Filtered Learning: Middle Schoolers Take the Plunge

By Eve Pranis, June 23, 2008

When middle-school students in Greenwood, AR, set out to create a schoolyard habitat and outdoor classroom, they knew four habitat components were essential: food, water, shelter, and places to raise young. But with no water source in sight, their vision could have dried right up.

To solve their problem, the young stewards decided to create their own pond, bog, and waterfall. A donated liner, a submersible pump, lots of elbow grease, and a gift of rocks from a contractor helped bring their 3,000-gallon pond to fruition.

"Students wanted to have a healthy habitat that would support fish, then learned that they'd need to have some sort of filtration system to remove the ammonia and nitrate that came from the fish waste," says parent volunteer Karen Underdahl.

So with encouragement from their sixth grade science teacher, the students designed both an upper and a lower pond with a connecting waterfall, and used plants to clean up the act. Water from the lower pond is pumped to the upper one, where, students theorized, submerged plants (such as anacharis) and those with floating leaves and long roots that grow down (such as water hyacinths) will filter waste by taking in nutrients through their leaves and roots, respectively. "We transferred many native plants from a local lake, then decided to purchase a few exotic ones from a water garden supply catalog," says Karen.

Meanwhile, Karen's daughter, with an eye to a science project, tried creating a more elaborate "natural vegetable filter" in their home pond. (Don't get confused. It's actually aquatic plants, rather than vegetables, that filter the water!) This involved building a structure with concrete blocks in one end of the upper pond, lining it, putting gravel in the base, then filling it with submerged and floating-leaved plants. Water is pumped from the main pond up to the vegetable filter, where the highly concentrated aquatic plants take in nutrients before the water cascades down to the lower pond.

To test their filtering theories, students used the Pondwater Tour kit to check the levels of dissolved oxygen, pH, ammonia, and nitrates in both ponds, at the base of the waterfall, and in a control container of distilled water. The plant filters were, in fact, successful. (You can often obtain water test kits from a local conservation or Fish and Wildlife office.)

Before students actually moved fish in, they used math to determine how many should be added. One rule of thumb is that a pond can support 1 inch of fish per square foot of surface area. "Although the kids at first figured that they could stock the pond with 90 1-inch fish, they soon realized that they'd need to estimate how fast and large they'd grow, then adjust their numbers accordingly," explains Karen. She adds that they don't need to feed their fish (mostly carp) since they live on algae, plant roots, and insects during active periods, then become inactive during the winter. But at one point, her careful observers were concerned that the fish might be hungry because the pond was too clean. Armed with the knowledge that algae multiply faster in sunny spots, and tend to grow along plant stems, students tried rotating some of their potted plants to follow the sun.

Besides enjoying the macro-experience of the pond ecosystem, Karen's students delighted in using microscopes to explore the algae and native snails they'd imported with the pondwater, and the ramshorn snails they'd purchased. "These kids have never gotten to do anything like this," says Karen. "The teacher and I were sure that it had been worthwhile when a fifth grade girl said, 'It's like taking the science book and bringing it to life with our own hands-on work!'"

Today's site banner is by Dutchlady1 and is called "Plumeria 'Texas Rose'"