy of floating islands in removing excess nutrients from the water. Broken into groups, the students sifted through the fancy terminology and scientific structure of the article to find the key points to summarize.

     Sylvia had the students write unfamiliar vocabulary words on the board. She addressed the first word off the bat as it pertained to the experiment's first design.

     "Batch mesocosm can be one of those stressful words," Sylvia said. Mesocosm just means medium-sized-world, as the experiment was not on a tiny scale or a large scale. "Batch" referred to the way the researchers included water in the system. They created batches of water that mimicked the composition of natural pond or creek water and held it motionless in containers. This is contrast to the second design, which allowed that water to flow.

     The students described the batch mesocosms as being a box shape with a native macrophyte on the small floating island. Sylvia then explained another vocabulary word. "A microphyte is basically a small plant." Like algae. She compared the prefix to the word "microscope" which the students were familiar with, to make the connection that microscopes help you see tiny things such as microphytes. A macrophyte, then, is an aquatic plant that is large enough to see, like grass. So each little island had a native plant living atop it.

     "Why do you need a control?" Sylvia asked.
     "What's a control?"
     Sylvia explained that one of the batches did not have a floating island or any plants, just a metal sheet to produce the same amount of shade. This plantless batch was the control.
     "So, to compare to the other one," a student said.
     "Why do they have three experimental tanks?" These would be the batches with plants.
     "Alter them to see different effects," a student suggested. Indeed, a researcher would use their experimental tanks in comparison to the control to see which variables produce measurable changes and which do not, or which produce the most change. Sylvia got to a more basic point though, which is that researchers want to make sure their "data is stable." This means that they gather enough data from the experimental batches that they can find a pattern across all of them. This ensures that if, for example, one of the plants is dying, it doesn't determine all of the results. Having extra batches helps make sure the correct variables contribute the desired information.

     Then they moved on to the next design. The flow tanks took place in "a giant shipping container" divided in half lengthwise with water flowing through each side. Here the researchers wanted to know if the rate of water flow mattered, so one side had slow-moving water and the other had fast-moving water. The steel container had a special coating to prevent metal ions from entering the water, since they can be toxic to plants. Each side also had soil at the bottom. Sylvia reminded the students that soil can have nutrients in it, especially when it contained decomposed materials, so the soil imitated natural conditions where the nutrients came from within the system.

     "One of them absorbed more than the other," a student noted.
     "Yes, definitely. Which one?" Sylvia asked.
     "I think it was the high flow," the student answered.
     Sylvia reviewed the previous lesson on the phosphorus and nitrogen cycles. Both of these are nutrients that are important to plant growth, but in large amounts they can lead to the overgrowth of microphytes: algae blooms. The side with high water flow had more of these nutrients removed by the floating island plants, indicating that faster streams are better at preventing algal overgrowth.

     With the design out of the way, Sylvia broke the experiment down into its most basic parts. She guided the students through identifying the dependent and independent variables of each design. The students identified the dependent variables fairly quickly (the presence or absence of a floating island in the batches, and water flow in the flow tanks), but had trouble with the independent variable. Sylvia emphasized that the answer was simple, and the same across both designs, since the researchers were interested in one thing. This was the level of nutrients (phosphorus and nitrogen) in the water, as lower levels mean the floating islands are removing them more effectively.

Mini floating island models
     These experiments both took place in a laboratory environment, which is useful because "you can control what happens," as one student said. In other words, there are fewer variables at play because you can choose which ones to include, which then "makes [the data] easier to interpret," as Sylvia said.

     Field experiments have advantages too. "You get all the natural things that could happen right there," another student said, so you don't have to try to replicate all the variables that do exist and interact. If an experiment left some of these variables out, the results might not accurately reflect what would happen in a real ecosystem when one variable changes. This helped the students realize that an effective laboratory experiment should aim to be as much like the natural system as possible.

     Sylvia set the students to work in small groups on a worksheet that asked them to identify other variables that could be important in designing a floating island experiment. I wandered around to guide their thought processes, without giving away any answers.

     Sylvia's repeated use of small groups was intentional, as it can help students develop leadership and social skills through conversation practice. It also helps them consider multiple angles and the ideas of their classmates when dealing with complex issues.

     Many of the students looked sleepy and stayed quiet through most of the lesson, as I remembered I was during my high school mornings, but the answers on their worksheets gave away the connections they were making with the material. Sylvia was thrilled.

-Cassie Sevigny
AmeriCorps Team Member
Media Coordinator

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