Engage: Show students pictures of water chestnut and the native water celery plant. Ask: What do you know about the water chestnut invasion? View aerial photos of the water chestnut beds. Ask what they think happened to the native water celery population, and the impacts of the changes on different factors such as dissolved oxygen, fish populations, etc. Record their hypotheses on the board.
Explore: Hand out the “How does water chestnut impact the Hudson River?” worksheet. The beginning asks the students to draw a few things on the graph to make it easier to understand. You may want to guide them through this process using the PowerPoint associated with this activity.
Explanation: 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. It can also induce severe dissolved oxygen depletion beneath the plants. Because the plant’s leaves cover the water surface instead of being equally distributed throughout the water column, much of the oxygen produced by the plant is released to the air instead of being dissolved into the water. The plant also effectively crowds the water’s surface and blocks sunlight to phytoplankton or plants beneath it, such as water celery, that would otherwise also produce oxygen. The water can become anoxic (oxygen-free) during low tide, with the oxygen being replenished only when the tides change. The native water celery however, has leaves distributed throughout the water column and distributes its oxygen directly to the water column. For this reason, oxygen production is influenced far more by diurnal cycles than tides.
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 some invertebrates, increase fish diversity, and effectively remove nutrients from the water. There have been attempts to eradicate the plant, but it is now more abundant than it has ever been.
Extend: Students can view the animation to understand how a location with water celery is different from one with water chestnut. They can also read the Fact Sheets provided with this lesson to gain more background information.
Evaluate: Students will complete the graph interpretation exercise and answer the questions on the worksheet. Make sure students are able to answer the question of how the native water celery plant differs in its oxygen regime from the invasive water chestnut plant.