Teacher Notes

Why Is the Water Green?

Student Laboratory Kit

Materials Included In Kit

Bristol’s prepared algae media concentrate, 100X, 100 mL
Nitrate solution, 100 mL
Phosphate solution, 100 mL
Coverslips, 100
Culture containers, snap-top, 40
Microscope slides, 72
Pipets, 60

Additional Materials Required

Algae culture, 400 mL
Cups or beakers, 10
Graduated cylinders, 25 mL, 10
Markers, permanent, 5 (each to be shared by 2 groups)
Microscope, 40X objective

Prelab Preparation

Set up the stock algae culture by obtaining 400–500 mL of algae from a local pond, aquarium, or purchase a stock algae culture, such as Desmids, available from Flinn Scientific, Catalog Number LM1045. (Try to get a concentrated sample—the greener the water the better). A purchased culture is typically about 150 mL. Since 400 mL of culture is required for this lab, dilute the culture to reach the necessary volume with spring or dechlorinated tap water. If time allows, let the culture sit in sunlight for a few days before beginning the activity.

Thoroughly shake the bottle of 100X Bristol’s prepared algae media and dilute by adding 10 mL of the concentrate to 1 L of distilled or spring water.

Safety Precautions

The algae cultures, nitrate and phosphate solution are not considered hazardous but always use safe laboratory practices. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Algae cultures, including those containing nitrates and phosphates, may be poured down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. The microscope slides and coverslips should be thrown away in a glass disposal bin only. The snap-top containers maybe saved for future use.

Lab Hints

  • This kit contains enough materials for a class of 30 working in groups of three.
  • The initial culture setup on the first day should take about 30 minutes. Establish a schedule for students to check their cultures over the next 10 day period. Cultures should be checked and the appearance recorded in Data Table 1 on the first day and then four times more times throughout the course of this activity. It is best if the second observation is made no earlier than four days after the cultures are started. This should only take a couple minutes at the beginning or end of lab. Activity 2 will require an additional lab period.
  • Place the beaker containing the stock algae culture and the nitrate and phosphate solution bottles in a central location since all groups will need to use them.
  • To achieve a more concentrated stock culture, simply allow the culture to sit in sunlight for a few days before beginning the activity.
  • Algae are extremely useful for classroom study and most are easy to maintain, since algae grow in diverse environments and under a variety of conditions. Common algae species thrive well in a laboratory setting.
  • 70% isopropyl alcohol may be used to remove the ink from the lids of the snap-top containers.
  • As a general trend, at the end of the activity students are likely to observe a light green in the control samples, a medium level of green in the samples containing only nitrate or phosphate and the darkest green in the combined nitrate and phosphate sample. However, some green algae species may yield the darkest green in the phosphate only container.

Further Extensions

  • Due to variation in green algae types, some samples may be easiest to view differences in the amounts of growth after settling for some time while others after shaking the container.
  • Algae range greatly in size. Certain species may require a 100X objective to view individual algae cells.
  • Alternate activities may involve having students bring in individual green-algae rich water samples rather than having the entire class test samples from the same location.
  • Commercial fertilizer may be substituted for the nitrate and phosphate solutions, however most fertilizers contain both nitrates and phosphates so it may be appropriate to do simply two samples—a “control” and a “runoff” containing fertilizer.
  • Compare cultures grown in dim light or colored lights, and compare with those cultures grown in a sunny location.
  • Try varying concentrations of nitrates and phosphates in the cultures. Is there a point where the concentration becomes too high for the algae to flourish?

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Asking questions and defining problems
Analyzing and interpreting data
Planning and carrying out investigations
Engaging in argument from evidence

Disciplinary Core Ideas

MS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems
HS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems

Crosscutting Concepts

Cause and effect
Scale, proportion, and quantity
Energy and matter

Performance Expectations

HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.

Answers to Prelab Questions

  1. How do algae contribute to the well-being of the environment?

    Algae are responsible for production of a large portion of the Earth’s oxygen, and marine and aquatic food chains rely greatly on their presence.

  2. Define, in your own words, an algae bloom.

    Accept all reasonable answers.

  3. List a few reasons algae blooms can be detrimental to an ecosystem.

    Student answers may include some of the following:

Thick blankets of algae block out sunlight needed by other photosynthetic life below the surface.

Bacteria levels therefore increase to decompose the plants and other organisms killed directly, or indirectly, by the lack of sunlight. In turn, dissolved oxygen levels decrease as bacteria populations increase.

Lack of dissolved oxygen can be detrimental to larger life forms (e.g., fish, turtles and amphibians) and indirectly can even affect humans.

Sample Data

Activity 1


Activity 2

Counts will vary with species used.


Answers to Questions

Activity 1

  1. What purpose did the control culture serve in this activity?

The control sample did not receive any “runoff” chemicals and provides a baseline level of growth for comparison with the samples that did receive nitrates and phosphates.

  1. Which culture appeared to have the largest amount of algae growth? What factors are responsible for rapid algae growth?

The nitrate and phosphate combination culture. Although the same amount of nutrient was added, the combination nutrient of yields higher growth than samples containing a single nutrient. Note: Some species may yield the highest growth in the phosphate only culture.

  1. The algae in this activity color the water samples green. What component of their makeup creates this coloration? Hint: Algal cells are photosynthetic like plants.

Chlorophyll, which is the site of photosynthesis, creates the green coloration of both the green algae this experiment and in plants.

Activity 2
  1. Sketch what you viewed under the microscope for each culture.

Student sketches will vary.

  1. Which culture had the highest microscopic algal count?

Typically the nitrate and phosphate combined culture have the highest microscopic algal count.

  1. Assuming one drop is one twentieth of a milliliter and the algae cultures each contain 30 mL, approximately how many algae cells are present in each culture?

Student answers will vary depending on algal cell counts.

  1. Did this agree with your results from Activity 1?

Yes. The “greenest” culture also yielded the highest microscopic algal cell count.

Student Pages

Why Is the Water Green?


Ever notice how bodies of water differ in color, odor and clarity? Algae are among the most abundant life forms present in all bodies of water, saltwater and freshwater alike and are a major contributor to water quality.


  • Pollution

  • Algae blooms
  • Growth rates


Algae are a special group of organisms that can be found nearly everywhere—in saltwater, freshwater, damp soil, ice, on rocks, lichens and even in the air. Considering that algae are photosynthetic organisms, they can basically survive anywhere moisture, essential nutrients and sunlight are present. Algae are responsible for production of a large portion of the Earth’s atmospheric oxygen and marine and aquatic food chains rely greatly on their presence. Algae are broadly defined by their pigment color, which are often described in color terms (e.g., blue-green, green, brown, red, brown-yellow). Algae come in many sizes from single-celled organisms to very large seaweed varieties.

Nitrates and phosphates are necessary for plant and algae growth. Since soil often becomes stripped of these nutrients due to constant farming, fertilizers are often added to replace these lost nutrients. However, these fertilizers seep into the ground when it rains and eventually reach groundwater. Groundwater flows to larger bodies of water where algae thrive. Unnaturally high levels of nutrients may cause rapid, excessive algae growth referred to as an algae bloom. Students often question, why is this such a bad thing? If algae produce oxygen and are at the bottom of the food chain, then having more around should be a good thing, right? This is not the case for several reasons. First, if a thick blanket of algae is present on the surface, it blocks out sunlight needed by other photosynthetic life inhabiting the water below the surface. As plants and microorganisms die off from lack of sunlight, bacteria levels increase. Algae require oxygen and other nutrients to live, so as the populations increase, dissolved oxygen levels decrease. Dissolved oxygen is used by other life forms (e.g., fish, turtles, amphibians). If the levels of dissolved oxygen fall below 3 ppm (parts per million), larger organisms begin to die off, creating stress on the entire ecosystem in the area. It may even indirectly have an effect on humans—for example, if the area is frequented by fishermen. When certain fish are not readily available, market prices go up or the fish may not be available for consumption at all.

Experiment Overview

In this lab, algae cultures will be treated with nitrates and phosphates to simulate bodies of water that have been contaminated with chemical runoff. Will algae bloom in these microenvironments?


Activity 1
Nitrate solution, 3 mL
Phosphate solution, 3 mL
Algae culture, 40 mL
Algae media, 80 mL
Culture containers, snap top, 4
Graduated cylinder, 25 mL
Marker, permanent
Pipet, wide-stem

Activity 2
Algae cultures from Activity 1, 4
Coverslips, 4
Microscope, 40X objective
Microscope slides, 4
Pipet, wide-stem, 4

Prelab Questions

  1. How do algae contribute to the well-being of the environment?
  2. Define, in your own words, an algae bloom.
  3. List a few reasons algae blooms are detrimental to an ecosystem.

Safety Precautions

Wear chemical splash goggles whenever working with chemicals, heat or glassware. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.


Activity 1

  1. Label the lids of four culture containers “Control,” “Nitrates,” “Phosphates” and “Nitrates and Phosphates” using a permanent marker. Also include your group number or group member’s initials.
  2. Obtain 40 mL of algae culture in a cup or beaker.
  3. Measure out 10 mL of algae culture using a graduated cylinder and pour into the “Control” container.
  4. Repeat step 3 for the remaining three culture containers.
  5. Add 20 mL of algae media to each culture container using the same graduated cylinder. Each container should now contain 30 mL of diluted algae culture.
  6. Using a wide-stem pipet, add 2 mL of nitrate solution to the container labeled “Nitrate” and 1 mL to the “Nitrate and Phosphate” container. Note: 1 mL is approximately 20 drops using a wide-stem pipet. Close the lid. Rinse the pipet, inside and out, with water.
  7. Using a wide-stem pipet, add 2 mL of phosphate solution to the container labeled “Phosphate.” Close the lid.
  8. Add 1 mL of phosphate solution to the “Nitrate and Phosphate” container. Close the lid.
  9. On the data table, write your observations in the “Start” column.
  10. Place the four culture containers where they will receive plenty of sunlight per your teacher’s instructions.

Student Data Table 1. Observations of Algal Growth

In the data table, write in the day the observation was taken (example: Day 4, if it has been four days since the culture was started) and the respective observed algae growth level according to the following descriptions. Note: Place the cultures on a white sheet of paper to facilitate observation.

Tint—Slight green algae growth
Light—Noticeable amount of green algae growth
Medium—Very noticeable green color but not quite deep green in color
Dark—Deep green in color, possibly a layer or ring forming on the surface of the solution or container


Activity 2

  1. Obtain four microscope slides and label them “C,” for control, “N” for nitrate, “P” for phosphate and “NP” for the sample containing both nitrate and phosphate solution with a permanent marker. Note: Label the slides off the far right or left-hand corner of the slides leaving the middle of the slide open.
  2. Check the lid of the culture to ensure it is on tightly. Shake the control sample gently to disperse the algal cells.
  3. Using a pipet, place a drop of the solution on the slide labeled “C” and slowly lower a cover slip onto the sample so that air bubbles are minimized.
  4. Using a clean pipet, repeat steps 2 and 3 for the nitrate, phosphate and combination culture samples.
  5. Set up the first slide (control) on the microscope stage and focus in low power.
  6. Carefully turn the nosepiece of the microscope so that the 40X lens is in place for viewing the sample.
  7. Sketch the magnified view of the algae under 40X on Activity 2 Worksheet.
  8. Count an approximate number of algal cells in two different fields of view and record each count in the Student Data Table 2. Note: Algal cells tend to clump together—try to count individual cells.
  9. Repeat steps 5–10 using the other three algae samples.
  10. Determine the average counts for each sample and record the value in the data table.
  11. At the culmination of the activity algae cultures may be poured down the drain. Rinse out the snap-top containers with water and consult your teacher for storage or washing instructions.

Student Data Table 2. Algae Cell Counts


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