Ocean - Session II.8 Phytoplankton et zooplankton


Using magnifying glasses and microscopes to make observations, pupils learn about the microscopic organisms living in the ocean.

Key ideas

  • Marine environments are not only inhabited by “large” plants and animals, but also by microscopic organisms.
  • To observe these organisms, pupils need magnifying instruments such as magnifying glasses or microscopes.

Inquiry methods



  • Magnifying glass, microscope
  • Biological material to study; for example phytoplankton or zooplankton (available in bottles from aquarium supply stores), or the eggs of small animals such as brine shrimp (also sold as live food for aquarium fish)


Cultivation, plankton, phytoplankton, zooplankton, microscope


Variable depending on the selected option

Teacher’s note

  • This session may not be relevant for primary school, but it is for middle school, where making observations (especially using a microscope) and studying the growth of living organisms (depending on available resources) are part of the curriculum.

Introductory question

The teacher goes over ideas covered in previous sessions and asks pupils about the size of the marine organisms they have studied (either using cards or the observations from the intertidal zone field trip, if applicable). Some organisms are very large, like the blue whale, and others very small, like the sandhopper. Either the teacher or pupils mention plankton, described in cards 25 and 26. “Plankton” refers to organisms that are suspended in water and cannot swim against the current. The teacher tells the class that, in this session, they will study microscopic organisms that live in the ocean.

Science note

  • Plankton represents 98% of oceanic biomass. In food webs, it only takes 15 days for all the molecules in this biomass to change organism. Plankton includes animals and plants, but also bacteria, protists and viruses. One litre of seawater contains between 10 and 100 billion viruses, 1 and 10 billion bacteria and 0.1 and 1 billion protists.

Research: observing or cultivating animals

For this session, the teacher should choose one activity – observing or cultivating animals. The following examples are suggestions. The teacher may also choose other biological models.

Example A: Using a microscope to observe live phytoplankton or zooplankton

To carry out these observations, the class requires simple microscopes ("Jeulin Collège" microscopes or binocular loupes are suitable). The material to be observed can be purchased in aquarium supply stores as live food for fish. It is available in diluted or concentrated form. Generally, these aquarium kits are called names such as “microfauna booster” or “plankton booster”. They are available for zooplankton and phytoplankton. Recommended material includes slides, coverslips and flexible pipettes to isolate small organisms.

If the teacher wishes, he/she may consult one of the many aquarium supply websites with protocols for phytoplankton cultivation. This activity could be a good introduction to the observation activity. It ensures pupils understand the needs of these primary producers, which they studied as part of food webs.

Exercise B: cultivating and observing brine shrimp

Brine shrimp (Artemia salina) are small shellfish that mostly live in lagoons. They are also found in saltwater lakes and salt marshes. These animals can swim but cannot resist currents. They are therefore considered zooplankton.

Brine shrimp are interesting because they can produce cysts. In other words, larvae have shells that protect them from harsh environments (especially drought). Consequently, they can survive extreme weather conditions, such as temperatures below -190° or boiling water for over two hours.  These cysts are sold by aquarium suppliers. It is easy to cultivate brine shrimps (Siberian brine shrimp eggs are the most robust) for fish to eat.

 The process is as follows:

  • Leave one litre of tap water to sit in a large bowl at room temperature for 24 hours (so the chlorine evaporates). Put some water in another bowl and let it sit also.
  • Put four teaspoons of sea salt in the large bowl and stir.
  • Soak the cysts for one hour in the non-salted water, then pour them into the bowl of salt water. Stir so they sink.
  • From time to time, aerate the water by blowing into it with a straw or using a "bubbler".
  • The cysts hatch between 1 and 5 days later, producing larvae called nauplii that later become adult brine shrimps.
  • Feed the larvae with phytoplankton powder sold at aquarium supply stores (one pinch each day for the whole bowl). Dunaliella salina is the most commonly used phytoplankton strain.

Adult brine shrimps measure between 8 and 10mm (0.3-0.4 in) and can be observed using a magnifying glass. Their bodies have 20 segments (1 for the head, 11 for the thorax and 8 for the abdomen – their three body parts). They have ten pairs of foliaceous legs. They have a lifespan of several months.

The behaviour of brine shrimps is interesting to observe: nauplii are attracted to lamp light, but adult shrimps tend to gather in dark areas. The brine shrimp's life cycle (which includes a larval stage and metamorphosis) can be studied with pupils, as can its position in the food chain (pupils will be interested to know that these shrimps are the reason flamingos are pink).

Written conclusion: descriptive texts and observational drawings

To keep a written record of these observations, pupils could write short texts describing the organisms studied. This is a good exercise for developing vocabulary, because they must find the words to describe shapes and matter. Similarly, pupils could produce observational drawings.

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