Giant Kelp Forests

Macrocystis Pyrifera - Forests under the Sea
By Scott Gietler and Dr Bill Bushing

There is nothing quite like swimming underwater through a giant kelp forest. Schools of fish around you, sunlight beaming down, it is a majestic, religious experience. I consider myself lucky enough to have these beautiful underwater forests just a few miles from my house in California.

Giant Kelp forests - range

Giant kelp is found from central & southern California south to Bahia de Magdalena, Baja California and also in South America, New Zealand, Tasmania and the Sub-Antarctic islands. Its unusual distribution in both the northern and southern hemispheres may reflect oceanographic events occuring during the Ice Ages. It is a temperate water alga, preferring water 68 degrees or less, and is therefore not found in the subtropics and tropics where corals dominate.

giant kelp forest

Kelp Forest Underwater Photography Tips

  • The best kelp forests are found off the west coast of Baja, San Diego, Malibu, the Channel Islands, and Monterey. San Clemente, Catalina and Anacapa islands offer perhaps the best places for underwater photography, with the healthiest kelp forests and the best visibility.

  • Use as wide of a lens as possible

  • Early morning on a sunny day, during the fall or winter will offer the best conditions for kelp forest underwater photography.

  • Mid-range or macro lenses can be used for artistic and creative kelp closeups.

  • Kelp forest photos can have a large dynamic range, try to hide the sun behind kelp to reduce the contrast of the shot.

  • For your wide-angle underwater photography, find a foreground object,and use the kelp as your background. A sea fan, anemone, starfish, or a large fish all work well as a foreground object. Make sure to take portrait shots.

  • For wide-angle shots, use diagonal lines and leading line compositions in your images. For close-ups, find healthy pneumatocysts and try some s-curves or spirals

  • Swim slowly, control your breathing and head into the deeper part of the reef for a chance encounter with a black sea bass, or go shallow for harbor seals. Bat rays, schools of fish and sea lions can appear anywhere. Read more about navigating kelp safely.

Giant Kelp Forest Photos

sheephead and giant kelp at catalina

Sheepheads and Giant Kelp at Catalina Island

giant kelp forest at santa cruz island

Kelp at Santa cruz island. 


Giant kelp and a sea fan, catalina

Giant kelp and sea fans.

Giant Kelp Forests - a story by Dr Bill Bushing

The history of Kelp

Naturalist Charles Darwin (yes, the evolution guy) was impressed with the diversity of life he observed when he first saw kelp forests. Kelp serves as both food and habitat for at least 800 species of marine animals and 300 marine algae. A number of fish such as halfmoon and opaleye, as well as invertebrates like snails and sea urchins, feed directly on the kelp. Many others feed on drifting pieces of kelp or particles from decaying organic matter including kelp known as leptopel. Kelp itself produces its own food through photosynthesis, similar to land plants. Many fish live within the protection of the kelp forest while invertebrates such as bryozoa, hydroids and worms actually attach to the blades of this alga once they stop growing. Kelp may be attacked by diseases like black rot.

kelp forest on san clemente island

Giant kelp forest at San Clemente Island

Giant kelp has an interesting natural history involving two different types of plants. Most people are familiar only with the giant sporophyte plants forming the kelp forests, but there is a second generation involving tiny gametophyte plants found on the bottom. The giant sporophytes may produce as many as 500,000 spores an hour. Most settle to the bottom near the parent plants, but some may drift in the ocean currents for a brief time. The spores develop into the male and female gametophytes which produce sperm and eggs that fertilize and develop into the giant sporophytes. Of course only a small percentage of the spores and young gametophytes actually survive to reproduce or we'd be overrun by giant kelp!

There are several different structures on a mature sporophyte. These include a root-like holdfast which serves to attach the plant to the bottom substrate (usually rock). The holdfast is not a true root since it does not absorb water or nutrients from the soil as in land plants. The stipe is the vine- or stem-like strand that originates at the holdfast level and reaches towards the surface. Stipes are covered with leaf-like blades which are where nutrients are absorbed and photosynthesis largely takes place. At the point of attachment of the blade to the stipe is a floatation bulb known as a pneumatocyst which buoys the alga towards the surface where sunlight is abundant. Although some have mistakenly said these float bulbs contain mainly methane, there are actually a number of different gasses inside them. A single stipe with its blades and pneumatocysts is called a frond.

Under optimum conditions of water temperature, nutrients and light, giant kelp can grow as much as 24" a day and reach lengths of 200 feet. Growth starts near the base of the holdfast where the individual stipes form. Since these growth regions are at the base, they are not directly affected by kelp harvesting which is limited to the top four feet of the water column. Growth regions known as apical meristems can be seen at the end of each stipe where the individual blades are splitting to form new blades and pneumatocysts. Individual fronds survive for about six months but the kelp holdfast, and therefore the alga itself, may survive several years.

Giant kelp has not been harvested commercially off Catalina for many years. Historically kelp has been harvested by man for a variety of uses. Prior to our involvement in World War I, the United States conducted a major survey of the kelp resources on the West Coast. The reason was to find a replacement for the potash that came primarily from German mines prior to the Great War. Why? It is a primary ingredient of gunpowder. Some say that kelp won the war!

Beginning in the 1930's kelp was harvested for the sodium alginate in it. Alginate was an important gelling and colloidal agent. It has been used in a wide range of products including beer (you can see where my priorities are), toothpaste, ice cream, cake mixes and frostings, shampoos, health foods, lubricating oils and welding rods. Due in part to the ecological value of giant kelp, synthetic chemicals have been developed to reduce the need for alginate and kelp harvesting.

The structure of Kelp

Morphologically, kelp is divided into three structural elements: the leaf-like blades, the stem-like stipe and the root-like holdfast. The broad, flat blades are responsible for most of the photosynthesis. They are corrugated (known technically as rugosity) and have spines along the edges. It is believed these structures slow down water flow and allow higher rates of transfer for nutrients and gases. To keep the blades near the surface where sunlight is most intense, each one has a hollow float bulb known as a pneumatocyst at the base. Contrary to popular opinion, these are not filled with methane, but with a combination of several gases.

The blades are attached to the stipes which may number one to several hundred on an individual kelp. Together, a stipe with its blades and pneumatocysts is called a frond. Each frond lives about six months, and new ones are created at the top of the holdfast to replace the ones that die and slough off. They grow upward towards the surface by having the end blade divide and form many new blades. This region of growth is called the apical meristem. All blades have another growth region at their base, which has cells that divide in three directions causing each blade to grow longer and thicker as it ages.

sea lion in the kelp

Sea lion swimming in kelp, Anacapa Island, California

Kelp Reproducton

The obvious members of our kelp forests are the spore producing or sporophyte generation. Spores are created in structures known as sporangia on specialized blades known as sporophylls clustered just above the holdfast. The sporangia are often grouped together on the blade in a whitish patch known as a sorus. Spores contain one half the normal compliment of chromosomes. Fifty percent of them are "male" and fifty percent are "female." The large alga may continually release spores from spring to fall. Estimates of spore production by a single plant have been given in various units. One source stated that at maturity, millions of spores are released. Another stated as many as 500,000 spores are produced per hour. Still another reported a single mature sporophyte may have many sporophylls, each containing "billions" of spores... and it wasn't Carl Sagan.

The microscopic spores usually fall relatively close to the adult sporophyte. However their small size and slow rate of sinking may allow them to be dispersed short distances by ocean currents. Large numbers are produced because relatively few survive. They may be consumed by filter feeders as they drift downward, or by grazing animals living on the ocean floor.

The spores grow into male or female gametophytes, which represent the sexual generation in the kelp life cycle. These tiny microscopic algae become reproductively mature within two weeks... very precocious! They may also remain in a vegetative state for up to a year if environmental conditions are not right. For example, light levels may be too low until a storm removes some of their parent's canopy. Female gametophytes produce eggs which release a chemical pheromone known as lamoxirene. The male gametophytes release motile sperm that follow this pheromone's increasing chemical gradient to the egg.

The sperm are so small that the male and female gametophytes must be within 1mm, or 1/25th of an inch, of one another for successful fertilization. Now that's getting up close and personal! Giant kelp may cross fertilize with other species of Macrocystis and even other kelps like the elk kelp, producing fertile hybrids, but despite this they remain morphologically distinct species. The fertilized egg starts a new sporophyte generation. It grows on top of the female gametophyte. The sporophyte first becomes visible with a single, heart-shaped blade. Within a few weeks, at a size of 3-4" it splits into two blades. After 12 to 15 months it is a mature giant sporophyte, ready to start the cycle all over again.

The huge sporophytes we see are perennial, and may live six years or more. An individual frond, composed of a single stem-like stipe and its attached blades, usually has a maximum lifespan of six months. They are constantly being replaced by new ones growing up from the holdfast region. The older fronds break away in a process known as sloughing ("sluffing"). A young kelp forest has many individual sporophytes with only a single stipe growing very close together. Dense mature forests studied on the mainland usually have about one sporophyte every two square meters. However, studies at some locations on Catalina Island show densities about ten times that.

To summarize this complex reproductive strategy, sporophylls at the base of the mature sporophyte generation produce spores asexually. These mature into male and female gametophytes that create sperm and eggs. These gametes fertilize and becoming tiny sporophytes that grow into the 200 foot giants to start the process all over. The gametophytes and very young sporophytes are tiny organisms living near the ocean floor, while the mature sporophytes have most of their biomass in the upper water column near the surface. Thus they must live and grow under markedly different environmental conditions. I guess being an amphibious individual, I'm much the same... living half my life in the ocean and the other half on land!

Although the holdfast looks like a root on land plants, it does not serve to collect water and nutrients since the alga is surrounded by both in the water column. It's primary function is to attach the kelp securely to the bottom and keep it there in the face of storms, surge and wave action. It attaches by forming root-like structures known as hapterae (singular haptera). Initially purplish to red in color, these turn yellow or brown once they actually contact and adhere to the rocks. The holdfasts are perennial, and may live as long as six years.

Visit Dr. Bill Bushing's Web Site at

Further Reading

Navigating Kelp Safely

California Marine Life

Diving Catalina Island

Diving the Channel Islands

Black Sea Bass

Kelp Resources

Orange county kelp restoration

Giant Kelp Forests
Scott Gietler and Dr Bill Bushing
Macrocystis Pyrifera - Forests under the Sea


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