Bull Kelp

Nereocystis luetkeana
Photo: Jackie Hildering, TheMarineDetective.com

Redwoods of the Ocean

Bull Kelp (Nereocystis luetkeana) is the majestic kelp that grows up to sixty feet in one season, to reproduce and be washed away by winter storms each year. It is the signature species making up the vastly productive kelp forests of the Northern Pacific coastline from Central California through Northern California, the Pacific Northwest, British Columbia, Alaska, and halfway through the Aleutian Islands. It is the kelp within which herring school and spawn, rockfish abound, seals seek prey and refuge from sharks, and young salmon adapt to their salty home. It is the canopy under which countless other seaweeds grow including the understory kelps and pink rock-encrusting coralline algae that attract the settling of urchin and abalone and young bull kelp. Under the gestural swaying largess of the bull kelp is a colorful world with sea stars and nudibranchs and sponges. There are other large kelps in the mix—Macrocystis pyrifera, or giant kelp, dominates the kelp forests south of Monterey clear to Mexico, and understory kelps such as feather boa kelp and bladder-chain wrack will be found washing ashore—but from Central California to Alaska, bull kelp is the major player, massive and hefty, thriving in the nutrient-rich, cold waters of the Northern Pacific. 

What is The Bull Kelp Forest?

Bull kelp is the algal species that creates the rich, underwater ecosystem where hundreds of species thrive. These ocean forests are as diverse and abundant and important for life on earth as any of our terrestrial forests. Bull kelp grows just a few hundred yards from shore in what is known as the subtidal zone, needing a rocky bottom to hold onto and enough sunlight to promote growth. The continual sloughing away of the bull kelp blades creates a snowstorm of organic matter (known as detritus) that feeds countless organisms, such as shrimp, isopods, snails, crabs, urchin, and abalone. These low-level detritivores are important food sources for herring, rockfish, juvenile salmon, lingcod and then these in turn build the food web important for marine birds and mammals such as seals, sea otter, murres, pelicans, and even whales. Bull kelp detritus thrown onto the beach, known as wrack, is decomposed by kelp flies and amphipods, which are an important protein source for shorebirds. The web of interdependencies is known as the trophic food web (who eats whom), and the majestic bull kelp is the underpinning of all this complexity in the coastal waters of the Northeastern Pacific Ocean.

Three groups of seaweed
Three Color Groups: Green, Red, and Brown
By Josie Iselin

What Color is your Kelp? 

A seaweed’s color is determined by the combination of pigments housed in its chloroplasts or plastids, which collect photons from different wavelengths of light. This energy activates the process of photosynthesis.

The Greens

Green algae (or seaweed) (phylum Chlorophyta) like plants, have chiefly green chlorophyll a and b 
in their chloroplasts. They are typically bright Kelly green.

The Reds

Red algae (phylum Rhodophyta) have red and blue accessory pigments that overshadow the chlorophyll a pigment and combine to make an array of pink, magenta, and red seaweeds. There are over 6,000 or so species of red algae. 

The Browns

Brown algae (class Phaeophyceae) include the kelps and rockweeds. They have a brown pigments that combine with the green chlorophyll to make seaweeds that range from olive green to golden brown to yellow orange. Bull Kelp is in this brown group. When viewed from below looking up at the ocean surface with sun coming through its blades, bull kelp glows a pure gold, drying to chocolate brown on the beach and slowly bleach to white as they dry in the sun. The kelps evolved many millions of years after the green and red algae.

How Does Bull Kelp Grow? 

Kelp have the most differentiated bodies of all the seaweeds and bull kelp has, perhaps, the most extraordinary form of all the kelps. The bull kelp stipe is a long rope of tough, cortical material that can stretch up to 38% to give and take with the ocean current. It emerges from a small holdfast anchored to a rocky substrate and ends in a single bulb, or bladder, shaped like an oversized avocado. The blades, streaming out from the bladder—or pneumatocyst—multiply at four source points into as many as sixty ribbons of thin golden fronds. The entire kelp body is called the thallus. In springtime, baby kelps, in various stages of development, reach for the surface, their golden bladders no bigger than your thumbnail, their four golden blades, catching the sunlight, radiating its wonder in their molten translucence. These young Nereocystis use the power of sunlight and the nutrients of the ocean to perform one of the greatest feats of metabolic growth on our planet; they will become a massive and majestic bull kelp in only a matter of months. 

Nereocystis luetkeana
By Josie Iselin

Nereocystis luetkeana is, generally speaking, an annual. The entire organism grows anew each season typically starting in late winter or early spring, and for six months that growth is astounding. The long singular stipe (or stalk) grows in a reverse taper, from thin at the bottom to thick towards the top. It can grow 6–10 inches a day and reach heights of 60–100 feet in a matter of months, slowing once the bladder senses a proximity to the surface. This stupendous growth is all in the service of efficient photosynthesis—getting the blades closer to sunlight to catch as many photons as possible. After about two hundred days, or with the onset of winter storms, the entire organism dislodges from the ocean floor, holdfast and all, and washes away, sinking as it breaks up, or tumbling onto sandy beaches. This bull kelp wrack in turn fuels other food webs. Some individuals, produced late in the season, may successfully overwinter and survive a second year, up to 18 months. These can be recognized as old growth, grandmother bull kelps, usually with plenty of other algae (epiphytes) and bryozoans growing on them.

Diagram after Mondragon showing the alternating sexual (microscopic) and asexual (sporophyte) generations of the bull kelp life cycle.

How Does Bull Kelp Reproduce? 

Bull kelp’s spore patches are the acorns of the kelp forest. The extended daylight of summer combined with the upwelling of nutrients, accelerates bull kelp growth and by the end of summer, a dark brown patch, 2–7 inches long, emerges on each blade. Called sori, these fertile patches are bundles of millions of spores. As growth continues, the maturing sori gravitate to the outer end of the blades, while new ones emerge closer to the bladder. Coordinated with the light of dawn, a whole sorus will abscise from its blade and the spore patch falls away to float to the ocean bottom and settle near the base of the parent alga. Within hours, millions of photosynthetic spores are released into the water column to disperse, the majority settling on the rocky bottom, near the parent from which they came. With forty to sixty blades each, an adult Nereocystis can produce over three trillion spores—thus it is called a sporophyte with two sets of chromosomes in each cell. By fall time, as the spore patches fall away, only the edges of the blades remain, ragged, hanging like stencil edges or the leftover dough after cookies have been cut out. Soon the entire thallus, or algal body, will loosen from the bottom and wash ashore. 

Bull kelp sori
Bull kelp sori
Photo by Abbey Dias

The tiny spores that successfully settle on the rocky bottom, germinate into the alternate phase of the Nereocystis life cycle: microscopic, thread-like sexual organisms, males and females, also known as gametophytes, which sport only a single set of chromosomes. These tiny organisms lodge in the cracks and crevices of the ocean bottom to wait out the rough weather and dim light of winter. While cultivating bull kelp in the lab is not difficult, studying this cryptic sexual phase in its natural environment is understandably hard and questions abound. It is not clear how long these filamentous microorganisms can persist. We do know that when the time comes, the sessile, or fixed-in-place, female releases a powerful pheromone that attracts the nearby roving sperm and fertilization initiates the development of a baby sporophyte that grows into the massive bull kelp we recognize. This new growth begins in late winter or early spring, and the annual cycle begins again. The regular rotation of wrack on the beaches each year, indicates the rhythms of robust growth out under the waves.

Photosynthesis and Ocean Nutrients: A Power Duo

Bull kelp is a primary producer, it is the oak tree of the underwater forest. Bull kelp collects spectra of light that filter through the ocean water which energizes the process that uses carbon dioxide (CO2) combined with water to create sugars, i.e. plant matter, while offgassing oxygen (O2). This remarkable process, called photosynthesis, is the engine for all life on earth, creating food for low level organisms, fixing carbon, and creating the oxygen that all the respirating creatures on Earth depend on. Every other breath we take comes from the oxygen that the ocean provides, oxygen created by single-celled diatoms and other marine plants. 

Bull kelp are some of the most efficient photosynthesizers on the planet. Every cell of the kelp also has access to the vast array of nutrients mineralized in ocean water. Sodium, potassium, magnesium, iodine and other essential minerals in the ocean directly feed the kelp, fueling its abundant growth. Transforming these inorganic compounds into organic matter, using up carbon in the process, is kelp’s superpower, and why kelp farming is on the tip of everyone’s tongues.

The molecules in cold water are packed more tightly together than in warm water, so cold ocean water holds a greater density of nutrients than warmer, tropical water. This is why kelp grows in temperate regions around the globe; they need the nutrient-dense, cold waters that fuel their prodigious growth.

Along the North Pacific Coast, cold nutrient-rich water upwells from the deep ocean onto the continental shelf and makes this regions some of the richest waters on Earth. Those thick 7 mil wetsuits make it harder to swim and study in these cold oceans, but the frigid water is a sign of abundance. Every degree of warmer ocean is a stressor on the kelps and seaweeds as they struggle to connect with the nutrients necessary for survival.


Along the North Pacific Coast of America, deep sea mineral nutrients are transported by the California Current and upwelled onto the continental shelf. This phenomena combined with springtime, wind-driven upwelling makes this region some of the richest waters on Earth.

What Is In a Name?

For humans, naming creates connections. Empathy for a seaweed or kelp starts with wonder, an attraction towards the beautiful and the strange. It is built upon by a lineage of learning across time: how it grows, what it tastes like, where to find it. In the western project of cataloguing the natural world it acquires a Latin name and place in the Linnean taxonomic system, so that everyone everywhere can talk about the same organism. There are many references to bull kelp across the globe but Nereocystis luetkeana refers specifically to the bull kelp growing along the eastern edge of the Pacific along the North American coastline. Its common name on this coast references its likeness to a bull whip. But there are other deep and place-specific relationships to seaweeds reflected in names from local language speakers. These names signify a plethora of relationships to the intertidal realm, relationships that value this world of richness and wonder. 

Lithograph of juvenile bull kelp

Juvenile bull kelp, lithograph from Illustrationes Algarum, 1840

By Alexander Postels

At the very north of California, the Tolowa Dee-ni' speakers call the bull kelp ghvtlh-k'vsh. Farther south, the Kashia Band of the Pomo know this same majestic kelp as cʰanamá. In Kodiak, Alaska this same foundational organism is nasquluq in the Alutiiq language; near Coos Bay, Oregon, Hanis speakers refer to it as qálaqas; and 19th-century Russian sailors called it sea otter cabbage. This multiplicity of names and descriptors highlight bull kelp as intrinsically of its ocean world, a universe we humans try to interpret with language and words.

Nereocystis luetkeana was named in Latin by Franz Josef Ruprechtand and Alexander Postels in a spectacular oversized folio published in 1840. Nereocystis translates to “mermaid’s bladder,” and luetkeana honors Captain Feodor P. Lütke, the leader of the 1829 Russian exploratory expedition to Alaska during which bull kelp was collected in Sitka Sound and brought back to Russia to be described and named. No record of this “type specimen" exists today, but the print folio of the seaweeds collected in Alaska that accompany the detailed description of the bull kelp organism are breathtaking, rendered with passion and commitment to their likeness by the young naturalist named Alexander Postels. Bull kelp is rendered as a majestic mature bull kelp encircling itself, replete with tiny holdfast clinging to a rock, and as a playful, jazzy, juvenile duo, barely contained within the confines of the page. 

Collecting and naming the type specimen of such an iconic species as Nereocystis is quite a claim to fame, but in fact, there was an earlier description of bull kelp made by Captain William Clark and Meriwether Lewis. They encountered an odd “fuci” at the western most point of their famous journey. Found when inspecting their salt works on the Oregon Coast in the winter of 1806, they describe what is now known to be a bull kelp with some feather boa kelp attached to it. [Lewis and Clark journals, March 17, 1806]