Many believe that nutrient loading -- from natural and human-derived sources -- is responsible for Harmful Algal Blooms (HABs). For example, excess nutrients can selectively grow certain species of toxic phytoplankton. Some coastal waters -- for example, those with sewage treatment plants -- may form excess nutrients and associated algal blooms. When these organisms die, they sink to the bottom. As they decompose, they use up the oxygen at the bottom and adversely impact benthic (i.e., bottom-dwelling) organisms including lobsters and clams. Fisheries in New York's Long Island Sound have been disrupted by such HABs.

This activity is designed to demonstrate how adding nutrients can affect the total water column. Although it does not simulate true processes that happen in nature, it is a dramatic way to help students better understand the connections between nutrient input, phytoplankton growth, decomposition and loss of oxygen.
  • Understand which "ingredients" are needed for phytoplankton growth and reproduction
  • See how "nutrient loading" at the surface may affect benthic organisms
  • Extend learned concepts to assess the idea of "fertilizing the oceans"


Water; 500 ml beaker or large glass vase; About 0.5 cups of light brown sugar; About 0.25 cups of corn syrup; Candy "critters" such as "gummy lobsters,""gummy worms," "gummy fish"; About 0.5 cups of cooking oil; About 4 tablespoons of green granulated sugar

Note that the amount of water, brown sugar, corn syrup, cooking oil and green sugar depend on the size of the container being used. This experiment can also be conducted by teams of students using smaller containers. If students do sprinkling the green sugar to get the desired visual effects (see photos below).
  1. Put a layer of brown sugar (about 0.5 inches thick) at the bottom of a 500 ml beaker or glass vase.
  2. Add corn syrup until it completely covers the brown sugar. You may need to swirl the corn syrup around to completely cover the brown sugar. (See photo) Together these layers form your benthic habitat. Are marine benthic habitats hard like the floor of building? Or are they soft and gooey?
  3. Ask the students: "What kind of critters live in benthic habitats?" Place a "critter" on the benthic habitat; good examles are "gummy Lobsters," "gummy Worms," "gummy Fish," etc. Try to place the "critter" near the side of the glass so you can watch it as the demonstration progresses. (See photo) If you'd like the students to remember the term "benthic," name your gummy critter "Ben."
  4. Gently add cold water until it is about 3 - 4 inches deep. (See photo)
  5. Air bubbles should begin to rise from this "healthy" benthic environment. (See photo)
  6. Add a 1-inch layer of cooking oil, which will sit on top. (See photo) This is a great opportunity to introduce students to the photic zone. Ask which part of the ocean receives more sunlight: the bottom or top? The yellow "sunlit" layer floats because of a difference in density between the oil and water. In the oceans, which is generally warmer: the bottom or the top? When heated, seawater volume expands and density decreases. So, Sun-warmed surface waters can float on top of colder, denser waters below. This leads to layering of water -- or stratification -- by temperature. In temperate oceans, such stratification exists only during certain times of the year (usually spring and summer).
  7. Show the class the green granulated sugar. Explain that each crystal represents two different things: nutrients and phytoplankton. In order to grow and reproduce, phytoplankton need carbon, sunlight and nutrients. In this demonstration, there is ample carbon dioxide in the water and the "photic zone" is well-sunlit. So, every "nutrient" (i.e., sugar crystal) becomes one phytoplankter in the water column. Using GREEN SUGAR also makes it easy to remember the following: The process of converting sunlight (i.e., light energy) into chemical energy -- i.e., photosynthesis -- occurs with the help of light-absorbing pigments such as chlorophyll; In this process carbohydrates, such as sugars, are produced within phytoplankton cells and oxygen is released.
  8. SLOWLY sprinkle "nutrients" into the photic zone. (See photo) Remind the students that the growth of the phytoplankton population is tied to nutrient availability. Discuss your observations as a group. At first, the phytoplankton will primarily stay in the photic zone. This represents a good balance between nutrients and sunlight. At some point, the phytoplankton may concentrate at the bottom of the photic zone. (See photo) This often happens in nature, as well; discuss why this might be the case. (HINT: in marine waters, gravity sinks nutrients below the photic zone, enriching the waters below.)
  9. As you add nutrients, some phytoplankton may drop below the photic zone. What is the fate of these phytoplankton? Do they have all the elements that they need to grow and reproduce?
  10. Keep adding nutrients. After a minute or so, the continued addition of nutrients will cause "green blobs" to sink downward. (See photo) Ask the students: "How might phytoplankton sinking into the benthic habitat impact critters who live there? What will happen as these phytoplantkon decompose?"
  11. Soon thereafter, large air bubbles will rise from the bottom. (See photo) Ask the students what these bubbles might represent. (These bubbles represent the loss of oxygen from the benthic habitat as the phytoplankton decompose.)
  12. After adding all the nutrients, the benthic layer will have a thick layer of "goo." Ask the students: "If you were a filter-feeding benthic critter, how could you survive in this 'gooey' layer?
  • People have suggested that we actively "fertilize the oceans" with nutrients to decrease the carbon dioxide concentration in our atmosphere. How could critters deal with the loss of oxygen from the benthic environment?
  • Another effect of excess phytoplankton (or biomass) is that there is too much food for some filter feeders. Instead of taking in some phytoplankton and water and filtering out the food, they are taking in too much food and not enough water. Because it has become too difficult to feed, the filter feeders may stop even trying to feed and starve to death.
    • Given the lessons learned in this demonstration, do your students feel that this is a good or bad idea?
Bigelow Laboratory for Ocean Sciences, Copyright 2000
Create 0.5" layer of brown sugar and cover with corn syrup.
Add critters.
Add water until...
it's about 3-4" deep.
Note rising air bubbles.
Add cooking oil...
...until 1" thick.
Small bubbles continue to rise.
Slowly sprinkle green sugar.
Sugar may collect at oil/water interface.
When more sugar is added...
"blobs" will drop to the bottom.
Eventually large air bubbles...
...will rise to the surface.