Dissolved Oxygen and Lake Stratification

Oxygen is the key to life — most organisms cannot survive without it, even those under water. Seasonal weather patterns and the physical properties of water can affect temperature and dissolved oxygen levels throughout the water column. Why is this important? Because the seasonal weather patterns and cycles are directly related to how much life an aquatic environment can support.

Grade Levels: Middle School 5th-8th grade

Performance Expectations:

  • MS-ESS3-4 Earth and Human Activity: Construct an argument supported by evidence for how increases in human population and per-capita consumption of natural resources impact Earth’s systems.
  • 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 Next Generation Science Standards Summary

Goal: Students will be able to describe how lake thermal stratification and dissolved oxygen levels relate to a lake’s ability to support animal life. This lesson and activity utilize the 5 E learning cycle. To find out more, check out this 5E Instructional Model Factsheet.

Objectives

Upon completion of this lesson, students will be able to:

  • Describe what thermal stratification is and why some lakes in temperate regions stratify.
  • Summarize how lake thermal stratification affects dissolved oxygen.
  • Construct and interpret graphs of dissolved oxygen and water depth.
  • Understand and define hypoxic zones, anoxic zones and dead zones.
  • Discuss the importance of dissolved oxygen to organisms.
  • Understand the connection between nutrient inputs and dead zones.

Terms to know by the end of the lesson:

  • Dissolved oxygen
  • Turnover
  • Atmospheric diffusion
  • Thermal stratification
  • Hypolimnion, metalimnion and epilimnion
  • Productivity
  • Hypoxic zone
  • Anoxic zone
  • Dead zone
  • Bacterial decomposition

Background

From late spring through early fall, some lakes in temperate climates experience thermal stratification, a phenomenon wherein lakes separate into three distinct thermal layers (Figure 1). The warming of the surface of the water by the sun causes water density variations and initiates thermal stratification. Cooler, denser water settles to the bottom of the lake forming the hypolimnion. A layer of warmer water, called the epilimnion, floats on top. A thin middle layer called the metalimnion (or thermocline) separates the top and bottom layers and is characterized by a rapid change in water temperature. This separation often is strong enough to resist mixing of the layers by the wind.

The most extreme thermal stratification occurs within lakes during the warm summer months. During fall turnover, the epilimnion cools, sinks and falls below the thermocline, resulting in mixing. Thermal stratification of a lake depends on the lake’s depth, shape and size. Some small, shallow lakes may not experience seasonal thermal stratification because the wind mixes the entire lake. Other lakes, such as Lake Erie, have a combination of geographic location and water depth that regularly produces thermal stratification.

13-407-Water-Temperature-Illustration
Figure 1: Water Temperature and Lake Thermal Stratification.

Oxygen can enter a lake via three different routes. The main mechanism is atmospheric diffusion where oxygen in the air is absorbed by surface water due to a difference in oxygen concentrations. Second, aquatic plants photosynthesize and release oxygen into the water. Finally, rivers and streams bring oxygenated water into the lake. In stratified lakes, the hypolimnion receives little oxygen from atmospheric diffusion and is too dark to support oxygen-producing plant life. Riverine input has only minimal impacts on the oxygen content of large water bodies such as Lake Erie. Thus, the deep hypolimnion receives very little dissolved oxygen during summer thermal stratification.

Lakes can be described by their productivity. This refers to the amount of nutrients available in a lake and the primary production, or plant and algal growth, they support. Defining trophic (nutrient or growth) status is a means of classifying lakes in terms of their productivity levels. Identified tropic levels are:

  • Oligotrophic (olig-oh-trof-ik) – An oligotrophic lake has low nutrient concentrations and low plant growth (e.g., Lake Superior). It is usually considered to have low productivity.
  • Eutrophic (yoo-trof-ik) – A eutrophic lake has high nutrient concentrations and high plant growth. (e.g., Lake Erie). It is considered to have high productivity.
  • Mesotrophic (meso-trof-ik) – Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes. They are considered to have average productivity.

In eutrophic lakes, such as Lake Erie, large blooms of algae grow at the surface during the summer. The algae need large amounts of nutrients in order to form these blooms. As the algae die, the bloom sinks to the bottom and is decomposed by bacteria. Decomposition by bacteria, or the biological separation of a substance into simpler elements, requires oxygen. Oxygen consumption and low oxygen input in the hypolimnion combine to create extremely low oxygen levels during thermal stratification.

LakeErieMap_DeadZoneDiagram

Figure 2. Dead zones in Lake Erie from 1970-2002.

Once dissolved oxygen levels drop below 2mg/l, the water is described as hypoxic. As it approaches 0mg/l, it becomes anoxic. A dead zone is an area within a lake that is either hypoxic or anoxic, and in which few organisms can survive. Oxygen-consuming organisms within dead zones either suffocate or leave the area. According to the Michigan water quality standards, a minimum oxygen concentration of 7mg/l is needed for cold-water fish and minimum of 5 mg/l is needed for warm water fish (MDEQ, 1994).

erie
Figure 3. Lake Erie Bathymetry Map (Credit:NOAA).

The shallow central basin of Lake Erie experiences dead zones. Scientists across the Great Lakes basin are monitoring the lake by collecting and sharing water quality data to better understand what contributes to the formation of dead zones. One of the most user-friendly data portals is the Great Lakes FieldScope project. Created through a partnership between Michigan Sea Grant and National Geographic, this project gathers water quality data from the Great Lakes region and allows users to enter their own data or explore regional water quality data through the use of graphs and maps. The program is user friendly and robust enough to make basic scientific inquiries — perfect for an introductory lesson on thermal stratification and dead zones.

The following lesson is applicable to middle school students (grades 6-8). It explores water quality data stored within the Great Lakes FieldScope project database and uses FieldScope-based interactive analytical and mapping tools. This lesson, along with the data worksheet and data worksheet key, also can be found on the Michigan Sea Grant website at www.miseagrant.umich.edu/lessons/. Click the Explore Lessons & Data tab then search for Oxygen in Water.

 

Engage

This part of the lesson should capture the students’ interest, connect with previous course work if possible and introduce the topic.

  1. Begin by asking if any of the students have swum in a lake or pond during the summer and felt cold water at their feet. If so, they may have felt thermal stratification. Ask students if they can define thermal stratification and then clarify what it is by using the background information above. Encourage students to ask questions about why water stratifies. Educators can relate thermal stratification to the layering that happens with oil and vinegar. Oil and vinegar have different densities, thus one floats on the other. This is similar to water at different temperatures. Cold water is denser than warm water. Denser water will sink and warmer water will float, thus creating layers. This is a good opportunity to present Figure 1 and provide an opportunity for students to ask questions.
  1. Ask if students know about dissolved oxygen. To help them understand the idea, ask if they ever have seen a bubble stone in an aquarium. If so, ask why they are used. Some answers might be: Bubble stones circulate water and increase oxygen levels in aquariums by directly inputting oxygen into the system and by increasing the amount of water coming into contact with the air. This promotes atmospheric diffusion of oxygen into the water.
  1. Now discuss air diffusion at the lake scale. In what ways might a lake receive oxygen? Discuss the background information provided above so students know about the three methods of oxygen diffusion. Most students know that plants produce oxygen, and educators can connect this idea to the aquatic environment. Ask students why they may think dissolved oxygen is important in a lake. Make sure students understand that, like terrestrial animals, aquatic animals require oxygen. Describe how much oxygen cold water and warm water fish need. Then explain how oxygen levels can become very low at certain times of year due to thermal stratification. Introduce the concept of dead zones. This is a good opportunity to display Figure 2.

Explore & Explain

 

Elaborate

In this section students are provided with additional resources on hypoxia. These provide information about how dissolved oxygen levels may impact important services such as drinking water and recreation.

Resources:

Discussion Questions:

  • How can dissolved oxygen levels influence organisms living in a lake?
  • How can human activities affect dead zones?
  • What impacts does hypoxia have on the ecosystem/food web of Lake Erie?

Students should combine everything they have learned up to this point to develop a mini report and present their results to the rest of the class. The report might include graphs, answers to worksheet and discussion questions, and information gained from this lesson and the resources provided below.

 

Evaluate

Evaluation is ongoing. This section of the lesson and activity gives the educator flexibility to asses and monitor student progress.

One way to evaluate if the students understand how dead zones are formed is to have them create a diagram of the steps involved in creating a dead zone in a eutrophic lake. The diagram might consist of boxes and arrows flowing through a lake. It would start with nutrient input, followed by an algal bloom that dies and sinks to the bottom. Finally, bacteria decompose the algae, which depletes oxygen levels leading to the formation of a dead zone. The diagram would also include the epilimnion, metalimnion and hypolimnion.

In addition, based on the activity and class discussion, students should be able to:

  • Describe what thermal stratification is and why some lakes in temperate regions stratify.
  • Comprehend how lake thermal stratification affects dissolved oxygen.
  • Construct and interpret graphs of dissolved oxygen and water depth.
  • Understand and define hypoxic zones, anoxic zones, and dead zones.
  • Discuss the importance of dissolved oxygen to organisms.

 

Activities

  • How dissolved oxygen varies with lake depth and thermal stratification (with Great Lakes FieldScope)
    Summary: Students will be able to describe how lake thermal stratification and dissolved oxygen levels relate to a lake’s ability to support animal life.
    Time: 50-minute class period.
  • Graphing Temperatures
    Summary: Graph Lake Erie water temperatures from the surface to the bottom of the lake.
    Time: One 50-minute class period
    Dead Zones – Lesson 3 Activity A: Standards and Assessment
  • Air Supply: Graphing Dissolved Oxygen
    Summary: Graph dissolved oxygen from the surface to the bottom of Lake Erie.
    Time: Two 50-minute class periods
    Dead Zones – Lesson 3 Activity B: Standards and Assessment

Additional Figures & Resources

 

Lesson & 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.

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.

Michigan Department of Environmental Quality (MDEQ). 1994. Dissolved Oxygen. http://www.michigan.gov/documents/deq/wb-npdes-DissolvedOxygen_247232_7.pdf

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.