Dissolved oxygen and algae blooms

In this lesson, learners will dive into the occurrence of algae blooms in water bodies with data and map analysis and water quality testing. Through a combination of classroom activities and hands-on outdoor exploration using Water Rangers’ data platform and equipment, students will explore the environmental factors that influence algae growth and the significance of dissolved oxygen for sustaining aquatic life.

All of our lessons are designed with two parts — an in-classroom and a field component. You can choose to do the lessons in succession over multiple days, at a convenient time, or teach either part as a standalone lesson. The activities are presented as small group activities to facilitate and encourage active participation. At the bottom of the page, you will find a section on ocean and real-world connections, downloadable slides to accompany the in-classroom activity, and a glossary of terms.

Recommended grades

This activity is recommended for grades 11 to 12. We’ve included tips and tricks to adapt the activity for a wide range of students throughout the lesson.

Learning outcomes

  • Identify key environmental factors influencing the occurrence of algae blooms in water bodies.
  • Analyze maps and data to investigate the spatial distribution of algae blooms and their correlation with landscape features.
  • Measure dissolved oxygen levels at a local water body using scientific tools.
  • Recognize the significance of dissolved oxygen in supporting aquatic life.
  • Acknowledge the role of community-based water monitoring in supporting healthy aquatic ecosystems.


What is dissolved oxygen?

Dissolved oxygen is the amount of gaseous oxygen that is dissolved in water. Water directly absorbs oxygen from the atmosphere, and currents, known as turbulence, mix water with the air and increase the oxygen levels. Water also absorbs oxygen that aquatic plants produce during photosynthesis. Dissolved oxygen is measured in milligrams per litre (mg/L) as a concentration. This measurement tells us how much oxygen is dissolved in one litre of water. The concentration of dissolved oxygen can vary, and different levels have different effects on aquatic life. Here are some general guidelines:

0-2 mg/LNot enough oxygen to support most aquatic life.
2-4 mg/LOnly a few types of fish and insects can survive.
4-7 mg/LSuitable for many aquatic animals, but may not be enough for some cold-water fish.
7-11 mg/LVery good for most stream fish and other aquatic organisms.

Why is dissolved oxygen important?

Like all living things, fish and other aquatic organisms need oxygen to survive. Dissolved oxygen is vital for the health of aquatic ecosystems. Fish, plants, and other organisms rely on sufficient levels of oxygen in the water to breathe and carry out their life processes. In a water body, oxygen gets used up by different organisms. Plants and algae, just like land plants, also respire or breathe, using up oxygen during the night or when there’s no sunlight for photosynthesis. Also, decomposers, like bacteria and fungi, use oxygen when breaking down dead plants and animals. When dissolved oxygen levels drop too low, it can harm the survival and well-being of aquatic organisms, disrupting the delicate balance of the ecosystem.

What are algae blooms?

Algae are tiny plants that live in water and use sunlight to grow. An algae bloom is a rapid increase in the population of algae in a water body. When conditions are just right, like warm temperatures and high nutrient levels, algae can grow quickly and form blooms. The nutrients that algae need are nitrogen and phosphorus, which can come from different sources. It can come from runoff carrying fertilizers from agricultural areas, wastewater discharge, and atmospheric deposition. The availability of these nutrients, along with favourable environmental conditions, can cause lots of algae to grow and form blooms in water systems.

How do algae blooms harm the environment?

Algae blooms can harm water bodies in several ways. They can decrease the amount of dissolved oxygen in the water, which can make it hard for fish and other creatures to survive. Algae blooms can also make the water appear cloudy or change its colour. This impacts water’s temperature levels and the amount of sunlight that can penetrate the water and prevents underwater plants from photosynthesizing effectively.

Certain types of algae blooms, such as blue-green algae (they are actually cyanobacteria and not algae), can create toxins (cyanotoxins) that are dangerous to humans and animals. These Harmful Algal Blooms (HABs) can lead to health issues if people and animals come into contact with the polluted water. It’s important to understand why algae blooms happen and how they affect the environment to safeguard the well-being of lakes and rivers. By keeping an eye on and controlling nutrient levels and other factors, we can work towards preventing and lessening the impact of harmful algae blooms.

Identifying algae

Green and blue-green algae are commonly found in water bodies. Sometimes pollen in water can be mistaken for algae. Some ways to tell them apart are:

  • Algae lack roots, stems, or leaves like aquatic plants.
  • Green algae is light green to grass green and often has hair-like strands in clumps or looks like cotton candy. It is attached to rocks/plants or floats in water.
  • Blue-green algae is blue-green to pea soup in colour, and looks like paint that’s been spilled on the water’s surface or dots/globs.
  • Pollen is typically yellow and appears in the spring/early summer whereas blue-green algae blooms late summer to early fall.

Investigating Big Rideau Lake

Location: In the classroom

Duration: 1 hour

Objective: Investigate how landscape characteristics of a lake may influence the occurrence of algae blooms by analyzing maps and datasets.

Groups: We recommend groups of 2-3 students for this activity.

Materials needed

  • Computers or tablets with internet access
  • Google Maps or any other online mapping tool
  • Water Rangers Issue Report on an algae bloom
  • Presentation software or poster-making materials (optional)
  • Note taking materials


This activity includes complimenting slides to support you in delivering the lesson. Ideally, each group should have access to a computer or tablet and you can share the slides ahead of time so they have access to the links. This activity can also be done as a class, where you project your screen and explore the maps and data together.

Download the slides:


The Big Rideau Lake Association tests in many locations around the Big Rideau Lake. They have over 500 observations and data going back to the 1980s!

Teacher tip!

When students are exploring the data platform, they should only focus on the observation summaries. Although they might find the incident reports ahead of time, their hypotheses should still be backed with reasoning.

  1. Start by asking the question: “How does the landscape surrounding a lake affect the likelihood of algae blooms?”
  2. Encourage students to discuss the question in their groups and share their initial thoughts.
  3. Instruct students to draw a picture or diagram that illustrates their hypothesis on where algae blooms might occur, and create a table for recording observations based on the characteristics of the lake’s landscape.
    • Students should consider quantitative and qualitative observations; biotic and abiotic factors, urban and natural landscapes, and human activities that may contribute to algae blooms. They can label the regions to be investigated North, South, East and West.
  4. Use Big Rideau Lake as an example. Show students the lake on Google Maps or a similar mapping tool. Explain that Big Rideau Lake is a popular summer cottage lake with significant boat traffic.
  5. Guide students through exploring the map of Big Rideau Lake using Google Maps. Encourage them to zoom in and out, navigate the map, and observe various features of the lake and its surroundings.
  6. Ask the questions:
    • How do you think the runoff differs between the North and South shores of Big Rideau Lake? 
    • How would this difference influence algae blooms?
  7. Introduce the Big Rideau Lake Association. They are an organization committed to fostering a safe and healthy environment in the lake community and use our data platform to store data on Big Rideau Lake going back to 1986!
  8. Use their data to explore different sections of the lake. They have many observation locations, so focus on:
  9. Have the students explore the data, record observations in their tables, and update their maps based on their findings.
  10. Ask students to predict whether they think the bloom occurred on the North or South shore of the lake and provide reasons based on their observations from Google Maps and the incident report
  11. Have a class discussion where students share their visual representations of algae blooms and collectively determine the location of the blooms. Encourage them to explain their reasoning and speak to any qualitative or quantitative data they recorded.
  12. Share the issue reports from the Big Rideau Lake monitoring group. Zoom out on the map on the left hand side of the reports to see where the blooms occurred. They have documented algae blooms in:
  13. Instruct students to revisit the Google Map to locate the issue areas in the southeast part of the lake. If they are having trouble they can use the search bar to search the location (e.g. Klautz Point Big Rideau Lake). As a class, discuss the natural and built environment by examining the Google pins and consider what may have contributed to the blooms in that area.

Discussion questions

Teacher tip!

The discussion questions can be used to evaluate students’ knowledge.

  • Did the observed data support the reported algae bloom issues? Why or why not? Think about factors like the dates of the reports, data collection methods, etc.
  • How might human activities around the southeast lake region contribute to the algae blooms? How does this align with your initial predictions?
  • How can the occurrence of more algae blooms be prevented in the lake?
  • How does community monitoring help to address environmental threats like algae blooms in water bodies?

Testing for dissolved oxygen in a local water body

How to use Water Rangers dissolved oxygen kit

In this video, Kat explains how to use the dissolved oxygen kit, how to accurately read the comparison chart, and how to dispose of the glass ampule.

Location: In the field at a local water body

Duration: 45 minutes

Objective: Test for dissolved oxygen and other water quality parameters at a local water body.

Groups: We recommend groups of 5 students maximum for this activity.

Materials needed

  • Water Rangers dissolved oxygen kit
  • Notepads
  • Ampoule disposal jar

Do you have your Education testkit?

All materials for outdoor activities can be found in the Water Rangers education testkit.

Education testkit

Always refer to the safety tips in your educator’s guide, as well as our testing location guide and general tips for choosing a sample location when testing outdoors.



Suggestions for enrichment

Test for all water quality parameters included in your testkit! Explore as a class how the different parameters influence each other, and how the abiotic factors being tested for (e.g. pH, temperature, dissolved oxygen) influence the biotic factors observed.

  • Encourage learners to observe their surroundings.
    • What do they see/notice?
    • What sounds do they hear?
    • What do they smell from where they stand/sit?
  • Ask students to note how oxygen enters the water body, identifying factors like plants, turbulence, sunlight, etc.
  • Discuss potential factors that could reduce oxygen levels, such as the time of day, decomposing organic matter, or water temperature (cold water holds more oxygen!).
  • Encourage students to note any other qualitative observations of the environment.

Testing for dissolved oxygen

Testing for dissolved oxygen can be tricky! We suggest that you demonstrate to the class how to conduct the test before the students participate. The test must be done quickly and requires using the small sample cup to scoop the water. If you are unable to access the water, you can use the regular sample cup and reacher stick, but it will impact the measurement. There will be mixing of the sample with the air, so your reading will be higher. To save resources, you can also be the only one to do the test and compare it with the chart as a class.

  1. Take out an ampoule and a sample cup. Notice the ridges at the bottom of the sample cup and the white tip on the ampoule. This is the weak point to break with the ridge.
  2. Collect a water sample from below the surface (15 cm or deeper).
  3. Immediately insert the ampoule into the sample, break off the tip by inserting it in one of the ridges and pinching the ampoule and sample cup together.
  4. Invert the ampoule to make the air bubble go up and down twice.
  5. Allow the ampoule to develop for 2 minutes using the sand timer in the kit.
  6. Use the comparison chart to determine the dissolved oxygen level and record the result.
  7. Dispose of the used ampoule and tip in the glass disposal container included in your kit.

Discussion questions

  • Do you think the measured dissolved oxygen level is suitable for supporting aquatic life in the water body? Why or why not?
  • How did the physical observations made by the students relate to the measurements recorded?
  • How can water quality testing contribute to a better understanding of aquatic ecosystems and their health?

Ocean connections

The ocean plays a critical role in regulating dissolved oxygen levels, which are essential for the survival of marine life. However, various factors can disrupt this balance. Human-induced nutrient pollution disrupts this balance by introducing excessive nitrogen and phosphorus into coastal waters. This nutrient influx fuels the growth of algae, leading to eutrophication, where dense blooms block sunlight from reaching underwater habitats like coral reefs, causing coral bleaching and weakening corals, making them more susceptible to disease and death.

Moreover, as these algae blooms die and decompose, they consume large amounts of oxygen, further depleting dissolved oxygen levels in the water. This oxygen depletion can have devastating effects on marine organisms that rely on oxygen to survive, leading to fish kills and the creation of hypoxic or “dead zones” where marine life cannot thrive. Estuaries, where freshwater rivers meet the ocean, are particularly vulnerable to nutrient pollution and eutrophication, impacting biodiversity and ecosystem health.

Learn more about eutrophication

Real world connections

The article “On Big Rideau Lake, the Clock Ticks” captures a fuller story of what’s going on at Big Rideau Lake. Changing demographics and increased development are reshaping the shoreline, raising concerns over environmental impacts — particularly water quality and shoreline preservation. Challenges with phosphorus contamination leading to algae blooms has prompted environmental organizations like the Big Rideau Lake Association (BRLA) conduct scientific research on water quality. Their work includes assessing contaminant pathways to the lakes and advocating for lake protection in municipal planning processes.

Read the full article

Downloadable resources

Investigating algae blooms presentation slides


Abiotic factors: Non-living parts of an ecosystem, like temperature, sunlight, water, and minerals, that affect the environment and living things in it.

Atmospheric deposition: The process where particles, gasses, and pollutants from the atmosphere settle onto land or water through precipitation or being directly absorbed from the surface.

Biotic factors: Living parts of an ecosystem, including plants, animals, fungi, bacteria, and other microorganisms, that interact with each other and their environment.

Cytotoxins: Substances or agents that are toxic to cells, often causing damage or cell death upon exposure.

Eutrophication: The process where a water body becomes saturated with nutrients, leading to accelerated algae growth and eventual oxygen depletion.

Hypoxic: Conditions in a water body characterized by low oxygen levels.

Turbulence: Chaotic and unpredictable movement of water or air.