Dead Zones

Lake Erie algal blooms, August 2011Some eutrophic lakes (and coastal regions of the ocean) may develop what is referred to as dead zones in the summer. Dead Zones are areas without enough dissolved oxygen to support fish or zooplankton. Lake Erie is especially prone to developing dead zones. This lesson explores what contributes to dead zones, how it can affect the life in a lake and where and how to spot one using data clues.

Grade Levels:

  • National Science Education Standards, 5th-8th grade
  • Michigan Grade Level Content Expectations, 5th-7th grade

Performance Expectations:

  • MS-ESS3-4 Earth and Human Activity. Construct an argument supported by evidence for how increases in human populations and per-capita consumption of natural resources impact Earth’s systems.
  • MS-LS2-1 Ecosystems: Interactions, Energy and Dynamics. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
  • MS-LS2-4 Ecosystems: Interactions, Energy and Dynamics. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.

For alignment, see: NGSS Summary

Lesson Objectives

  • Explain why and how dead zones affect aquatic organisms.
  • Describe how human activities may contribute to the creation of dead zones.
  • Use graphs to investigate the size and location of the dead zone in Lake Erie.
  • Communicate conclusions about the size and location of the dead zones using evidence.

 

Background

Less oxygen dissolved in the water is often referred to as a “dead zone” because most aquatic life either dies, or if they are mobile like fish, they can leave the area. Dead zones typically occur in eutrophic lakes and parts of the ocean – meaning the parts that are quite productive and normally support a good amount of life. A dead zone can occur when there is actually too much life, which can deplete the levels of dissolved oxygen found throughout the water column.

Technically, a dead zone is hypoxic (water with low levels of dissolved oxygen) or anoxic (water that does not contain dissolved oxygen) areas without enough dissolved oxygen to support most aquatic life. Dead zones can form after waters become stratified in the summer and surface and bottom waters do not mix. The thermocline cuts off bottom water (hypolimnion) from new supplies of dissolved oxygen from the air until fall. Adequate concentrations of dissolved oxygen are necessary for the life of fish and most other aquatic organisms. Therefore, the size of the hypolimnion affects the ecology of a lake.

Dead zones occur in many areas throughout North America, particularly along the East Coast, the Gulf of Mexico and the Great Lakes.

What Causes Them?

Dead zones can occur naturally, but they can also be created or enhanced by human activity. There are many factors that combine to create dead zones, but excess nutrients entering the water is a primary cause of dead zones in the Great Lakes. These are considered human-influenced and can be linked particularly to excess nutrients that run off land or wastewater that can overflow into rivers and lakes.

The nutrients from these sources can stimulate algae growth, more than the lake would naturally support, which then sinks and decomposes in the water. The decomposition process consumes oxygen and there is none to replace it because of the temperature and water density gradient during the summer. The dissolved oxygen supply available for aquatic life is depleted until the lake “turns over” or the stratified layers of water mix.

Invasive species like zebra and quagga mussels are also thought to contribute to the development of dead zones, specifically in Lake Erie. The mussels filter out nutrients and green algae early in the year and release fecal pellets (excess nutrients) late into the summer. They also reject the blue green algae, which contributes to harmful algal blooms (which in turn can contribute to dead zones).

See Properties of Water and Oxygen in Water for more information on how the physical properties of water contribute to dead zones.

Lake Erie Dead Zone

LakeErieMap_DeadZoneDiagram
Figure 1. Researchers compared the Lake Erie dead zone area in the 1970s with dead zones in 2001 and 2002.

 

Activity

Lesson Sources

Scavia, D., Professor of Natural Resources and Environment and Environmental Engineering, Graham Sustainability Institute Director and Special Counsel to the University of Michigan President for Sustainability. Donald Scavia has studied coastal dead zones for 25 years and led the first Gulf Dead Zone scientific assessment on behalf of the Clinton White House.

Articles and Other Resources

  • NOAA Ocean Service Center – Dead Zones-Ocean Facts, Accessed April, 2013
  • Scientific American – Sept. 25, 2012 – What Causes Ocean Dead Zones?
  • Gristmill – June 19, 2013 – Dead zone could break records in Gulf this year
  • Louisiana Marine Education Resources – Gateways to Aquatic Science. On Again, Off Again – The Dead Zone. Louisiana Sea Grant. Louisiana State University, Baton Rouge, LA 70803. Authors: Lindstedt, D. Website, accessed December 1, 2009.
  • The Life of the Lakes, A Guide to the Great Lakes Fishery. 2003. MICHU-03-400. Michigan Sea Grant. University of Michigan, Ann Arbor, MI 48109. Authors: Dann, S, Schroeder, B.
  • Water on the Web – Monitoring Minnesota Lakes on the Internet and Training Water Science Technicians for the Future, University of Minnesota-Duluth, Duluth, MN 55812. Authors: Munson, BH, Axler, R, Hagley C, Host G, Merrick G, Richards C. Website, accessed December 1, 2009.

Data Sources

Great Lakes Coastal Forecasting System. NOAA-Great Lakes Environmental Research Laboratory (GLERL) Ann Arbor, MI 48108. Authors: Schwab, DJ, Beletsky, D, Bedford, KW, Lang, GA.

Great Lakes Water Data Sets for Teachers, Eastern Michigan University, Ypsilanti, MI 48197. Project supported by the Office of Education and Outreach at NOAA’s Great Lakes Environmental Research Laboratory, Ann Arbor, 48108. Authors: Rutherford, S, Coffman, M, Marshall, A, Sturtevant, R, Klang, G, Schwab, D, LaPorte, E.