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Knowing Nature . . .

Combing for Seaweed

January 6, 2006
Jocie Ingram

Turkish Washcloth
Turkish Washcloth: photo © Dave Ingram

Beachcombing is always good after a winter storm. Lately I’ve noticed a lot of seaweeds along the shore, washed up from the ocean depths. There is a rich assortment of medieval colours: reds, browns and greens. Held up to the light they are somewhat transparent, like looking through stained glass. Forms and textures vary, some have a knobby texture and some are rippled like puckered silk. A few are iridescent, giving off a pearly sheen of colour, similar to the effect of oil on water.

In the summer, seaweeds can rot, letting off a foul odour. They can be treacherously slippery to walk on, and some people avoid ocean swimming entirely due to a fear of brushing up against a piece of seaweed (these folks obviously weren’t raised here). Despite a few reasons for disliking seaweed, there is much to appreciate about these beautiful marine plants

Seaweeds are plants, but they are different from land plants. Land plants have roots that draw water and nutrients from the soil. They have specialized tissues for transporting food and water through the plant, called a vascular system. These ‘vascular’ plants usually produce seeds, which can grow into a whole new plant.


Seaweed Medley
Seaweed Medley: photo © Dave Ingram

More primitive than land plants, seaweeds belong to a group of non-vascular plants called algae (al-jee, singular alga).  Algae are found in both fresh water and salt water. Many ocean algae are microscopic, and unicellular. These are called phytoplankton, and have a free-floating life in the ocean. Filter-feeders like clams, anemones, and even some whales depend upon phytoplankton. The larger ‘macroscopic’ algae that we call seaweed, is also crucial to ocean ecosystems.

Seaweeds, or marine algae, have several features that distinguish them from land plants. They have a root-like structure called a holdfast, but the holdfast has one purpose only; to anchor the seaweed. At the base of the holdfast a small disc secretes a glue-like substance, which sticks the holdfast to a rock, or the sea floor. It takes a strong force, such as a big storm, to dislodge the seaweed from its holdfast. Leading up from the holdfast is a stem-like ‘stipe’, which connects to the leaf-like ‘blade’ of the seaweed. Seaweeds never have leaves or flowers, nor do they produce seeds as land plants do.

However, like land plants, seaweeds have green chlorophyll that captures the energy of sunlight, and converts it into chemical energy. During this complex process, called photosynthesis, carbon dioxide is taken in and oxygen is released as a byproduct.

Seaweeds are divided into three types: green, brown and red. Only 13% of green algae are found in the ocean, the rest occur in fresh water. Though there are fewer marine green algae than fresh water types, green seaweeds are common in our area, and sea lettuce (Ulva spp) is easily found on the beach.

Sugar Wrack Kelp
Sugar Wrack Kelp: photo © Dave Ingram

Brown seaweeds have a pigment, which masks green chlorophyll. The Pacific Northwest is home to some of the largest brown seaweeds in the world. These large seaweeds, called kelp, can be up to 120 feet long. There are many kinds of kelp, but the most well known is the bull kelp (Nereocystis luetkeana). The long, whip-like stipe of bull kelp is often found coiled up on the shore. At the head of the stipe is a round flask-shaped structure called a pneumatocyst, to which long ribbon-like blades are attached. The pneumatocyst contains carbon monoxide, and acts as a float to raise the blades to the ocean surface where there is more light. When light levels are high and nutrients abundant seaweeds like kelp grow extremely fast, up to 6 cm a day. Most seaweeds are annual, meaning that the entire life cycle is only one year.

Red seaweeds, like brown seaweeds, have pigments that mask green chlorophyll. The pigments allow red seaweeds to capture light at lower densities. Red algae are more common subtidally, where ocean depths are greater and light levels lower.

To date, scientists have identified as many as 650 species of seaweed in the Pacific Northwest. Of these, 530 occur in British Columbia, which makes our coast a very rich and diverse place for seaweeds. Algae are the primary food source for many marine animals. In addition to this, seaweeds provide animals with protection from predators. Kelp beds are like an underwater forest, a special home to scores of creatures.

Seaweeds are important to humans for a variety of commercial uses. Alginates from kelps, and agar and carageenan from red seaweeds are used as thickeners and stabilizers in many products including toothpaste, beer, ice cream, chocolate milk, non-drip paints, cosmetics and medicine capsules. Seaweeds are also used as an excellent garden fertilizer.

People in many parts of the world consume seaweed directly. In Japan, the harvesting of nori, a red seaweed used in making sushi, is an important part of the economy. In the maritime provinces of Canada, red seaweed called dulse is commonly eaten.

All things considered, there are few reasons to dislike seaweed. And after a storm, it is worthwhile to walk the shore, admiring the diversity of seaweeds washed up from the ocean’s gardens.

Click on a link below to view the CVNS newspaper column.

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