Nitrates and Phosphates and Algae, Oh My!

Part A: Making an Algal Bloom

Microscopic view of tetraselmis algae. Image source: Food and Agricultural Organization of the United Nations (FAO)

Because algae and other planktonic species are the primary producers upon which the rest of the marine food chain depends, it is important to understand why algal blooms occur and what effects they have on the surrounding water and marine life. In this part of the investigation, you will make your own algal bloom and test what factors influence the occurrence and duration of the bloom.

1. Prepare a "Control" container and a "+ Nutrients" container for your algae samples.
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   • algal culture
   • water (fresh or salt depending on the type of algae)
   • two 250-500 mL clear glass or plastic containers
   • light source (sunny window will work)
   • droppers or pipettes
   • liquid plant fertilizer
   • wax pencil, tape, or stickers to label containers
   • microscope
   • Label one container with the word "Control". Label the other container with "+ Nutrients". Then, label both containers with your group number.
   • Add an equal amount of water to each container.
   • Add 3 of drops of algal culture to each container.
   • Add 1 teaspoon of liquid fertilizer to each container.
   • Decide on a small unit (such as one half or one third of a pipette) that will be your standard base unit of fertilizer. As a class, decide how many units of fertilizer mixture each group will add to their "+ Nutrients" container. Add the proper number of units of liquid fertilizer mixture to your "+ Nutrients" container. Write the number of fertilizer units you added to the container on the label.
2. Take a drop of water from each sample and look at it under a microscope. 10x magnification will be sufficient. Count and record the number of phytoplankton visible in each sample. Have each member of your group repeat this and find the average number of phytoplankton per drop in both the "Control" and "+ Nutrients" containers.
3. Place both containers near a window, out of direct sunlight, but where they will receive approximately equal amounts of light and are kept at the same temperature (don't place some containers on a heater and others near an open window).
4. Every day or two over the course of 1-2 weeks, make observations (e.g., color, opacity, smell) on the algal cultures in each of the containers. You should also observe one drop of water from each sample and count the number of phytoplankton per drop as you did in Step 3. Carefully record your findings.
5. Analyze your findings.
   1. For each treatment ("Control" and "+ Nutrients"), create a graph to illustrate algal growth (determined by number of phytoplankton cells per water drop) on the y-axis versus number of days on the x-axis.
   2. Compare your findings to those of other groups using different amounts of fertilizer. Discuss any differences or inconsistencies.
   3. As a class, create a graph of final algal cell density versus fertilizer concentration for each nutrient treatment.
   4. Write a paragraph or two about how algal blooms like the ones in this experiment might occur in lakes and oceans in the real world.
   5. Which treatment had the highest final algae cell count?
   6. What do algae (and other plants) need to grow and survive?
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      Water, carbon dioxide, sunlight, and food (nutrients).

Now that you've seen what an algal bloom looks like on a small scale in the laboratory, let's take a look at what happens on larger scales in the ocean.