Contrary to popular belief, seagrass beds are not algae but flowering plants. These flowering plants, which grow entirely underwater, cover less than 0.2% of the ocean floor… yet produce 10% of the oxygen we breathe! A veritable factory of life, invisible from the surface.
Prenons la zostère, reine des côtes bretonnes : ces herbiers stockent d’énormes quantités de carbone, bien plus qu’une forêt terrestre de même taille, tout en oxygénant l’eau. Mais leur rôle ne s’arrête pas là : ils abritent une biodiversité foisonnante (larves et juvéniles de poissons, coquillages, crevettes) et atténuent la force des vagues, protégeant ainsi les littoraux de l’érosion.
But before reducing them to a mere ecological role, did you know that…
👉 Seaweed beds are archives of the past!
Their roots form a dense network in the sediment, known as a matte. This matte is made up of roots, rhizomes (underground stems), and sediments that have accumulated over time.
- The matte can accumulate over thousands of years, forming thick layers (sometimes several meters). It is highly resistant and can persist for a long time, even after the above‑ground parts of the plant have died.
- The roots and rhizomes grow slowly but continuously, spreading both horizontally and vertically through the sediment.
By studying these layers, scientists reconstruct the history of the oceans, much like reading the rings of a tree.
💡 They inspire medicine!
Some molecules extracted from seagrass beds have anticancer or antibacterial properties. Posidonia, an aquatic plant commonly found in Mediterranean seagrass beds, produces compounds that could help combat antibiotic-resistant bacteria.
🌊 An ecosystem in danger
Yet these treasures are disappearing at an alarming rate: between 1990 and 2006, nearly 30% of Mediterranean seagrass beds were destroyed by anchors, pollution, or global warming. Their silent decline threatens marine biodiversity as a whole.
Why don’t we take action?
Here are two ready-to-use activities for exploring these topics in class, with your students, or as a family:
“1. Sample the seagrass meadows using quadrats (from cycle 3 onward)
Objective: To understand what sampling is and learn how to use a transect, and to observe the biodiversity associated with seagrass beds.
Materials: 1 m² quadrats (PVC or wood), clipboard or board, data sheets, pencils.
Procedure:
- Identify an area in the field where to sample the seagrass beds.
- In groups, toss the quadrat randomly within the defined area (random sampling).
- Count the number of seagrass “clumps” found within the quadrat.
- Cycle 4: estimate the surface covered (in %).
- In the same quadrat, count the small crustaceans, mollusks, etc.
- Repeat this exercise at least five times.
To take this exercise a step further, you can compare a protected area and an anchorage zone by taking two samples.
2. Observe the difference between flowering plants and algae (from Cycle 2 onwards)
Objective: to identify the parts of a plant, to understand that eelgrass is a flowering plant, and to observe how it differs from seaweed.
Materials: eelgrass or an image of eelgrass; a diagram of a plant; seaweed or an image of seaweed.
Procedure:
- Observe and identify the various essential parts of a plant (roots, stem, leaves, etc.).
- Observe the different parts of a seaweed (stipe, frond, holdfast, thallus, etc.).
- Compare these two plants.
- Understand that eelgrass is a flowering plant.