SOS Lesson Plans/Student Worksheets

Teachers Guide to Impact of Exotic Species on Lake Ecology

Lesson Introduction

Zebra Mussels (Dreissena polymorpha)
Zebra mussels, a European species, were first discovered in Lake St. Clair (Detroit) in June 1988 and brought into the U.S. accidentally through the ballast water of ocean traveling ships. They are now well established in North America. Zebra mussels were first detected in Seneca Lake during the summer of 1992. They apparently made their way from Lake St. Clair. They are notorious for filter-feeding large volumes of algae or phytoplankton each day. They also colonize "new" territory quickly due to their planktonic (free-floating) larval stage, prolific reproduction (females can lay over one million eggs in a spawning season) and lack of predators in this part of the world. They quickly colonized Seneca Lake, and were well established by 1995.
Source: John Halfman, HWS, Science on Seneca

Chlorophyll a and Zebra Mussels
Chlorophyll is the pigment that allows plants to convert sunlight into energy through photosynthesis. It is found in the chloroplast's of plants including phytoplankton or algae. Chlorophyll-a is the parameter that is measured to provide an indication of the mass of phytoplankton in a waterbody. Phytoplankton are an important part of the aquatic food web because they are the primary producers of food for zooplankton and foraging fish. Without adequate phytoplankton, the food web in an aquatic system would collapse. When zebra mussels are introduced into a waterbody such as Seneca Lake they can impact this food web by competing for food with other organisms that depend on the phytoplankton. Chart A below illustrates the disruption that zebra mussels may have on the food web. Their long-term impact on the fishery is not known.

Lesson Outcomes

Through this lesson and accompanying activity students will:

  • Understand how invasive species are introduced and the impacts that invasive species can have on lake ecology
  • Understand population dynamics of the exotic zebra mussel
  • Determine averages and how to graphically represent data collected from Seneca Lake
  • Be able to form hypothesis about the role that exotic species may have on the food web in a lake

MST Standards

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

Lesson Objectives

In this lesson students are asked to graph data that show Chlorophyll-a (Chl a) and secchi disk concentrations in Seneca Lake from the years 1991-2006. They will use these graphs to explain some of the ecological changes that may be occurring in Seneca Lake since the zebra mussel's introduction to this waterbody. Students will then answer the questions A-D using their generated line graph, Graph 1, and the information from the web site references and articles. At the end of this lesson students should have:

  • A Line Graph: Average Chl a and Secchi Disk Depths - Seneca Lake
  • Answered questions A-D

The students' Line Graph should look like this:



Questions for Students:

A. What does this line graph tell you about the relationship between secchi disk depths and Chl a concentrations in Seneca Lake?
There is a relationship between the depths at which the secchi disc was seen in the water column and the decrease in Chl a concentrations over time. As secchi disc depths went up the Chl a concentrations went down.

B. What are two possible explanations for the rise in secchi disk depths over the past ten years?
Loss of primary producers such as phytoplankton either from increased predation from zebra mussels or lack of nutrients in the water for phytoplankton.

View Graph 1 below which is a graph of the numbers of zebra mussels in Lake Ontario and the corresponding Chl a concentrations from 1990-2004.

C. What does Graph 1 tell you about the impact of zebra mussels on the primary production in Lake Ontario?
Zebra mussels increased rapidly in the early-to mid '90s. Chl a is an indicator of primary productivity, the concentration of Chl a fluctuates as the rise of zebra mussel densities increased in Lake Ontario. This shows that zebra mussels were consuming phytoplankton, and removing Chl a from the water column. As their mussels declined the Chl a concentrations began to rise again.

D. If you were a scientist that researched Seneca Lake ecology would you be concerned about the impact of Zebra Mussels? Why or Why not?
Yes, because there are indications that zebra mussels have affected Seneca Lake. The bar graph shows that the water is more clear now and the concentrations of Chl a have gone down during the same period of time that zebra mussels invaded Seneca Lake. If Lake Ontario is a model of what could happen in Seneca Lake then we would expect that primary productivity in Seneca Lake would decrease and the food web in Seneca Lake would be affected. In addition, now that light can penetrate deeper in the water column more plants are able to grow and the lake could become more weedy.

On the other hand, as the article about Cayuga Lake points out (see references below) - there have been many introduced species into the Finger Lakes and the ecological impacts are not always so profound.


Preparing Students to Lead Lives of Consequence.