Has any well-intentioned person ever told you that giant kelp, Macrocystis pyrifera, is the world’s fastest growing plant? Unfortunately, if they have, you have been misinformed. 

So, does this mean that there is another plant that grows faster than giant kelp? Or was giant kelp never the fastest growing plant? Maybe your informant meant to say that giant kelp is the world’s fastest growing marine plant. Still not true.
Confused? Many are. Here’s the skinny: Not too many years ago your informant would have been spot on. But not today. That’s because giant kelp is no longer considered to be a plant. Instead, giant kelp is a type of brown algae.
For many years taxonomists separated living organisms into two kingdoms; one of plants and the other of animals. But today insight provided by DNA testing has provided far more accurate classification of living organisms, although the system can appear to be much more layered and complex to lay people.
For the purposes of this discussion I simply want to make the point that giant kelp is a type of brown algae, seaweed described in the kingdom Chromalveolata. That kingdom was not recognized until a few decades ago. Somehow it doesn’t sound quite as sexy to think of giant kelp as the world’s fastest growing seaweed, or algae, but these days that is exactly what it is.
The Big Picture
Although a variety of kelp species such as bull, palm and ribbon kelps thrive in many kelp forests, the mainstay of kelp forests in southern and central California kelp forests (aka kelp beds) is giant kelp. These undersea forests occur from Santa Cruz, California in the north southward to Turtle Bay, Mexico, roughly halfway down the Pacific side of the Baja peninsula. 
Giant kelp thrives in nutrient-rich areas where the substrate is rocky reef, water temperature is between 42 and 70 degrees F, the water is relatively clear, there is ample sunlight, and sufficient current and water exchange. The healthiest and densest kelp forests are usually found near upwellings. 
Giant kelp grows from a maximum depth of approximately 130 feet. But individual fronds attain lengths close to 200 feet as the fronds grow upward from the sea floor and then spread out on and just under the surface forming the surface canopy.
A Cathedral In The Sea
There is no question among those who have explored a giant kelp forest on a sunny day when water conditions were favorable that these forests rank high on the list of the world’s most alluring dive sites. On those days a kelp forest is a magical place where shimmering rays of sunlight dance among towering golden fronds that are surrounded by an inviting sea. As you descend to only a few feet below the surface canopy the forest takes on the feel of a natural cathedral. The golden canopy and sections of blue sky form the ceiling while the fronds and water column bear resemblance the open space of a main chamber. The colors of the rocky reef and eel and surf grasses below add to a stunning scene. 
As you orient yourself to your surroundings, you become aware that the forest is rhythmically swaying back and forth keeping time with the waves and swell. Magical indeed!
Kelp Forest Ecosystem
More than 800 species of animals have been documented to inhabit a healthy kelp bed. Perhaps even more astonishing, a single kelp frond can support north of one million organisms. However, many of these creatures are too small to see with the naked eye. 
On a typical dive in a giant kelp forest divers are routinely greeted by bright orange garibaldi, a damselfish that has been designated as our state’s marine fish, and schools of silvery jack mackerel and salema making their way through the forest maze. In some kelp forests divers are routinely treated to the sights of curious California sea lions and harbor seals that move in to check out their human counterparts. Migratory fishes such as barracuda and yellowtail might also reveal themselves. So might a black sea bass, a once threatened species that has made a very encouraging comeback in recent years.
The rocky reefs on the forest floor are alive with a variety of rockfish, cabezon, sculpin, painted greenlings and gobies galore. Those reefs also provide living quarters for moray eels, California spiny lobster, the colorful corallimorphs commonly referred to as corynactus anemones, a variety of sea stars, and brightly colored sea fans. And on some kelp forest reefs where the prevailing current is sufficient you might spot stands of the brightly colored, slow growing hydrocoral known as purple coral.
All of these creatures are common sights in forests of giant kelp. And you just never know when you might catch a glimpse of a massive California gray whale or a soupfin shark silently passing through.
As the mainstay species, giant kelp provides places of attachment, food, and shelter for a wide variety of marine creatures. Gastropod mollusks known as Norris’ top snail can often be found slowly making their way along a kelp stipe while lion’s main nudibranchs perch atop the blades. Several species of anemones also use kelp as a place of attachment.
Encrusting colonies of hydrozoans and bryozoans grow in such dense concentrations on the blades that the blades sometimes appear white and furry. As a result of the collective weight of those tiny invertebrates the kelp fronds need to continuously shed their blades to prevent being dragged to the bottom. The constant sloughing feeds a detrital community, a group that includes sea cucumbers, sea stars, sea urchins, lobsters and more. 
Superbly camouflaged giant kelpfish, opaleye and surfperch often seek refuge by hiding amongst the fronds. And senorita fish can often be seen picking tiny invertebrates off the blades when they are not providing their cleaning services to blacksmith fish, black sea bass and other kelp forest inhabitants. 
Grazers such as snails, abalone, and numerous crustaceans along with a variety of fishes feed upon the kelp. These animals serve as links from the kelp to higher trophic levels in the food web. With creatures such as sea urchins being both detritivores and grazers, the animals in a kelp forest ecosystem are all part of a very large, complex food web rather than separated food chains.
Life Cycle
A single frond can live as long as seven years given favorable water and growing conditions, and can increase its length by as much as two feet per day in ideal conditions. 
Giant kelp reproduces through a cycle known as alternation of generations. The complete cycle requires two generations; a sexually reproducing generation and an asexually reproducing generation. The fronds that create the kelp forests we enjoy are the asexual form known as sporophytes. 
Sporophytes release one-celled spores that develop into male and female gametophytes, the microscopic, sexually reproducing form that we don’t see. When a healthy gametophyte egg is fertilized, the resulting organism has a chance to develop into a mature sporophyte, thus completing the reproductive cycle. 
If a juvenile sporophyte is able to successfully attach to the bottom, it will soon develop into a one-bladed frond. When it grows three to four inches high the blade begins to split in two. Before this split is complete, the two new blades also begin to divide. As the process continues the fronds develop and gas bladders appear at the base of the blades. At the tip of each frond is an apical blade. This blade continues to produce new blades as the frond grows upwards. 
When Kelp Attacks
On many late-night television shows giant kelp fronds that appear to reach out and grab divers, snorkelers and swimmers. This makes both bad television and bad science.
However, it is possible to get entangled in kelp. Entanglements are generally quite easy to avoid. First of all, you want to avoid swimming or crawling over the surface canopy whenever you can. That is an experience that wise divers only do once in a lifetime as crawling over a thick canopy can be a lot of work while simultaneously providing your dive buddies with fodder to abuse you with for years. 
When entering a kelp forest from a boat it is best to enter from the edge of the forest, or through one of the many large openings in the surface canopy with hoses and gauges neatly tucked in by your sides. And when ending your dives you want to be sure you have enough air to swim under the canopy to your exit point. 
It is worthwhile to note that kelp is highly elastic. It has to be to survive daily wave action and the thrashing of winter storms. The elasticity also means that most people cannot break a stipe by stretching it. So, if you do get ensnared and find yourself dragging a long strand of kelp, the best strategy is to stop, achieve neutral buoyancy, and without spinning around and around see if you can simply lift the kelp away from you. If you can’t do that, it is very easy to snap a stipe in two by bending it back and forth a few times. 
The Anatomy of a Kelp Frond
A single kelp frond is comprised of a root-like holdfast, stipe, gas bladders and blades. A holdfast consists of a number of spaghetti-like looking strands called haptera that grip onto the rocky substrate, thus anchoring the frond. 
The stipe is the stalk, or stem-like, structure to which the holdfast, gas bladders and blades attach. The gas-bladders, or pneumatocysts, are bulbs of gas-tight structures that buoy the frond so it can receive the maximum amount of available sunlight for photosynthesis.
The blades are the numerous leaf-like appendages. Unlike the leaves of terrestrial plants the blades have no top and bottom sides. Having no top or bottom side makes it possible for the blades to conduct photosynthesis no matter how they are flipped about by waves and wind. In fact, the entire frond conducts photosynthesis, and all parts of kelp plants absorb nutrients.