Hudson River Food Webs

Preparation

1. Organisms, live or prepared specimens, Part 1
Place orders for prepared slides or live animals in advance. Check with your supplier to find out when to order live animals. Preparing live slides takes some time, although students can help if you have dissecting and compound microscopes. Making slides with live animals can be done during class with students or ahead of time. A product such as ‘Protoslo’ helps calm the organisms down, and is available for around $6 from any scientific supply company. A company that provides “demoslides” of live organisms is Connecticut Valley Biological Supply; each demoslide costs around $3, and will allow live organisms to be viewed without advance preparation of specimen slides. To collect your own plankton, a great option is to use a plankton tow, which can be easily made from household items.

2. Food Webs of the Hudson River, Part 1
You may want to view the animations students will be viewing during Part 1. This will provide information and review about the food webs you will discuss with students.

3. Mini-graph sets, Part 2 and 3
During Part 2 students will be split into six teams. Each student will need the set of mini-graphs for their team’s own organism. (For example in a class of 30, five students will get Chlorophyll mini-graphs, five will get Nauplii mini-graphs, five will get Rotifer mini-graphs, five will get Copepod mini-graphs, five will get Cladoceran, and five will get Unionid mini-graphs.) Each student will need his or her own zebra mussel mini-graphs. All the mini-graphs can be printed on cardstock or laminated for easy reuse. NOTE: Chlorophyll measurement is a well-established method to assessing the quantity of phytoplankton in an aquatic environment. Scientists measure the amount of carbon as a proxy for phytoplankton production.

4. Videos and Readings for Students, and Background Knowledge for TeachersThe American Museum of Natural History’s “River Ecology” program has excellent supplemental videos and readings focused specifically on the zebra mussel invasion. The "Overview" and “Curricular Materials” links bring you to readings and videos.  Videos feature Dr. Strayer and other Cary Institute scientists.

Part 1: Identifying Food Chains in a Hudson River Food Web

Engage: Begin by asking a formative assessment question: What lives in the Hudson River? Ask students to sketch, individually or in pairs, a food web for the Hudson River. Once they have drawn their food webs, students can view and critique other food webs. Have groups share their food webs. Discuss what students have created, and create a class-wide food web on the board. Ask students to think about which parts of the created web may be missing organisms or missing links between organisms. Students often leave out important parts of the food web, especially invisible or hidden organisms and detritus. Students should save their webs in order to add to them later.

Explore: Introduce four habitats that are subsets of the larger Hudson River habitat: marsh, brackish channel, freshwater channel, and fresh water shallows. Allow students to go online to view the four different food webs of the Hudson River animations, or distribute copies of the food webs to students. Distribute Student Worksheet packets. Students will use information from the website to complete page 1. Review answers with students. Compare the food webs in each habitat. What organisms are in all of the diagrams? What organisms seem to specialize in one or two habitats? Point out micro- and macro-invertebrates.

Students should view live organisms or prepared slides at the microscope stations in order to help understand what forms the base of the Hudson River food web. On page 2 of their packet is a place to sketch the organisms. They can use the Microbe ID pages to find the names of the organisms. Students can find information about food/prey and predators by referring back to the website.

Explain: Discuss what students have discovered. They should know that they have been looking at organisms at the ‘base’ of the Hudson River food webs – the producers (algae and phytoplankton) support the primary consumers that feed the larger consumers in the ecosystem. Make sure that all students have noticed the detritus and bacteria in the food web – these components of the ecosystem are a more important food source than phytoplankton.

Extend: You may want to show the two video clips titled “The Problem” and “Observation” https://www.amnh.org/learn-teach/curriculum-collections/river-ecology/curricular-materials from the American Museum of Natural history. These video clips could serve as a preview of the topic of the next part of this lesson. Students will see scientists collect and analyze information about the early years of the zebra mussel invasion in the Hudson River. The “Curricular Materials” link brings you to the videos.)

Evaluate: To complete page 3 in their packet, students should use the Hudson River Food Webs Reading assignment. This can also be done while students are rotating through the microscope stations. You may want students revise their original food webs, which can be used as a formative assessment. Lower level students may need help understanding how bacteria, detritus, and phytoplankton are involved in the food web. Make sure that these components are included.

Part 2: The Invasion of the Zebra Mussels -- Population Changes

Engage: Formative Assessment: How might the Hudson River ecosystem change if a filter feeding organism (one that eats phytoplankton and small zooplankton it filters from the water) were introduced into the Hudson River? What changes might occur to the food web? This question can be answered individually or as a class – if you are doing this as a class, ask students to answer the question first on whiteboards so that you can check for understanding. Lower level students will focus on predator-prey relationships, or one-step relationships, such as the fact that if a new mussel is introduced, there will be fewer phytoplankton in the river. More advanced students will be able to make additional links and will start to think about the “ripple effect” of a new consumer in the system.
(Optional: If you did not show the two video clips at the end of Part 1, you may want to do so now.) Show the first portion of the “Hudson River Food Webs” PowerPoint, including the slide entitled: Zebra Mussels arrived in the Hudson River in 1992 to introduce students to zebra mussels. This slide asks students to consider: How do you think this affected other organisms? Why do you think zebra mussels affected other organisms? Ask students to make predictions about other groups of organisms in their food webs. This slide also has photos of the organisms students will learn about in teams.

Explore: Introduce the six groups of organisms used in this activity: Phytoplankton (Chlorophyll), Nauplii (immature copepods), Rotifers (microzooplankton), Copepodsand Cladocerans (both are macrozooplankton), and Unionids (native mussels including the pearly mussel). (Note: Since it is difficult to count phytoplankton because they are so small, scientists have used measurements of chlorophyll instead. Daphnia, which students may have looked at in Part 1, are cladocerans.) Divide students into six teams, one for each organism. Each team will examine graphs showing population data for their organism. The graphs display data collected by scientists over approximately twenty-five years. Each year is an average of bi-monthly sampling trips to the river. Ongoing data collection continues to this day. Student teams will look at the first two graphs during Part 2 of the lesson and look at the final graph during Part 3.

Students will use pages 4 and 5 in their Student Packets for this activity. Distribute the first graphs for 1987-1991. These are pre-invasion population numbers collected before the zebra mussels arrived in the Hudson. Orient students to the graphs by having them look at the labels for each axis. Have them note the scale. Review or introduce the term ‘trend’ which is used on the data sheet. In teams, students analyze their first graph and complete the corresponding portion of their worksheet. Discuss. Repeat with the 1993-2004 graphs that display early invasion population numbers.

Explain: Reconvene so that teams can report on what they learned from the graphs. These graphs are contained in the PowerPoint so they can be used during the discussion. (Red vertical lines on the graphs mark 1992, the year when zebra mussels began to be prevalent in the estuary, and 2005, when a major shift in zebra mussel populations was recorded by scientists.) Students should begin to fill in the ‘1993-2004’ column in the chart “Trends in the Living and Non-Living Environment” in their packets as their classmates provide the information. NOTE: A key to this chart is provided at the end of this lesson plan. Show the slide with the graph of zebra mussel population numbers. Now have students record their predictions about how the zebra mussels will affect two abiotic factors (water transparency and dissolved oxygen), fish and submerged aquatic vegetation and record those predictions on their charts. Show the remaining PowerPoint slides for Part 2.

Help students understand that living organisms can change abiotic factors in the environment, and these changes in turn can cause biota in the water to respond. Phytoplankton, algae, and submerged aquatic vegetation increase the dissolved oxygen level in the water, while heterotrophic animals decrease the dissolved oxygen. Filter feeders such as mussels can increase the clarity of the water by removing some of the suspended solids (organisms, silt, etc.). The enormous number of zebra mussels efficiently and rapidly filtered the Hudson River, increasing the transparency of the water, allowing sunlight to penetrate more deeply. This allowed submerged aquatic vegetation to grow at greater depths, become more plentiful, and increase in biomass.

Students should now complete the questions for Part 2 in their packets.

Extend: Show the video clip “Results” from the American Museum of Natural History. The video shows scientists in action as they collect data in the field and gives further information about the effects of zebra mussels.

Part 3: Small, Medium, & Large Zebra Mussels – The Later Invasion Years

Engage: Formative Assessment: How did zebra mussels affect the dissolved oxygen and water transparency of the Hudson River? Students should be aware of the decrease in dissolved oxygen and an increase in water transparency from the previous part of the lesson. Ask: Are the changes caused by the zebra mussels “good” or “bad”? Explain that in ecology, we don’t use these words to describe ecosystem changes – we just explain the changes. It also depends on your ‘point of view’ – if you are a phytoplankton and your population has declined due to extensive filter feeding by zebra mussels, the invasion was “bad”. However, if you are a submerged aquatic plant and your population has because water transparency has increased due to filter feeding by zebra mussels, the invasion could be “good”.

Explore: Ask: What do you think has happened to the populations of your organisms since 2005? Ask students to explain their answers and give their reasoning. Tell students that a major change in the zebra mussel population occurred about 2005. Before they learn the details, have students reassemble their teams and examine the final graph (2005-2009) in the series and answer the first question in Part 3 in their packets. Have teams report out their information and begin to fill in the ‘2005-2009’ column in the chart they began in Part 2. Complete the chart by using the slides in the PowerPoint.

Explain: Show the American Museum of Natural History video, “Going Further,” to introduce the changes in zebra mussel class sizes first noticed about 2005. You may wish to show this short video two or three times, discussing the information after each viewing. Have students complete the student packet. Discuss possible future changes in zebra mussel class sizes, organism populations, or abiotic factors.

Extend: For a more in-depth analysis of the changes in different population groups in the Hudson, use the lesson called “Graphing Zebra Mussel Data” in the Invasive Species module of the Changing Hudson Project curriculum.

Evaluate: The crane fly questions on the last page of the student packet can be used as a summative assessment. You may also return students’ food webs from the first part of the lesson, and ask them to add details to the Hudson River ecosystem (they should include at least two abiotic factors, dissolved oxygen and water transparency), then have them note which parts of the ecosystem have been affected by the zebra mussels and how they changed. Alternatively, students can write a 1-2 page paper about the effects of the zebra mussel invasion on the Hudson River.