Bull Kelp
Nereocystis luetkeana
Bull kelp is an algae that grows close together rooted to the seafloor. At the top of their long stem-like stipe is a large gas-filled bulb (pneumatocyst), which acts as a center for up to 100 leafy blades. Bull kelp uses photosynthesis to convert sunlight into usable energy.
This algae forms a canopy and provides shelter for many animals. Seals have even been known to anchor themselves to help to take a nap!
Bull kelp has seen a large decline in Northern California due to climate change and an increase in their predator, sea urchins. The Aquarium of the Pacific is one of many organizations helping to preserve genetic material for future restoration efforts of bull kelp.
Originally published: March 14, 2025
Last updated: October 13, 2025
Credit: Courtesy of Dr. Sophie McCoy
SPECIES IN DETAIL
Bull Kelp
Nereocystis luetkeana
CONSERVATION STATUS: Not evaluated
CLIMATE CHANGE:
Geographic Distribution
Bull kelp grows along the Pacific coast of North America, from San Luis Obispo and northward to the Aleutian Islands in Alaska.
Habitat
Bull kelp grows at low intertidal to subtidal zones along the coastlines of Northern CA, Oregon, Washington, British Columbia, and Alaska in temperatures ranging from 5 to 20 degrees Celsius. They form thick beds on subtidal rocks in areas of high current waters.
Physical Characteristics
Bull kelp attaches to rocks with a root-like holdfast. From that, its stipe grows upward like a stem until its blades reach the surface of the water. These long, narrow blades fan out around the pneumatocyst, or bulbous float, and enable photosynthesis (turning sunlight into a food source). Blade size and shape can be affected by the force of water current. Slower moving waters encourage ruffled-edged blades and persistent currents elongate the blades, causing them to be heavier and less ruffled. The blades’ optimal growth reaches up to 13.72 cm (5.40 in) per day.
Size
Their stipe can reach a maximum length of 35 m (115 ft). The blade length and weight depends on water current conditions.
Diet
Bull kelp use photosynthesis to gain energy. Long, wide, flat blades rest on or just below the ocean surface and absorb sunlight. The sunlight is converted into usable energy within each alga by converting carbon dioxide and water into sugar and oxygen.
Reproduction
These algae use spore reproduction. The kelp’s life cycle begins as mature spores fall to the seafloor from their blades. These spores develop two flagella that allow them, like a tail, to have some movement along the seafloor. They settle clustered together near their parental alga and produce egg and sperm. Fertilized eggs will grow into adult kelp and restart the life cycle.
Behavior
Bull kelp helps build kelp forest habitats, providing shelter, and opportunities for food and nursery areas to hundreds of marine species.Their forests capture up to twenty times more carbon than terrestrial forests. Bull kelp uptakes carbon dioxide during photosynthesis, which has been shown to locally mitigate the ocean acidification stress.
Rooted in place offshore, kelp also absorbs some wave action energy, protecting the coastline.
Adaptation
These algae grow from the seafloor helped by a specialized root-like structure called a holdfast. The holdfast attaches onto rocky areas and keeps the algae in place. A pneumatocyst, a single, gas-filled bulb, floats at the ocean’s surface and allows up to 100 golden-colored blades to form a canopy and absorb sunlight.
Longevity
They live one year to up to 18 months.
Conservation
Northern California has seen a 95% decline in bull kelp population from two critical factors: ocean waters warming due to climate change which interrupts their reproductive cycle, and a growing population of purple sea urchins that consumes them. The Aquarium of the Pacific is one of several conservation organizations preserving and storing genetic material to be used for future restoration efforts. This material could be used to grow new live bull kelp and then outplanted to the ocean to repopulate an area.
Special Notes
Bull kelp forests can capture up to 20 times more carbon dioxide than terrestrial forests.
Their blades can grow up to 13.72 cm (5.40 in) per day, and their stipes can grow up to 10 cm (3.93 in) per day.
They help locally mitigate ocean acidification stress in the waters surrounding them through photosynthesis and their ability to remove carbon dioxide.
These algae are sometimes called a mermaid’s bladder.
SPECIES IN DETAIL | Print full entry
Bull Kelp
Nereocystis luetkeana
CONSERVATION STATUS: Not evaluated
CLIMATE CHANGE:
Bull kelp grows along the Pacific coast of North America, from San Luis Obispo and northward to the Aleutian Islands in Alaska.
Bull kelp grows at low intertidal to subtidal zones along the coastlines of Northern CA, Oregon, Washington, British Columbia, and Alaska in temperatures ranging from 5 to 20 degrees Celsius. They form thick beds on subtidal rocks in areas of high current waters.
Bull kelp attaches to rocks with a root-like holdfast. From that, its stipe grows upward like a stem until its blades reach the surface of the water. These long, narrow blades fan out around the pneumatocyst, or bulbous float, and enable photosynthesis (turning sunlight into a food source). Blade size and shape can be affected by the force of water current. Slower moving waters encourage ruffled-edged blades and persistent currents elongate the blades, causing them to be heavier and less ruffled. The blades’ optimal growth reaches up to 13.72 cm (5.40 in) per day.
Their stipe can reach a maximum length of 35 m (115 ft). The blade length and weight depends on water current conditions.
Bull kelp use photosynthesis to gain energy. Long, wide, flat blades rest on or just below the ocean surface and absorb sunlight. The sunlight is converted into usable energy within each alga by converting carbon dioxide and water into sugar and oxygen.
These algae use spore reproduction. The kelp’s life cycle begins as mature spores fall to the seafloor from their blades. These spores develop two flagella that allow them, like a tail, to have some movement along the seafloor. They settle clustered together near their parental alga and produce egg and sperm. Fertilized eggs will grow into adult kelp and restart the life cycle.
Bull kelp helps build kelp forest habitats, providing shelter, and opportunities for food and nursery areas to hundreds of marine species.Their forests capture up to twenty times more carbon than terrestrial forests. Bull kelp uptakes carbon dioxide during photosynthesis, which has been shown to locally mitigate the ocean acidification stress.
Rooted in place offshore, kelp also absorbs some wave action energy, protecting the coastline.
These algae grow from the seafloor helped by a specialized root-like structure called a holdfast. The holdfast attaches onto rocky areas and keeps the algae in place. A pneumatocyst, a single, gas-filled bulb, floats at the ocean’s surface and allows up to 100 golden-colored blades to form a canopy and absorb sunlight.
They live one year to up to 18 months.
Northern California has seen a 95% decline in bull kelp population from two critical factors: ocean waters warming due to climate change which interrupts their reproductive cycle, and a growing population of purple sea urchins that consumes them. The Aquarium of the Pacific is one of several conservation organizations preserving and storing genetic material to be used for future restoration efforts. This material could be used to grow new live bull kelp and then outplanted to the ocean to repopulate an area.
Bull kelp forests can capture up to 20 times more carbon dioxide than terrestrial forests.
Their blades can grow up to 13.72 cm (5.40 in) per day, and their stipes can grow up to 10 cm (3.93 in) per day.
They help locally mitigate ocean acidification stress in the waters surrounding them through photosynthesis and their ability to remove carbon dioxide.
These algae are sometimes called a mermaid’s bladder.