Water Chestnut & Dissolved Oxygen

Engage:

If you have completed a previous lesson related to water chestnut, ask: How does the invasive water chestnut plant affect the Hudson River? as a formative assessment of student learning.  Students should diagram or write out the changes on a whiteboard or notecard so that you can quickly check for understanding.  Then, have a volunteer come up to the board and draw the changes in dissolved oxygen in a water chestnut bed over 24 hours. 

If you have NOT completed a previous lesson related to water chestnut, use the “Water Chestnut Intro PowerPoint” to introduce students to water chestnut and what it looks like in the Hudson River. There are notes to help you.  Then, walk students through the animation that demonstrates how oxygen levels change.  Have students keep track of the oxygen levels using a chart like this:

Based on this animation, students should notice that what matters for the levels of oxygen is the TIDE, not the time of day.  You can use the last slide in the powerpoint to introduce the idea of DO changing through time.

Explore:  

Students complete the worksheet, answering questions to understand how water chestnut plants change the dissolved oxygen levels spatially across the plant bed and temporally through the growing season. 

Explain:

As water chestnut leaf out in late spring, they grow underwater, releasing oxygen into the water column.  However, by mid-summer, water chestnuts have leafed out completely, forming a dense bed of floating vegetation through which little sunlight can penetrate. The oxygen that these plants release mainly goes up into the atmosphere, instead of into the water.

            Students may need some help interpreting the graphs.  The first graph pair shows two years of data; the blue and red lines show data from 2005 and 2006, respectively. Dotted lines show data taken from the channel, and solid lines show data taken from the Trapa natans  (water chestnut)bed. The students should be able to understand that the DO is much lower within the water chestnut bed once the plants leaf out.  As the summer progresses, it is very obvious that the dissolved oxygen levels within the beds drop dramatically, even though the channel itself never becomes hypoxic.

            The photos of the water chestnut bed with the accompanying graph, depicting DO measurements across the bed, from the shore to the main channel, should help solidify students’ understanding of the way the plants affect DO levels.

Extend:

You can either have students do this brief activity individually or project the graphs on screen to begin a class discussion.

• Use the HRECOS Current Conditions link https://www.hrecos.org/ and the National Estuarine Research Reserve System (NERRS: http://cdmo.baruch.sc.edu/get/export.cfm) to compare DO in two sites: one which has a monitoring station highly influenced by water chestnut—Tivoli South Bay on NERRS, and one which has a monitoring station with little influence from the water chestnut—Norrie Point on HR-ECOS. While Norrie Point has water chestnut beds nearby, the monitoring station is further into the channel, so the effects of the DO changes are not seen.  Students can compare this with the trends they see in Figure 2 on the worksheet and in graphs from previous lessons, where the channel DO is higher and more stable than it is within the water chestnut beds. 

• On the HR-ECOS site, choose Norrie Point (hydro) as your site 1, and choose ‘Dissolved Oxygen %’ for the parameter. Set the same date range on both HR-ECOS and and NERRS. To see the greatest differences, choose a 3 month window that includes late summer or early fall, such as 2012/06/01 – 2012/09/01.  Then select “Plot 1.”  
• On the NERRS site, select the Hudson River, NY site from the map. Then, select Tivoli South Bay from the list of monitoring sites and click "proceed with this station".  Then select "Graph Data", put in the same date range as on the HR-ECOS site, and select "Dissolved Oxygen (%) from the drop down menu under Choose 1st Parameter and click "Graph!". 
• By comparing the graphs, the students should be able to see that the DO % is lower at Tivoli, and has more variability. If students have questions about the variability in the data observed from the Tivoli Bays site (DO exchange in tidal cycles), they can complete the “How does water chestnut impact the Hudson River” lesson.

Evaluate:

Assess student understanding by their answers to questions on the worksheet and the depth of discussion during the Extend activity.