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Seaweed study

Students learn the parts of rockweed (a common seaweed) and the functions of the parts.
Science content (2016 curriculum): 
Biology: Features, Adaptations of Living Things (K, 1, 3, 7)
Science topic (2005 curriculum connection): 
Life Science: Characteristics of Living Things (grade K)
Life Science: Needs of Living Things (grade 1)
Lessons activity is in: 
  • A bucket of rockweed - washed up pieces soften up if soaked in seawater overnight; check that the bladders still float though.
  • Piece of rockweed cut into pieces: some parts have bladders and some don't
  • Tray of seawater for floating rockweed pieces in
  • Scissors
  • Rockweed living on rocks to find

Students each find a piece of rockweed. Discuss what it is, where it grows, and that it is an alga (not a plant).
Students look closely at the parts of the rockweed, draw it in their notebooks (draw around a small piece), then label the parts: holdfast, bladders and blades.

Predict of what the parts of the seaweed might be for.
Test the function of each part of the seaweed by tearing the rockweed into pieces, comprising either blade or a bladder, then see if each float in a tray of water.
The blades should sink, and the bladders float (unless the air has been popped out of them). Do many pieces until a pattern is seen.

Discussion of the parts.
Bladders: they keep the tips of the seaweed floating up in the water, to maximally expose all parts of the seaweed to sunlight. The bladders can be torn open to find the gas bubbles among the jelly that make it float.
Holdfast: it keeps the seaweed anchored to a rock, so that it is not washed ashore.
Blades: wide to catch sunlight for photosynthesis.

Students hunt for living seaweed on rocks and washed up seaweed at the tide line.

Beach hoppers live under dried seaweed, and can be used for further discussion if there is time (information and images of beach hoppers, 6th image down).

Grades tested: 
Gr K
Gr 1
Gr 2
Gr 3
Carole Murray
Christy Wong
Diane Merchant
Julie Kawaguchi
Kecia Boecking
Mari Matsuo
Monika Pilat
Teaching site: 
McBride Elementary
Sexsmith Elementary
Simon Fraser Elementary
Weir Elementary
Activity originally developed and delivered: 

Scientist in Residence Program, Vancouver School District with teachers Ms. Kawaguchi and Ms. Merchant.
Original lesson plan at