Activity 2 - DO and Seasonal Variations


Seasonal variations in DO level occur when warmer temperatures during the summer speed up the rates of photosynthesis and decomposition of aquatic plants and algae. When the plants die they accumulate on the bottom of the lake and induce O 2 consumption by bacteria and other decomposers. Dissolved oxygen concentrations may change dramatically with lake depth depending on the trophic status of a lake. At the surface or epilimnion of a lake DO may be influenced by wind and wave action, and oxygen production during photosynthesis. At the lake bottom, or hypolimnion, sunken organic matter decomposes and photosynthesis is limited because light cannot penetrate down to these depths.

In this activity you will be making a line graph to compare two variables: DO levels and time of year. The data was collected in the year 2000 in Onondaga Lake and Seneca Lake. The water samples were taken from the surface and at the lower layer of water in the lakes or hypolimnion. After making your graphs visit the web site links at the bottom of the page to answer questions D-I.

Before conducting this exercise learn more about how line graphing works by visiting the following website: Line Graphing.


To conduct this activity you will need to follow these steps:

1) Link to the Seneca-Onondaga Lake data and save the excel sheet to your computer; you will see two sets of data: Hypolimnion and Surface. Then follow the directions in the link below.

Activity for Users of Excel Version 2007

14) Answer the following questions:

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

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

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

G. If you were a fish in Onondaga Lake – where would you most likely go in the summer?

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

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

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


Preparing Students to Lead Lives of Consequence.