Ocean - Session II.1 The blue planet


Reviewing documents, pupils will be exposed to the vastness of oceans and the diversity of life therein. They will realise how large and poorly understood oceans are.

Key ideas

  • Oceans cover more than 70% of the Earth’s surface.
  • Oceans hold more than 300 times the habitable volume of the Earth’s surface, but only 5% of it has been explored.
  • We have identified more than 250,000 ocean species and estimate that there are 4-10 as many left to discover. Because they are more difficult to access, marine species are generally less well known than land-based species.
  • The phytoplankton in the oceans supplies approximately 80% of the oxygen in the air we breathe.

Inquiry methods

Document review, math


For each pupil

For the class


Habitat, dioxygen, species, bacteria, plankton

Durée :


Introductory Question

The teacher asks the pupils: Some of our planet's life forms live in the ocean. In your opinion, is there more habitable space on land than in the ocean, or vice versa? Has all of this space been explored? Do we know everything about the life forms found there?

The pupils’ ideas are summarised on the board.

Research (Document Review)

In order to learn more, the teacher will distribute a copy of Worksheet 8 to each pupil. The class will read the text and then some time is left for each pupil to answer the questions individually.

Teacher’s Note

  • The document proposed for this review is available in two versions:
    • One version not requiring the calculation of area and volume, for fourth graders
    • One version that asks pupils to calculate for themselves the habitable volume of the ocean, for fifth through seventh graders. This activity could also be done in collaboration with their math teacher.

The teacher is free to choose the version that is appropriate for his or her pupils. However, both documents present a mathematical approach to the concept of percentages.

  • The volume of the ocean can be approximated to that of a cuboid (which pupils learn about starting in fifth grade). In order to calculate the volume of a cuboid, you multiply the area A of the bottom face (the side the cuboid sits on) by the height h of the cuboid. In the case of the ocean, A = the surface of the Earth’s oceans and h = their average depth. Make sure to convert each measurement to the same unit (km) because the average depth of the ocean is often expressed in metres.
  •  If the teacher is concerned that pupils may tear the worksheet by colouring several layers in marker, he or she may wish to propose using a crayon for the light blue, then a marker for the dark blue.

Scientifc notes:

  • Earth’s major forests, especially the Amazon, are frequently referred to as the “lungs of the planet” because they play a major role in producing dioxygen and absorbing CO2, through photosynthesis. In reality, however, it is the biomass of photosynthetic phytoplankton in oceans that is the primary producer of dioxygen on a global scale, and the primary absorber of atmospheric carbon dioxide (which has taken on a critical role in the context of the significant CO2 emissions produced by humans)!
  • It is worth noting, however, that this term is misleading: while lungs absorb oxygen and emit CO2, the “lungs of the planet” do the opposite: They emit dioxygen and absorb CO2.
  • For elementary pupils, the teacher may want to use the term “oxygen” instead and perhaps introduce the term “dioxygen” starting in middle school.

Group Discussion and Conclusion

The class will discuss their answers. The teacher may wish to copy the table proposed in the document on the board in order to better discuss the answers as a group. It seems that the majority of the Earth’s surface is covered by oceans, that the Pacific Ocean accounts for the largest portion, and that only a small part of these spaces have been explored by humans.

Fifth graders will have calculated the total volume of the ocean at 1.37 billion cubic kilometres (361,000,000 x 3.8 = 1,371,800,000). By comparing this with the volume of land-based habitats given in the document (4,560,000 km3), they will have realised that the ocean has 300 times more habitable space than land. For fourth graders, this information is directly provided by the text.

They will also realise that, of the millions of species that scientists believe to exist in the ocean, 770,000 are yet to be discovered.

Finally, they will learn that the tiny plants suspended in water, called phytoplankton, account for approximately 80% of the production of dioxygen in the Earth’s air, or four times greater than the dioxygen production of land-based plants, and that the ocean better deserves the title “lungs of the Earth” than the Amazon Forest, which commonly receives this honour.

The teacher will finish by presenting the map in Worksheet 10 and the class will discuss the topography of the ocean floor: from the continental slope to the oceanic trenches by way of the abyssal plains and the mid-ocean ridges, the morphology of the ocean is no less varied than that of dry land!

Using these ideas as a starting point, the class will come up with a collective conclusion that will be written in their science logbook.

Example in a 4th grade class

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