Engage: Show students a picture of piles of leaves, or ask them to think about their yards in the fall. How is their yard different from a forest? What happens to all of the detritus that ends up in a stream, pond, or lake? Allow students to brainstorm for a few moments. Depending on their understanding of decomposition, they may know that aquatic organisms feed on detritus, slowly breaking it down. They may not know how that process affects the dissolved oxygen levels.
Explore: Each group of students should receive a bag of dried leaves, and they should use the scale to measure a fixed amount (5 g works well). Allow students to design the basics of the experiment themselves. Most students will submerge the leaves in their measured water sample, and cover it with plastic wrap. Encourage the creation of control groups. If you are interested in having them measure other parameters, they can do that as well.
Leave the containers in a well-lit space for at least a week (this works within 3 days if using partially decomposed leaves). Allow students to check the DO at pre-determined time periods, and run the experiment for about two weeks. With careful measurement, DO levels drop to less than 1 mg/L by the end of two weeks. Encourage students to be careful when obtaining the water samples to test, especially if they are using a kit that requires them to pour water into a bottle. If possible, have them submerge the bottle in the water sample instead of pouring it from the container.
Explain: When decomposition takes place in an aquatic ecosystem, oxygen is depleted because the bacteria and other decomposers require it in order to function. Without the action of moving water (currents, waves) and by sealing off the interface between the air and the water’s surface, no additional oxygen can enter into the water. Thus, dissolved oxygen levels are quickly depleted. Encourage students to think about how this would change in different aquatic ecosystems, and how this is connected to the process of eutrophication.
Extend: If you had easy access to an outdoor stream, you can create leaf packs from the dried leaves and place them for two weeks in the stream. Be sure to have students weigh the samples beforehand. Carefully remove the leaf packs from the stream, placing them in a bag or other closed container. Return them to the classroom, where students can observe the leaves for signs of predation. Allow the leaves to dry (best to use a dry oven) and re-weigh the leaves. Students should see a response. For more information on this procedure, see the lesson titled “Decomposition in Freshwater” on Learn NC at http://www.learnnc.org/lessons/HeatherLanier5232002011 .
You can also measure Biological Oxygen Demand (BOD) if time permits. There is an inverse relationship between BOD and DO levels, and this is a good way for students to think about the metabolic requirements of aquatic organisms.
Evaluate: Students should complete a lab report and be able to discuss their results in class.