1. Discuss indicators of the health of a body of water such as the Chesapeake Bay.

Discuss with students about the value of a healthy watershed. Ask: In what ways do you think the Chesapeake Bay watershed is important to humans? (Student responses may include the seafood we eat, the water we drink from reservoirs, and fishing, hunting, and other recreation in and around the waterways.) Ask: How can we know if the Chesapeake Bay and other waterways in the watershed are healthy and safe? (Answers may include that there is abundant wildlife, including birds, fish, crabs, and oysters, and that pollutants are not found in the water.)

Explain that scientists monitor the health of the bay and watershed by analyzing what is in the water and also the populations of species that live in and around the bay. Scientists are continuously measuring the amount of oxygen in the water, among other measures.

2. Have a whole class discussion about dissolved oxygen and the health of the Chesapeake Bay and its tributaries.

Ask students why oxygen is important in underwater ecosystems. Help them recognize that a variety of organisms need oxygen to survive, and as part of food webs, the balance of species in an ecosystem depends on appropriate levels of oxygen as well as other needs. Explain that scientists measure dissolved oxygen to monitor conditions in these underwater habitats, checking if they are healthy or not. Dissolved oxygen is a water quality measurement that changes over time, and depends on the temperature of the water and other factors. Underwater ecosystems are sensitive to changes to dissolved oxygen levels, and these levels can fluctuate greatly and be affected by human activities on land.

Review together the Chesapeake Bay Program’s webpage with an overview of dissolved oxygen. Ask:

• Why is dissolved oxygen an important water quality measure to monitor? (Almost all life found in the underwater habitats of the Chesapeake Bay depends on the availability of dissolved oxygen in the water.)
• Why do you think dissolved oxygen levels change over time? (Changes in air and water temperature naturally affect the amount of dissolved oxygen in the water, but human impacts from farming and urban development can lead to very low, unhealthy levels of dissolved oxygen in the bay.)
• What do you think a dead zone is? (Low dissolved oxygen levels can lead to areas where almost no life can survive due to lack of oxygen, referred to as dead zones.)

Explain that these low-oxygen dead zones occur often in bodies of water around the world, including the Chesapeake Bay. Project the National Geographic encyclopedia entry for dead zones, and view the illustrations and captions together. Have students use this entry and Chesapeake Bay Program’s Dissolved Oxygen webpage as references for this activity.

Explain and write on the board these key measures:

• 5.0 mg/L or higher is an ideal dissolved oxygen level for an estuary such as the Chesapeake Bay.
• Below 5.0 mg/L, aquatic organisms can become stressed. The lower the level, the more organisms are unable to survive.
• 0 to 0.2 mg/L  of dissolved oxygen is the measure for a dead zone.

3. Have students analyze dissolved oxygen level data from the Chesapeake Bay.

Tell students they will use FieldScope to examine the dissolved oxygen levels in the Chesapeake Bay and its tributaries. Distribute a copy of the handout Analyzing Watershed Health: Dissolved Oxygen to each student. Have students work in pairs or small groups at computers for this exercise.

Students will work on Part 1 of the worksheet.  Students look at a color gradient that changes over a year’s time. Pause the activity after Part 1 to discuss as a class the seasonal changes of dissolved oxygen levels.

Students should infer that in summer months, warmer temperatures result in lower dissolved oxygen levels. Warm water can hold less oxygen than cooler water; so generally, Chesapeake Bay waters hold less oxygen in the summer than in winter. Also, spring rains can bring a lot of nutrient pollution into the bay, causing algal blooms that can lower the amount of dissolved oxygen in the water.

In winter months, cooler water temperatures and less runoff from agriculture help keep a more healthy dissolved oxygen level.

4. Have students analyze data in the Chesapeake Bay Interpretive Buoy System (CBIBS) data layer.

Have students turn now to the Chesapeake Bay Interpretive Buoy System (CBIBS) data in the layers tab. In Part 2, students create a graph to visualize dissolved oxygen data for the previous year in the Chesapeake Bay.

In Part 3, students refer back to the earlier data set but focus on the area near their location. Help students think about impacts from upstream and downstream where their water flows into the bay.