When we spot a piece of seaweed on the beach, it’s usually dead and bedraggled. And yet, if we were to head underwater, we would be immersed in a forest of waving leaves and branches in colours ranging from bright green to purple to dark brown and in all shapes and sizes. Seaweeds’ origins and reproductive cycle are even more remarkable.


Long before plants were established on land, there was a profusion of growth in the rich intertidal zone where the ocean recedes and floods twice a day. Seaweeds are a subset of algae, simple-bodied organisms, most of which live in water. At some stage they acquired the ability to create energy through photosynthesis by acquiring bacteria that contain a light-harvesting pigment.

The continental shelf is close to shore in the Pacific Northwest, forming a steep transition to deep waters. When strong winds blow, they create an upsurge of cold water from the depths, which is rich in nutrients and can support a diverse ecosystem. As a result, the coastal waters support a large number and variety of seaweeds. There are 530 species off the coast of British Columbia and 650 throughout the region.


It’s easy to forget that seaweeds are not plants as their basic structure is very similar, particularly that of the brown algae. The holdfast attaches the seaweed to the sea bottom. It can vary from a tangle of root-like growths if the seaweed inhabits muddy areas to a flat surface that attaches itself directly to a boulder. Unlike plant roots, the holdfast’s only function is to anchor the organism; it doesn’t absorb nutrition.

The stipe is a stem-like structure that supports the blades, which are equivalent to the leaves on a terrestrial plant. They are responsible for absorbing sunlight. The blades vary greatly in size and shape. Witch’s Hair has an abundance of skinny branches, while Sea Cauliflower resembles a brain. You have only to read the names of some seaweeds to imagine what they look like: Feather Boa, Sea Lettuce, and Turkish Towel.

Some species, particularly the kelp, have gas-filled bladders that float, keeping the blades pointing upwards towards the surface of the water and the light.

Location & Colour

Seaweed can be found in two distinct coastal areas: the intertidal and the subtidal. The ocean meets the land in the intertidal area. Closest to the shore is the spray zone, which is only submerged during very high tides and storms. A little further away is the high intertidal zone which floods twice a day but remains out of water for extended periods of time. The middle intertidal zone is underwater for longer periods of time and is home to a great many plants and animals, including sea star and anemone. The low intertidal zone is almost always underwater and is only uncovered during the lowest of spring tides. The second coastal area is the subtidal, which is always underwater.

Seaweeds can be divided into three groups based on their colour. Green seaweeds rely on chlorophyll, which is most effective at absorbing daylight in shallow surface waters. As a result, green seaweeds are primarily found in the intertidal zone. Brown, red, and blue pigments have evolved to capture the shorter wavelengths of light that penetrate deep water. This is why you’ll find red and brown algae in the low intertidal or subtidal zones.

Sea Lettuce (ulva lactuca) is a particularly vivid green. Only two cells thick and containing only chlorophyll, there are no other pigments to distract from its essential greenness. Purple Nori (porphyra) is only two cells thick as well but remarkably tough. Pull on a piece of wet nori and it will stretch rather than break. Nori is prolific and can be found around the world and around your sushi.

Rainbow Leaf or Splendid Iridescent (mazzaella splendens) is common in British Columbia. Its iridescent sheen, similar to oil on water, is caused by “light reflecting and refracting through a series of alternating translucent and opaque layers that make up the cuticle or protective outer layer of cells.” [The Curious World of Seaweed]


Seaweeds’ life cycle is complicated and at some stages may appear to be two completely different organisms. For example, at one stage Turkish Washcloth (mastocarpus papillatus) forms a black tar-like crust on rocks, while at another stage it has bumpy, 6-inch-long brown blades. In some cases, reproduction is sexual, while in others it’s asexual. And in some species, such as Nori and Bull Kelp, there is both a sexual and an asexual reproductive cycle.

Bull Kelp (nereocyctis luetkeana) can grow to an immense size, particularly surprising as its lifespan is a mere 200 days. The stipe can grow 6-10 inches a day, reaching heights of 50-60 feet. The first stage of its life cycle is asexual. Dark brown patches of spores appear on the blades and gradually migrate to the edge, falling off at maturity. Up to 3 trillion spores will eventually fall to the sea bottom from each individual organism. Each spore has 2 copies of nuclear DNA and germinates into male and female gametophytes (sperm and egg). The gametophytes lodge in cracks to wait out the winter. When the time is right, the female gametophytes release a powerful pheromone that attracts nearby roving sperm. Fertilization produces an immature sporophyte that will eventually grow into a massive bull kelp.

Nori has a similar two-stage reproductive process as illustrated in American Scientist.


Seaweeds have found various ways to protect themselves. Turkish Towel (chondracanthus exasperates) has thinner blades when growing in sheltered areas, whereas in exposed areas the blades are thicker and stiff. Each cell of Acid Kelp (desmarestia herbacea) contains a tiny pocket of sulfuric acid, which protects it from being nibbled on by marine foragers.

Underwater Communities

Over 30 species of kelp can be found off the Pacific Northwest coast. In some areas, Giant Kelp and Bull Kelp form dense, many-tiered forests. Kelp forests “provide food and shelter for thousands of fish, invertebrates, and marine mammal species” and “harbor a greater variety and higher diversity of plants and animals than almost any other ocean community. Many organisms use the thick blades as a safe shelter for their young from predators or even rough storms.” [National Marine Sanctuaries]

When kelp forests are close to land, their biomass enriches the local food web with oxygen and nutrients.  They also protect the shore from erosion by decreasing the impact of the waves.

Note: This article relied extensively on information provided in The Curious World of Seaweed by Josie Iselin as well as other online resources as noted.

Photo credit: https://www.flickr.com/photos/apmckinlay/9368776395

For More Information

The Curious World of Seaweed, Josie Iselin

The Science of Seaweeds, American Scientist

An Introduction to the Seaweeds of British Columbia

Seaweeds of the Pacific Northwest

Common Seaweeds of the Pacific Northwest, Capital Regional District Fact Sheet