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

O 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 eutrophic lakes that have large populations of plants and algae one would expect to see dramatic variations in the diurnal cycle of DO concentration 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, 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 Objectives

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

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

  • DO and Temp Seneca
  • DO Surface Onondaga
  • DO Bottom Onondaga
  • DO Surface Seneca
  • DO Bottom Seneca
  • Answered questions A-I plus a bonus question using student generated charts, the charts provided and the web site links as references.