Life of marine plants
Plants need light to make food. So marine plants live in the sunlit zone: along the shore and in shallow sea. Light dies out fast in water, and by about 200 m it is dark. Below that, no plant can live.
Water holds them up, so most marine plants are soft and bendy. They bend with waves instead of breaking. They take in nutrients straight from the water.
Sea-grass and other flowering plants
Sea-grass is a real flowering plant that went back to the sea. It has roots, stem, leaves, flowers and seeds. It grows in sandy, shallow, calm water and forms meadows.
Sea-grass meadows are nurseries for baby fish, feed for turtles and dugongs, and they hold the sand so it does not wash away. Mangroves are trees with special roots that grow in muddy salty shores.
Seaweeds: the algae
Seaweeds are big algae. They have no true roots, stems or flowers. A holdfast sticks them to rocks, a stalk-like stipe holds up the leaf-like blade. Many have air bladders that make them float up to the light.
They take in water and nutrients over the whole body. They multiply by spores, not seeds.
Main marine plants and their colours
- Green seaweeds (Ulva, sea lettuce): shallow water, bright light.
- Brown seaweeds (Sargassum, kelp): mid depth, can grow very long. Kelp forests shelter many animals.
- Red seaweeds (Gracilaria, Porphyra/nori): deepest, as they catch the little blue light left. Gracilaria gives agar.
- Sea-grass (Zostera, Halophila): sandy shallows.
Every seaweed has chlorophyll. Brown and red ones have extra pigments that hide the green and use the light that reaches deeper.
Marine plants and people
Food (nori, wakame), jelly makers (agar, carrageenan, alginate), fertiliser, animal feed, and biofuel research. They also store carbon dioxide and slow waves. Too much fertiliser from farms can make seaweed overgrow, and harbour building and dredging destroy sea-grass beds.
Try it
In the 3D, move the depth slider and write down the depth where the light drops below 10%. Predict which colour plant lives there, then press the story step to check. At home: look at a piece of dried seaweed or nori in water: it is still green inside, though it looks dark.
Key formulas and definitions
- Key term: sea-grass = flowering plant with roots; seaweed = algae with holdfast
- Parts of a seaweed: holdfast, stipe, blade, (air bladder)
- Depth order: green (shallow) → brown (middle) → red (deepest)
- Sunlit zone: about 0 to 200 m
- Seaweed products: agar, carrageenan, alginate
Worked examples
1. A plant on the sea floor has roots, leaves and flowers. Seaweed or sea-grass?
Roots and flowers mean a true flowering plant: sea-grass. Seaweeds have no flowers or true roots.
2. Why do red seaweeds grow deeper than green ones?
Red light is absorbed first, blue light goes deepest. Red seaweeds have pigments that catch blue light, so they can make food deeper.
3. Light left is about 100 percent at the surface and about 37 percent at 12 m. What does this tell a seaweed farmer?
Rope farms should hang in the top few metres, where light is strong, so the seaweed grows fast.
Common mistakes
- Saying seaweeds have roots. They have a holdfast that only sticks, it does not take in food.
- Thinking red seaweed has no chlorophyll. It has chlorophyll hidden by red pigment.
- Thinking marine plants can live in the deep dark sea. Without light there is no photosynthesis.
- Calling sea-grass a seaweed. Sea-grass is a flowering plant, seaweed is an alga.