Please read the pages indicated by Z-25 and study Behavioral Objective #37. A decrease in water temperature causes an increase in water density up to the temperature of 4 degree Centigrade. From 4 degree C to freezing, water and ice becomes less dense.  In winter, as the surface temperature drops to 4 degrees Centigrade, the water becomes not heavier, but less dense. Remaining at the surface, this lighter water impedes circulation. The bottom is now warmer than the top. Because bacterial decomposition and respiration are less at low temperature and cold water holds more oxygen, there is usually no great winter stagnation. The formation of ice may, however, cause oxygen depletion and a heavy winter may kill fish.

The spring overturn occurs when the ice melts, and the heavier surface water sinks to the bottom. It must be said here that the density of water increases as the temperature decreases up to 4 degrees Centigrade. In spring as the ice melts and the water on the surface warms to 4 degrees Centigrade it becomes more dense and sinks to the bottom producing the spring overturn. After which, the water again circulates freely. Thus, in the spring and fall when the entire body of water approaches the same temperature, mixing occurs, and this results in a redistribution of oxygen and nutrients and often blooms of phytoplankton to appear. In your notes draw a cycle showing the water temperatures of a lake during each of the seasons. Explain in your notes fall tunover and spring turnover  in relation to the movement of nutrients. Draw a figure using arrows to show the movement of water  during spring and fall turnover. Indicate the effect this turnover has on oxygen  and nutrient levels.

In your notes write out definitions for the following terms describing freshwater lakes:  littoral, limnetic, and profundal zones; phytoplankton, zooplankton; oligtrophic, eutrophic lakes.

Activity: Write a paragraph describing which environmental factors are responsible for the characteristic adaptations demonstrated by the plants and animals in freshwater lakes and give examples of the flora and fauna which may be found in this ecosystem’s trophic levels.

Temperate zone lakes are thermally stratified with warm and cold layers along a boundary called a/an ______.

a. limnetic zone

b. littoral zone

c. thermocline

d. fall turnover

During which season would the bottom layer of a lake be colder than the top layer?

a. winter

b. fall

c. spring

d. summer