Activity 6: Dissolved Oxygen


  • Learn about dissolved oxygen, a water quality characteristic.
  • Learn about algal blooms and how they affect dissolved oxygen.
  • Use a watershed model to simulate how algal blooms form.

Learning Targets

  • Recognize that dissolved oxygen is important in water bodies.
  • Understand how dissolved oxygen affects species.
  • Model how nutrients get into a body of water.
  • Understand how nutrient runoff and dissolved oxygen are connected.


60-80 minutes.


Included in this kit:

  • 7 sets of Algal Bloom Pictures

Not included but needed for this activity:

  • Tap Water
  • The EnviroscapeTM Model (not included, but available for loan from the University of Maine Extension. Visit the Explore Maine 4-H STEM Toolkits page) note: comes in coastal and inland versions. For more information, visit the Enviroscape website.
  • Evaluations (choose one or more of the following):
    • Exit Ticket (1 for each youth)
    • KLEWS Chart (1 for each youth)
    • Journals (1 for each youth)


  • Dissolved Oxygen: the amount of gaseous, molecular (O2) oxygen dissolved in water. The form of oxygen that aquatic life can breathe.
  • Algae: Aquatic, plant-like, photosynthetic organisms. Range from phytoplankton to seaweed. (Difference from plants is due to structure, e.g. a holdfast instead of roots.)
    • Note: this definition is ever-changing in the scientific community.
  • Algal Bloom: a sudden increase in the number of algae, often caused by an influx of nutrients (nitrogen and phosphorus).
  • Dead Zone: an area of water with a dissolved oxygen concentration that is so low that animals suffocate or avoid the area as there is not enough oxygen to survive. Dead zones often form at the bottom of the water body. Dead zones may be caused by the decomposition of an algal bloom. The bacteria that decompose the algae use up most/all of the dissolved oxygen.
  • Toxic Algal Bloom (eg Red Tide): a bloom of algae that produce toxins, e.g. neurotoxins. Toxic algal blooms often result in closed fisheries, since shellfish that eat the toxin-producing algae are toxic to humans and animals that eat them.

Background Information for Facilitators

Note:  All links are provided purely for educational purposes. No responsibility is assumed for any content on the linked site(s).

Dissolved oxygen and algal bloom videos:

  • Dissolved oxygen (YouTube), MITK12Videos (length: 5:22)
    Summary: Aquatic animals also breathe oxygen, description of ‘dissolved’, how dissolved oxygen changes, how salinity and temperature impact DO2 and so the animals in the water, eutrophication (adding nutrients to water), how algae blooms result in decreased DO2



Before you begin, review the discussion norms that youth agreed to during the Introduction to Aquaculture activity. These norms are intended to help youth have productive, respectful conversations. Reminding youth of their norms helps youth to remember to follow their norms.

Engage: What is dissolved oxygen? (10 minutes)

Watch the following Activity 6: Dissolved Oxygen video: Candles Losing Oxygen (YouTube)

  • Journals: record observations on page 28.

Discuss what is happening. If needed, use the following questions to prompt discussion:

  • Why do the candles stop burning? Answer: the burning candle has used up all of the oxygen in the jar. No more oxygen = no more combustion = no flame
  • What would happen if there was an animal in the jar? Answer: the animal would also eventually run out of oxygen, and then die. (This assumes that the animal does not die of other causes, e.g. lack of food or water.)
  • What would happen if there was a plant in the jar? Answer: the plant would eventually use up all of the carbon dioxide in the jar, and then die.
  • So … animals on land need to breathe oxygen in order to survive … what about animals underwater? Plants underwater? Answer: aquatic animals also need oxygen to survive. Aquatic plants do not need oxygen to survive.
  • Where do aquatic animals get the oxygen they need? Answer: there is oxygen dissolved in water.
    • Note: while water contains oxygen and hydrogen, that oxygen cannot be easily used by animals – animals need molecular oxygen, which is two oxygen atoms. This molecular oxygen is what we are talking about when discussing ‘dissolved oxygen’.

As a group, agree on a definition of ‘dissolved oxygen’.

  • KLEWS: record definition of dissolved oxygen in S column.
  • Journals: record definition of dissolved oxygen on page 27.

Explain (10-20 minutes)

What changes dissolved oxygen

As a group, discuss how dissolved oxygen gets into water, and what can make it change.

  • KLEWS: brainstorm in the K column.
  • Journals: brainstorm on page 28.

How does dissolved oxygen get into water?

  • Equilibrium with atmosphere at water surface
  • Mixing, e.g. rapids or waterfalls
  • Photosynthesis of algae (or other aquatic plants)

What can change the amount of dissolved oxygen?

  • Time of day (more photosynthesis during the day than the night)
  • Number and kinds of plants and animals in the water

Algae Blooms

pages from the booklet about algae bloom

Is it possible to have too much oxygen in water? Too little? (Answer: yes, the dose makes the poison. What is good for one species may not be good for another.)

  • KLEWS: brainstorm in the K column.
  • Journals: brainstorm on page 28.

Describe an algal bloom:

  • There is normally some amount of algae in the water.
  • When there are more nutrients (phosphorus and nitrogen), you may have an overgrowth of algae … known as an algal bloom.
    • This bloom leads to a short-term increase in the amount of DO2 (mostly in the upper level of the water – thermocline).
  • Eventually, the algae will die, sink to the bottom.
  • Ask youth to recall the density box, and remind them that in lakes and the ocean the water is layered according to density – warmer less dense water floats on top of cooler, more dense water.
  • Algae is decomposed by bacteria … which uses up oxygen, leading to a lack of oxygen in the bottom layer of the water (a ‘dead zone’).
  • The water in the top layer with more oxygen doesn’t mix with the water in the bottom layer, so there’s no way to get more oxygen into that bottom layer of water
  • Things that need oxygen can no longer live on the bottom – they move (migrate) or die (suffocation).
  • Show youth the algae bloom images

Explore: EnviroscapeTM (30-40 minutes)

Use the EnviroscapeTM to model how nutrients get into a water body, which can lead to an algal bloom.

Here are the steps: (Have youth do things whenever possible).

  • Set up the EnviroscapeTM, putting the molded terrain on the clear plastic base. Be sure to line up the plug in the ocean/lake with the hole in the base.
  • Place buildings in their spots.
  • Place a maximum of three trees on the model. (The mountain has been deforested.)
  • Place the small plastic container underneath the plug in the ocean/lake.
  • Plug all wells with clay – the goal is to have the water flow to the ocean/lake.
  • Put some water in the ocean/lake.
  • Fertilizers (nutrients):
    • Discuss and agree on a definition for ‘fertilizer’: something to help plants grow often contains nitrogen and phosphorus along with other nutrients.
    • Sprinkle green powdered drink mix on places where fertilizer would be spread: the farm fields, the lawns, the golf course.
  • Pesticides:
    • Discuss and agree on a definition for ‘pesticide’: something meant to control pests, often by killing them; ‘herbicides’ are specifically for weeds (plants); ‘insecticides’ are specifically for insects; other types of pesticides can be targeted for fungi, birds, rodents, bacteria, viruses, snails, or algae
    • Sprinkle red powdered drink mix on places where pesticides would be spread: the farm fields, the lawns, the golf course.
  • Loose dirt:
    • Have a brief review of turbidity and how it can be impacted by the amount of loose dirt carried by rain from land into water bodies.
    • Sprinkle cocoa powder on places where loose dirt would be found: the mountains, the farm fields, the construction site, basically the brown areas of the model.
  • Ask the youth what they think will happen when it starts to rain.
  • Have youth take turns using the spray bottle to make it rain on the model.
    • Note: ask youth to name the area they want to spray and predict what they think will happen before they spray.
    • The water from the spray bottle should dissolve the powdered drink mix and cocoa powder, creating a colored liquid that … ends up in the lake/ocean.
  • Eventually, you will hit a point where most of the powders have ended up in the ocean/lake and connecting water bodies.
  • Ask youth what will happen to the ocean/lake
    • Nutrients from runoff may lead to an algal bloom, which may lead to a dead zone.
    • Loose dirt washed into water bodies will increase turbidity.
    • Pesticides can harm or kill aquatic organisms.

Next, see if youth can come up with a way to mitigate algal blooms. Here are the steps:

  • Clean off the EnviroscapeTM as much as possible. It does not need to be spotless, but change out the water in the ocean/lake, try to get the powder off of the land, and try to dry the land.
  • Ask youth to come up with methods that will prevent the fertilizers, pesticides, and loose dirt from getting into the water bodies. Youth will probably come up with some of the following ideas:
    • More trees = less loose dirt gets to rivers/streams.
    • More vegetation helps to slow down and collect the fertilizers/pesticides/loose dirt before they get to the water bodies.
    • Building embankments prevents runoff from getting into water bodies.
    • Do not put fertilizers and pesticides out right before it is supposed to rain – a lot will just wash into water bodies.
      Try out your mitigation methods!

When done with the EnviroscapeTM, please rinse things off and dry them (as much as possible) before packing everything away.

Extend: Aquaculture Project (10-15 minutes)

Use both supplemental resources to help youth with their research:

Look up the dissolved oxygen for your chosen water body. Is there a history of algal blooms and dead zones?
Will your chosen species tolerate these oxygen levels?

Evaluate (Assessment) (5 minutes)

Approximately 5 minutes before the end of the session, have youth finish up what they are doing, help clean up the materials, and do one of the following evaluation methods:

Have youth complete and turn in an exit ticket.

Have youth finish and turn in KLEWS chart.

After completing this activity youth should have the following in their journals:

  • Definitions for ‘dissolved oxygen’, ‘algae’, ‘algal bloom’, and ‘dead zone’ (page 27)
  • Aquaculture project: (pages 29 and 30)
    • Dissolved oxygen range of their species
    • Dissolved oxygen range of their waterbody
    • Comparison of the two


Additional information is available on the Enviroscape website.