Nutrients, Algae and Oxygen

Did you ever notice that some lakes and ponds are so clear you can almost see the bottom of the pond and others are so murky you can't see anything below the surface? The clarity of a pond's water is related to amount of algae living in the pond. The more more algae, the less clear the pond. That's because the even though an individual alga is so small you can't see it, lots and lots of algae will color the water and block your view. Algae populations are also related to the amount of dissolved nutrients in the water. More nutrients, more algae, less clear water. But, algae are also the base of the aquatic food chain, so having more algae means having more insects, fish, frogs, and birds, right? Well, sometimes it does. Why not try out the activity below and find out why sometimes more is less?

You'll need:

  • 7 quart-size canning, peanut butter or mayonaise jars
  • masking tape for labeling
  • tap water
  • a small amount of household plant fertilizer (ask an adult for help)
  • a roll of aluminum foil
  • 1 gallon of recently collected pond water with lots of life forms in it
  • 1 gallon distilled water
  • microscope (optional, but helpful)
  • soda straws (for putting drops of water on a microscope slide)
  • Identification guides for pond life

 

  1. Label the jars
    1. Control
    2. Distilled Water
    3. Tap Water
    4. Pond Water
    5. Distilled Water with Fertilizer x1
    6. Distilled Water with Fertilizer x10
    7. Distilled Water with Fertilizer x20

     

  2. Then put the following amounts of the type of water and/or fertilizer in its labelled jar. Make sure the pond water is shook up, so all the live creatures are evenly spread throughout the pond water. You want to have an equal amount of live creatures in each 1/2 cup of pond water you put in the jars, because you'll be measuring the life in each of the jars.

     

    1. Control--3 cups distilled water
    2. Distilled water--3 cups distilled water, plus 1/2 cup pond water
    3. Tap water--3 cups tap water, plus 1/2 cup pond water
    4. Pond water--3 1/2 cups pond water
    5. Distilled water with fertilizer x1--3 cups distilled water with the reccommended amount of fertilizer, plus 1/2 cup pond water
    6. Distilled water with fertilizer x10--3 cups distilled water with ten times the recommeded amount of fertilizer, plus 1/2 cup pond water
    7. Distilled water with fertilizer x20--3 cups distilled water with twenty times the recommended amount of fertilizer, plus 1/2 cup pond water

     

  3. Now cap the jars losely with aluminum foil. Don't seal the jars with the foil. Sealing the jars cuts off air that the aquatic animals and plants in the pond water need.

     

  4. Place the jars in one location. Don't place them in window with lots of direct sunlight, because the sunlight will heat the water and may kill the creatures. A good place might be a window with some sun and shade or in a sunny room away from the windows. Ask mom or dad if its okay first.
     

    What do you think will happen in the jars?

     

  5. This begins the four week observation period. Write down what you observe today, either using a microscope or just your eyes. If you have a microscope, then check out the creatures in the leftover pond water. Try and identify them. You'll be looking for them over the next few weeks.

     

  6. The next day check your 7 jars (the same way you did yesterday) again and record your observations. Don't get frustrated if you don't see any changes yet.

     

  7. Continue checking on the jars and recording observations daily for four weeks. You should see a change in the second week.
 

What happened?

Did some jars have more creatures than others? Were some more cloudy? The jars that had lots of nutrients (fertilizer added) represent eutrophic ponds--ponds with a lot of nutrients. These jars probably had lots of life and were pretty cloudy. The jars with few nutrients (tap water, distilled water) had few nutrients and probably didn't have many creatures. These jars represent oligotrophic ponds--ponds with few nutrients. Oligotrophic ponds are usually clearer than eutrophic ponds. Did you see a difference in your jars? How about the jar of pond water? Was it oligotrophic or eutrophic or right in the middle?

When the algae die bacteria use oxygen to break them down. If there are too many algae being decomposed by too many bacteria, then there isn't enough oxygen for the fish and other plants in the lake. Sometimes, too many nutrients can kill a pond or lake.

That's why we try to prevent fertilizers and nutrients from getting into the lakes and ponds. We do that by having buffer strips--strips of trees and grasses that use up some of the nutrients in runoff and rainwater before they get into the pond. We also stopped putting a nutrient known as phosphorus in laundry detergents. The phosphorus went from the washing machine to the lakes and ponds and killed fish and aquatic plants.

This activity is based on an activity called "Glass Menagerie" found in "Project Wild Aquatic: Education Activity Guide." Please see the educational resources section of the Iowa Wetlands and Riparian Areas Conservation Plan for ordering and contact information.