Objectives
Following this lesson, students will be able to:

• Describe how properties of water are related to Dead Zones
• Describe how dissolved oxygen and temperature levels can influence populations of organisms
• Graph dissolved oxygen levels and graph water temperatures
• Analyze water temperature versus depth graphs to answer questions
• Analyze dissolved oxygen versus depth graphs to answer questions

Background
Stratification in lakes prevents surface and bottom waters from mixing (Figure 1). During the summer, bottom water (hypolimnion) is blocked from new supplies of dissolved oxygen from the air until fall. Adequate concentrations of dissolved oxygen are necessary for the life of fish and other aquatic organisms. The size of the hypolimnion affects the ecology of a lake. For example:

• Big hypolimnion: In the Great Lakes, areas with a big (deep) hypolimnion (e.g., eastern Lake Erie, Lake Michigan) will have plenty of dissolved oxygen to last the summer.
• Shallow hypolimnion: Areas with a shallow hypolimnion (central Lake Erie, bays) have less total dissolved oxygen in bottom layer.
  
• Very shallow hypolimnion: In very shallow waters (western Lake Erie and most nearshore waters), the whole water column warms. As a result, a hypolimnion may not form at all and bottom waters remain well-oxygenated.

Figure 1. Water Temperature and Lake Stratification

Lakes have different levels of productivity – meaning the amount of nutrients available and growth that they can support. Defining trophic (nutrient or growth) status is a means of classifying lakes in terms of their productivity. For example:

• Oligotrophic: (ol-i-goh-trof-ik) An oligotrophic lake has low nutrient concentrations and low plant growth.
• Eutrophic: (yoo-trof-ik) A eutrophic lake has high nutrients and high plant growth.
  
• Mesotrophic: (meso-trof-ik) Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes.
  

Eutrophic lakes may have Dead Zones in the summer. Dead Zones are hypoxic or anoxic areas without enough dissolved oxygen to support fish and/or zooplankton. Increased organic matter from both internal inputs (e.g., algae production) and external inputs (e.g., sewage) can accelerate the depletion of dissolved oxygen in the hypolimnion in the summer. Organisms living and breathing in the hypolimnion and the decomposition of algae and other organisms can also speed up the loss of oxygen in the hypolimnion.

Ecological Impact of Dead Zones

• Fish: Lakes that become stratified in the summer may have a two-story fishery: warm- and cool-water fish living in the epilimnion and cool/cold-water fish in the cold, oxygen-rich hypolimnion (Figure 2, Ecological Impact of Dead Zones, above).
  
  
  
  Figure 3. Lake Trout
  
  Fish can be very sensitive to changes in water temperature or dissolved oxygen concentrations. (Example: Figure 3, Lake Trout, above). For example, yellow perch cannot tolerate low dissolved oxygen levels and need dissolved oxygen concentrations of at least 2.0-3.0 mg/l. When oxygen concentrations get too low in the hypolimnion, fish may move vertically or horizontally out of the hypoxic area. Fisheries scientists do not know exactly how these ‘dead zones’ impact the health of fish populations.

• Zooplankton: Zooplankton tolerate hypoxic conditions better than fish. Zooplankton can live in waters with dissolved oxygen concentrations as low as 0.1-0.2 mg/l, but not much less.

Key Terms: Anoxic, density, dissolved oxygen, epilimnion, eutrophic, eutrophication, hypolimnion, hypoxic, metalimnion, thermocline, trophic status

Lesson Sources:

• 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.
• Water on the Web - Monitoring Minnesota Lakes on the Internet and Training Water Science Technicians for the Future - A National Online Curriculum using Advanced Technologies and Real-time Data. 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.
  

• 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

Dead Zones - Lesson 3 Activity A: Graphing Temperatures

Grade levels:

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

Subjects: Science

Dead Zone Resources

• Lake Erie Dead Zone - EPA
• Facebook article: Dead Zone Growth is an Ecological Time Bomb, researcher Donald Scavia
• Nitrogen and Lake Erie Dead Zone - Ohio Sea Grant
• USGS Identifies Top Gulf Dead Zone Pollution Sources (PDF)

• Phytoplankton (PDF)
• Zooplankton (PDF)

• Lake Erie Dead Zone
• Mississippi River Dead Zone (source: Environmental Working Group)
• Lake Erie Bathymetry (source: NOAA)