Giant Kelp

Macrocystis pyrifera
Photo: Chad King
Giant kelp

Macrocystis pyrifera
Giant kelp is wide ranging–found from Alaska south to Mexico and also along the southern hemisphere coasts of Australia, South America, and South Africa.

Such a Beauty

“Kelp” is not really a scientific term but everyone uses it. For scientists, divers, beachcombers, and coastal wanderers the name refers to the big, fleshy marine algae that tend to be shades of golden or olive brown. Kelps are a subset of the class Phaeophyceae—the brown algae. Some kelps have gas-filled bladders, or pneumatocysts, to keep their bulky thallus afloat. Many of them tend to congregate beyond the surf zone in what has come to be called the kelp forest. Perhaps the most magnificent of these is Macrocystis pyrifera, or giant kelp.

A graphic array of bladders branch continuously off the main stipes of Macrocystis, and each bladder sports a luxuriously corrugated blade. This kelp is the main act of the Monterey Bay Aquarium’s central exhibition for good reason. Its exuberant growth and sheer beauty are breathtaking. It can reach lengths of one hundred feet or more in a single season. Scientists at the Scripps Institute of Oceanography in La Jolla have been studying the historic kelp forests just off the coast from the laboratories for many decades and now it is the Bay Foundation that leads the way with decades of restoration work in the giant kelp habitats of Southern California. 

Giant kelp cathedral
Photo by Abbey Dias

Macrocystis pyrifera and Nereocystis luetkeana, giant kelp and bull kelp, could be considered cousins. They are each a monospecific genus in the Laminaria family (Laminariaceae). They inhabit the same neighborhood of planet Earth, the eastern edge of the Pacific Ocean, but populate different blocks within that neighborhood, often growing next door to each other. Generally, bull kelp dominates the kelp forests north of the Golden Gate in California through Alaska and the Aleutians, and giant kelp dominates the warmer waters south of the Golden Gate all the way to Mexico, making historically massive kelp beds off of La Jolla and San Pedro near San Diego and Long Beach respectively. Macrocystis can also be found in the Southern Hemisphere, along the Chilean coast and around the southern tip of South America, South Africa and New Zealand, as well as throughout the Great Southern Reef of Australia. Generally it is thought that Macrocystis likes warmer, calmer waters, inhabiting bays and protected areas of the coast, while bull kelp likes the more dynamic zone and will be found out in the wave activated subtidal, in the rough waters of cold North Pacific. But these generalities are often turned on their heads and bull kelp is found in many inland waterways, and giant kelp is regularly found in areas thought to be exclusive bull kelp territory. 

The relationship of these two fast growing, massive and massively important kelps is an intertwined destiny. Around Big Sur and the Monterey Peninsula, Macrocystis and Nereocystis do an intertwined dance, their ranges intermingling. In Kodiak, Alaska, Macrocystis is emerging in hitherto bull kelp specific regions. Urchin barrens impact both Nereocystis and Macrocystis beds, but if regimes shift back from urchin to kelp dominated habitat (and might even include invasive species of kelp or seaweeds), there is no knowing what suite of algae will dominate; it is almost certain it will not be as it was before.

Ocean Delicacy

Like Nereocystis, giant kelp’s contributions to the health of our oceans cannot be overemphasized. Macrocystis pyrifera absorbs CO2 and creates oxygen, creates habitat, and provides protection against coastal erosion. Its blades serve as the platter upon which herring serve their roe. Each spring the Haida and other First Nations peoples of British Columbia wait with anticipation for the great schools of herring to lay their roe on the broad blades of the Macrocystis. These adorned kelp blades called kaaw (pronounced “gow”) are valued not only to eat fresh as a delicacy but to dry and trade.

Juvenile holdfast from the wrack: A tangle of haptera
Photo by Josie Iselin

Perennial vs. Annual

Macrocystis pyrifera is a perennial kelp, growing anew each spring from a holdfast that grows and persists through the winter. This is different from its annual cousin, bull kelp, that must grow anew, holdfast and all, each season. The giant kelp’s holdfast is a magnificent thing. As it ages, the newer, colorful haptera—they can be red, pink, even blueish—grow over the inner core, making a massive, cone-shaped edifice that becomes an underwater cathedral to hundreds of species that seek refuge in its catacombs. These complex structures are the base of a vertical wonderland of an ecosystem. Understory kelp such as Pterygophora grow between the giant kelps, and all of the algae oxygenate the surrounding waters—they are photosynthesizing machines that suck in CO2 to generate biomass and pump out oxygen at a rate unknown in our terrestrial world. Fish hide amongst the giant kelp’s fronds. Abalone and snails feed on kelp detritus that sloughs off, as do crabs and countless amphipods, isopods, and small crustaceans. Hydroids and bryozoan colonies take up residence on the giant kelp’s broad blades and stem, and fish nibble at these delicacies. Urchins are the most voracious kelp herbivore, able to attack the forest in hordes but also often just one of the thousands of organisms that thrive in the complex system that is the amber forest.

Baby giant kelp sporophyte

Baby giant kelp sporophyte emerging from the female gametophyte. 

Photo by Makena Rush

The giant kelp plant can live six years or more, but the reproductive cycle is similar in some ways to the annual Nereocystis, except the specialized reproductive blades are just above the holdfast, at the base of the rising, amber giant. These blades are smaller and thinner than their upper sisters, and release trillions of microscopic spores into the surrounding ocean waters. The photosynthetic spores waft slowly in the water column to disperse and settle on the ocean floor and, with enough light, develop into male or female gametophytes, microscopic organisms that produce either egg or sperm. Like the Nereocystis, a sperm follows a pheromone trail to find an egg, but we must remember scale here. Not only are these organisms tiny (microns big), but the female’s perfume will be instantly diluted by the ocean surrounding her. The male and female microorganisms must be within a half millimeter away from each other for the pheromone attractor to work. Once fertilized, the egg can wait until the opportune time, perhaps up to eighteen months, before it begins to divide, with half growing downward into the holdfast and half growing upward into a tiny, single-bladed sporophyte or baby kelp. This blade will split in two and split again, eventually making the various strands of the giant kelp that reach for the surface and develop into the giant Macrocystis. All of these intricate steps have evolved to occur in concert with the ebb and flow of tide and current, a true miracle.

Giant kelp sporophyll blades

Specialized sporophyll blades grow at the base of the giant kelp, producing the spores that will disperse and germinate into the alternate, microscopic sexual phase of the giant kelp life cycle. Giant kelp is a perennial, however, persisting from year to year while bull kelp is an annual.

By Josie Iselin

Two secrets of the giant kelp’s speedy development—up to two feet a day—is that it can grow from any number of points, and it can transfer energy from the top down. The very tip of the massive Macrocystis frond is called the apical scimitar, the flat, initially singular, curved blade that splits into individual fronds. This is where new growth occurs, extending horizontally once the ocean surface has been reached. The photosynthesizing blades at the ocean’s surface distribute their products down the stipe allowing new fronds to grow farther down the plant. The natural rhythms of the ocean cause the top blades to slough off within months and the younger blades then rise to the surface for more direct photosynthetic action. The entire giant kelp is a primary production conveyor belt. These multiple growth points mean that the giant kelp can lose, or have lopped off, its topmost vegetation and regrow with abundance. That is, if the oceans are healthy.