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# Activity: Sizing Up the Lake Erie Dead Zone

Summary: By analyzing temperature and dissolved oxygen data from stations in Lake Erie, students estimate the size and shape of a dead zone.

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## Procedure

Students will compare temperature and dissolved oxygen at each station in order to estimate the size and shape of the hypoxic area in Lake Erie.

### Part 1 – As a group

Analyze all temperature and dissolved oxygen data and graphs.

• Answer data sheet questions 1 through 9.

### Part 2 – Working in small groups

Each small group will draw Lake Erie and label all six stations.

• Draw Lake Erie, using NOAA map of stations as a guide
• Label each of the six stations (just estimate the location of the stations)
• Write the station number and depth of each station on map
• Label stations with hypoxic conditions using red
• Draw a circle around the area in Lake Erie which is likely to have hypoxic conditions
• Answer question 10 using the map
• Groups communicate conclusions to the class using data, graphs and maps as evidence.

### Part 3 – As a class

Compare data sheet answers and maps from each group and discuss:

Why do you think hypoxic conditions exist in this portion of Lake Erie?

• Hypoxic conditions may form in the central basin and not in the other basins because the central basin has a shallow hypolimnion and has a limited amount of dissolved oxygen at the bottom of the water column.
• The western basin is the shallowest part of the lake. Hypoxic conditions do not tend to form in the western basin because the western basin is often too shallow for a hypolimnion to form and the water column remains well mixed throughout the summer. Note: Dead zones do not typically form in nearshore areas for the same reason.
• The eastern basin is the deepest basin. The hypolimnion is very deep and will likely have plenty of dissolved oxygen to last the summer.

How might a dead zone in the central basin affect organisms?

• Fish and zooplankton need dissolved oxygen to live. Areas without enough dissolved oxygen may not be able to support fish and/or zooplankton.
• Cool/cold-water fish living in the cold, oxygen-rich hypolimnion, such as yellow perch, can be very sensitive to changes in water temperature or dissolved oxygen concentrations.
• For example, yellow perch cannot tolerate low dissolved oxygen levels and need dissolved oxygen concentrations of at least 2.0-3.0 milligrams per liter (mg/l).
• When oxygen concentrations get too low in the hypolimnion, fish may not thrive or may move vertically or horizontally out of the hypoxic area. Fisheries scientists do not know exactly how dead zones impact the health of fish populations.

How might human activities contribute to the creation of dead zones?

• Nutrients encourage growth in the epilimnion. When aquatic organisms (such as algae) in the epilimnion die, they sink to the bottom and decompose. Decomposition depletes dissolved oxygen in the hypolimnion.
• Excess nutrients from external sources (such as sewage) can accelerate the depletion of dissolved oxygen in the hypolimnion in the summer.
• Agricultural and industrial pollution put large volumes of nutrients into Lake Erie. Sources of nutrients include wastewater treatment plants and agricultural runoff (such as fertilizers).