Day 1: This can be done as a demonstration or as a cooperative activity. To prepare the experiment, set up several jars of aged tap water with sprigs of Elodea and place them in the dark overnight. This causes the levels of DO to drop due to respiration, and allows a better measurement of the change during the experiment. For a cooperative activity, each group needs two jars with Elodea. You should also provide jars of boiled sealed water (with no DO) that the students can test before and after to prove that the light is not affecting the water alone.
Day 2: Once students have tested their water after leaving the jars in different light conditions, they should be presented with the water chestnut problem. Once they have read the background information, they will be given several choices for how to mimic the effects of water chestnut: using a cover on the water, such as styrofoam or aluminum foil; adding algal cultures to create a floating plant ‘mat’; or using floating plants such as duckweed (teacher’s discretion). Allow one-two weeks for the creation and completion of the experiments.
Engage: Students should be encouraged to think about what life would be like if they had to live in a closet, or, if some imaginary large umbrella covered the sky and blocked out all the sunlight. Ask students if they know of any organisms that can survive without sunlight (they might think about animals in that live along the deep ocean vents). Discuss the kinds of adaptations these animals need to survive in such hostile conditions. Explain to students that today they will try and find out what happens to plants in different levels of light.
Day 1: Students will be given the jars with the elodea and the control jars. They should decide how to set up their experiment. Some students may want to cover their jars with a dark cloth, while others may want to place the jar in a closet or cupboard. Either way, make sure that temperature is not a confounding variable. Students should test the DO levels before they start their experiment. Leave the jars overnight and allow the students enough time to test the water again the next day. The jars have to be kept in very low artificial light if they are left overnight, because they might produce too much oxygen and saturate the system.
Day 2: After students test their jars, they should be shown the next set of materials: algal culture, floating plants, various types of covers for the jars, fertilizer. Each group can decide what concentration of fertilizer to use in their ‘pond’, but they should keep one jar as a control. The second jar will become inoculated with algal culture and the pre-determined amount of fertilizer. Jars should be placed in warm, well-lit place. This experiment should be allowed to run for two weeks. Replace water lost to evaporation with aged tap water. Students should continuously make observations on their ponds.
Explain: Water chestnut is an annual plant that consists of floating leaves that are attached to the sediment by a long, tough stem. The plant produces an edible nut that is hard and spiny on the outside, and can remain viable for a decade or more. Water chestnut was introduced to North America in the late 19th century by a well-meaning botanist who thought the plant was beautiful and useful as potential wildlife food. The chestnut quickly escaped the lakes where it was introduced, becoming a nuisance in the Hudson in the 1950s. It prefers slow-moving water up to 5m deep, and once established it forms dense, impenetrable stands of roots that even a kayak can’t get through.
There are a few problems with the water chestnut. First, because of the thick beds that it forms, it’s an obstacle for boaters and people who want to use the river recreationally. It has displaced native aquatic plants, changing the underwater habitat for fish. Because the plant’s leaves cover the water surface, blocking out sunlight, oxygen levels are reduced beneath plant beds. The roots and stems remove oxygen from the water to support respiration, while the photosynthesis that takes place happens only above the water’s surface. The water actually becomes anoxic (oxygen-free) during low tide, becoming replenished only when the tides change.
There may also be some positive aspects related to the water chestnut, although many of these are still being investigated. It may provide a good habitat for invertebrates, some studies show higher fish diversity in the water chestnut, and it removes nutrients from the water. There have been attempts to eradicate the plant, but, it is now more abundant than it has ever been.
Extend: Ask students to think about what their results might mean for aquatic animals living in a situation with low light. Ask the students to think about what kinds of conditions cause low light levels for submerged plants. This would be a good time to show the water chestnut photos and discuss the implications.
Evaluate: Students should complete their lab report and be prepared to give a presentation to the class about how an invasive floating plant can affect the aquatic ecosystem.