SOS Lesson Plans/Student Worksheets

Teachers Guide to Dissolved Oxygen Lesson

Lesson Introduction

Dissolved Oxygen and Aquatic Life
Dissolved oxygen (DO) is essential to all aquatic life. While humans and terrestrial animals breathe oxygen from the air, aquatic animals use oxygen that is dissolved in water. An aquatic animal breathes by absorbing free oxygen into its blood through its gills or directly through its body surface. Oxygen (O2) is consumed in surface waters by all aquatic organisms: fish, plants, algae, bacteria, and invertebrates. The oxygen molecule in dissolved oxygen is free (O2) which differs from the oxygen atom bound to hydrogen in a water molecule (H2O). Oxygen gets into water three ways: 1) diffusion from the surrounding air; 2) aeration (rapid movement); and 3) when released as a by-product of photosynthesis by algae and aquatic plants. There are a number of physical and biological factors that can impact the DO levels in waterbody such as a lake.

Factor 1 - DO and Photosynthesis
Photosynthesis by aquatic plants and algae may contribute significant amounts of O2 to a waterbody. During the day when light is present, aquatic plants and algae release O2 as a by-product of photosynthesis. On the other hand, the process by which aquatic life uses O2 is called respiration. Below are the formulas for photosynthesis and respiration.

H2O + CO2 + light energy ---> carbohydrate + O2

carbohydrate + O2 ---> CO2 + H2O + energy for respiration

DO levels in a water body will remain constant as the cycle of O2 consumption by respiration is replenished by photosynthesis, aeration and diffusion. At night however photosynthesis ceases but the rate of respiration remains the same, subsequently DO levels drop. Normally, this diurnal DO cycle will not harm aquatic life. However, in bodies of water that have large populations of plants and algae one would expect to see dramatic variations in the cycle of DO concentrations from day to night which may stress aquatic organisms. Below are two graphs: Graph A shows a typical diurnal cycle of DO levels; Graph B shows a cycle off balance.

Factor 2 - DO and Decomposition
Another factor which can affect the DO levels in a waterbody is the rate of decomposition. As plants and animals die they sink to the bottom of the lake where they are decomposed by bacteria, a process which uses up O2 and releases nutrients back into the waterbody. Although aquatic plants and algae are important for releasing O2 during photosynthesis, too much of a good thing can cause problems; lakes and ponds with excessive plant life can actually become more nutrient rich as plants die off, decompose and release nutrients. Nutrient rich lakes that harbor excessive plant life are called eutrophic lakes. Finally, organic matter entering into a waterbody also accelerates the rate of decomposition and DO loss. Organic matter comes from wastewater treatment plants that discharge into a waterbody, farmland runoff, and sediments from streams.

Factor 3 - DO and Temperature
Another factor that plays a role in DO levels in waterbodies is temperature: the colder the water, the more O2 it can hold. Think about drinking a glass of cold water versus a glass of lukewarm water. The warmer water tastes "flat" because the O2 has been removed through heating. When graphing DO and temperature one would expect to find an inverse relationship between the two variables, that is, as temperature goes up, DO goes down. Graph C illustrates this point.

Graph C

Increased temperatures also impact the level of biological activity in a waterbody. Warmer temperatures in the summer increases biological activity and the use of O2 by aquatic organisms, whereas in the winter, biological activity drops off with decreasing temperatures. Similar to the diurnal cycles of DO there are seasonal cycles of DO in lakes that are biologically productive or eutrophic.

Lesson Outcomes

Through this lesson and accompanying activity students will:

  • Understand what dissolved oxygen is and why it is important to aquatic life.
  • Understand what factors influence levels of dissolved oxygen in a lake.
  • Learn how to use MS Excel to make charts to show trends and correlations.
  • Interpret data and conduct independent research.

MST Standards

  • Standard 1 Key 1, Key 3
  • Standard 2 Key 1
  • Standard 3 Key 6
  • Standard 4 Physical Setting Key 2; LE Key 6, Key 7
  • Standard 6 Key 5

Lesson Objectives

In this lesson students will be using data on DO levels collected from Onondaga Lake in Syracuse, NY and Seneca Lake in the Finger Lakes for the year 2000. The water samples were taken from the surface and at depths from the lower layer of water in the lakes or hypolimnion. They are asked to graph charts to investigate the relationship between DO, temperature, and seasonal changes in the two lakes.

To do this students will be making 5 charts. Although the charts will help students answer some of the questions they will need to refer to the web site links found in the references on the activity pages. At the end of this lesson students should have completed:

  • Seneca DO Temp Profile
  • DO Surface Onondaga Year 2000
  • DO Bottom Onondaga Year 2000
  • DO Surface Seneca Year 2000
  • DO Bottom Seneca Year 2000
  • Answered questions A - G plus a bonus question using student generated charts, the charts provided and the website links as references.

Students will answer questions A-I:

A. Is there a strong correlation between DO levels and Temp for Seneca Lake? Is it positive or negative?

Yes - the correlation should be -.98 - a strong negative correlation.

B. The data for DO and Temp were taken at depths below 20 M - the hypolimnion - would you expect photosynthesis to contribute to DO levels at these depths?

At these depths it would be too deep for much plant life to receive sunlight (although there may be some phytoplankton) so photosynthesis would not be a major factor.

C. What can you infer from this graph about the DO levels in the hypolimnion of Seneca Lake?

Temperature plays a major role in DO levels, the deeper and colder the water the more oxygen it holds.

D. At what time of year do DO levels drop in each lake?

The charts show that DO drops in both lakes during the summer.

E. How do the seasonal fluctuations in DO levels compare between the two lakes?

The drop in DO levels in Onondaga are quite sudden and dramatic. The hypolimnion has no oxygen during a few months in the summer. The changes to DO levels in Seneca are very slight and do not last long.

F. Which lake system shows signs of a stressed environment for fish and other aquatic life?

Onondaga Lake loses all of its oxygen in the hypolimnion in the summer months making it difficult for fish and other aquatic organisms to survive at these depths. They either have to migrate to the upper layers where there is oxygen or they leave the lake altogether and stay in the Seneca River during this period.

G. What factors might play a role in determining DO levels in these lakes during the summer months?
Other factors affecting DO levels is Onondaga Lake are organic pollutants from the raw sewage that enters the lake from the sewers in the City of Syracuse. It is also a eutrophic lake so there is more plant life and more oxygen consumption when the plant life decomposes and during respiration. These factors are more pronounced in the summer months when the metabolism of bacteria and other decomposers increases.

Seneca Lake does not show signs of rapid declines in DO so decomposition and pollution would not play a major role in DO level fluctuations. Rather DO may be governed by the mixing between the upper and lower layers of water and colder temperatures.

H. What factors might play a role in determining DO levels in these lakes during the summer months at the surface?

At the surface, aeration, diffusion and photosynthesis all play a role in determing DO concentrations.

I. What factors might play a role in determining DO levels in these lakes during the summer months at the hypolimnion?

Onondaga Lake is a eutrophic lake. In the summer the hypolimnion is cut off from the epilimnion so there is little opportunity for diffusion or aeration. DO declines because of increased respiration and decompostion. Seneca has less biological activity in the hypolimnion so water temperatures play a role in keeping the DO levels stable.

Bonus Question:
What types of fish would you expect to find in the lower depths of Seneca Lake? Onondaga Lake? And why?

Seneca Lake has traditionally supported a cold-water fishery such as trout and salmon species. These species require high DO levels and colder water. At one time Onondaga Lake also had a cold-water fishery but over the past 100 years the habitat has changed for cold-water species and mostly warm-water species such as walleye and bass survive in the lake. During the summer when DO levels drop and temperatures rise these fish may leave the lake and enter Seneca River where DO levels are higher. Trout and salmon are not able to survive the fluctuations in DO levels.


Activity 1

Activity 2

The charts that students make should look like these:


Preparing Students to Lead Lives of Consequence.