Description of Lake Inferior

 

Introduction

Lakes have long been the subjects of intense study by ecologists. Indeed, the first important studies on energy flow in ecosystems were based on lakes. What is it about lakes that make them such good models for studying ecosystem structure and process? For one thing, lakes have distinct boundaries. For another, lakes often have an inflow and outflow point, so it is often convenient to monitor materials that both enter and leave the systems. Finally, the biological communities are often easy to sample, because the organisms are either floating in the water or are stationary in the mud at the bottom of the lake. In summary, the characteristics of lakes make them suitable subjects for introducing students to the study of ecosystems. Consider, for example, how much more difficult it would be to study a mature forest.

In this exercise two characteristics of Lake Inferior will be measured: water chemistry (including nutrient content) and composition of the planktonic community. Plankton are microscopic organisms suspended in the lake water.

Assessment of water chemistry can tell scientists many things about a lake and the factors that affect a lake. For example, the presence of high amounts of phosphorus or nitrogen may indicate that fertilizers or sewerage are polluting the lake. These same nutrients could result in very high rate of production of plants and algae. This increased lake fertility, referred to as eutrophication, can result in two long-term undesirable effects. The lake community may become less diverse, as a few dominant species out-compete all others. Also, as dead plants and algae sink to the bottom of the lake and decompose, the lake can become depleted of the dissolved oxygen necessary to support animal life.

 

Assessment of Water Chemistry

In this exercise we will measure the temperature, pH, and concentrations of dissolved nitrogen, phosphate, and oxygen in the water of Lake Inferior. We will compare water samples taken from the inlet of the lake with those taken from near the outlet. We will also take sample from the middle of the lake, and compare samples taken from the surface with those taken from depths of one and from two meters below the surface.

Work in three different groups. One group should sample the surface water near the inlet, another should sample the surface water near the outlet, and the third gets to row the boat into the middle of the lake in order to sample at varying depth. Some members of this third group should collect samples, one at a time, from the middle of the lake, and bring them to the remainder of the group onshore, who can then make the appropriate measurements.

Your instructor will show you how to use the sampling equipment. One member of your group should record the data (location, depth, temperature, pH, dissolved nitrogen, dissolved phosphate, dissolved oxygen). After the class returns to the lab, put your group results on the board, so that all students have access to all data.

 

Method for Assessment of Planktonic Community

The sampling device used for this part of the exercise is called a plankton net, and the sample is referred to as a plankton tow. The cone-shaped net, which is tossed into the water, strains organisms out of the water as it is pulled along the water surface. This sampling method is generally used in a quantitative manner, that is to determine how many of each type of organism is found per unit volume of lake water. However, in this exercise, we will only be concerned with identifying some of the types of organisms that comprise the planktonic community of Lake Inferior.

Your instructor will show you how to collect a plankton tow. Return the sample to the lab for observation under dissection and compund microscopes. Use the taxonomic guides to identify some of aquatic organism that you find. Place each of these types of organisms in one of the following four categories:

 

Rotifers Crustaceans Plant-like microbes Animal-like microbes

 

Questions

Are there any differences between the water samples taken from the inlet vs the outlet? If so, speculate about factors that may account for these differences? Discuss your ideas with your instructor and the rest of the class.

 

Does water temperature and/or chemistry vary with depth in Lake Inferior? If so, how does it vary ? What may account for the effect of depth? Discuss your ideas with your instructor and the rest of the class.

 

Based on the information gathered from your samples, would you conclude that Lake Inferior is polluted? What is the basis for your answer to this question?