Giant Perennial Kelp might be the fastest growing organism in the world. In California it can grow 35 cm (14 in) per day. Unlike species of Laminaria, Giant Perennial Kelp can store only a two week supply of nitrogen, so when environmental supplies run low, the kelp stops growing. Family Description: This family [Lessoniaceae] contains the giant kelps, and many members of the family have pneumatocysts (gas-filled floats). This family contains Lessonia, Macrocystis, Nereocystis, Pelagophycus, Postelsia and possibly Ecklonia, Egregia and Eisenia. In most members of this family, the intercalary meristem at the junction of the stipe and blade is split so that the stipe itself is branched, and there are numerous terminal blades. Midribs are lacking. Sori usually develop on blades or on sporophylls that develop from the blade side of the meristem rather than the stipe part as in the Alariaceae. Source: North Pacific Seaweeds (O'Clair and Lindstrom)

Species description:
This species is distinguished by its holdfast, which is tall and pyriform (pear- or pyramid-shaped) rather than flattened and creeping as in Small Perennial Kelp. In other respects it is very similar to M. integrifolia, and, since they can hybridize in nature, some specialists lump the two species together and call them both M. pyrifera. Most researchers, however, continue to recognize them as distinct species based on holdfast morphology.

A large amount of work has been done on this species because of its ecological and economic importance, and we summarize this briefly here.

In Southern California, the lowered nitrate levels accompanying the 1982-84 El Niño depressed the growth rates of Giant Perennial Kelp. When scientists experimentally exposed individuals to nitrate/phosphate fertilizer, however, growth rates increased to normal values. In normal (non-El Niño) years, the growth of Giant Perennial Kelp is limited by light rather than by nutrients. Considerable genetic differences can produce physiological differences between individuals from different geographic areas. For example, individuals from Santa Catalina Island in Southern California achieved maximum growth rates at nitrogen concentrations typical of Santa Catalina habitats, which were lower than those found at Santa Barbara and Monterey Bay. Such geographic differences are likely widespread in algae.

Other studies in Southern California have shown that blades have the highest photosynthetic rates (especially blades that float near the surface and blade tips, where the tissue is more photosynthetic and less structural), with sporophylls showing only half those rates and stipes much less. No photosynthesis was detectable in holdfast tissue. Within blades, organic molecules can be transported at rates of up to 22 cm (8.7 in) per hour. The main carbohydrates are mannitol and alginic acid.

The marginal spines and textured surfaces of Giant Perennial Kelp blades increase turbulence as water flows over them and facilitate the uptake of carbon dioxide and other molecules from the water.

Giant Perennial Kelp dies back to its holdfast each winter. In southern California, three quarters of the tagged individuals were lost between October and March, probably because they were torn off the substratum by winter storms. Individuals can live up to 7 or 8 years, but most individuals survive only 4 or 5 years. The holdfasts of older inviduals die in the center due to shading or to the accumulation of sediment. Red (Strongylocentrotus franciscanus) and, especially, Purple Sea Urchins (S. purpuratus) chew on the holdfast haptera, causing damage that also makes it easier for the alga to be torn off the rocks by storm waves.

Because it lacks tannins, Giant Perennial Kelp is more palatable than some other species of brown algae. The Bat Star (Asterina miniata) grazes young sporophytes and gametophytes in California, but once the blades are 1 to 3 cm (around 1 in) in height they are too large to be grazed by this sea star. The Northern Kelp Crab (Pugettia producta), and Red and Purple Sea Urchins all prefer to eat the Giant Perennial Kelp over species of Laminaria in California. The Concave Isopod (Pentidotea resecata) also eats Giant Perennial Kelp.

Many species of fishes depend on forests of Giant Perennial Kelp as nurseries for young stages and as habitat for adults. One study in Southern California showed that about one hundred species of fish are dependent on forests of Giant Perennial Kelp. Juvenile rockfish that hide in Giant Perennial Kelp forests prey on the larvae of nearby rocky shore invertebrates, giving this kelp an indirect role in structuring these nearby communities and tying intertidal community dynamics to offshore kelp forest health.

In Southern California, severe winter storms in 1980 removed all Giant Perennial Kelps but spared the understory species. With their preferred kelp gone, urchins then consumed understory species, disrupting the normal food chains. Then, in 1983, the urchins were eliminated by more severe winter storms and by the summer of 1984 the kelp forests had re-established (kelps 2 to 3 years old showed best survival in these storms). The effects of the storms therefore depended on the type of community present before the storms. When Giant Perennial Kelp is removed by storms, sometimes the opportunist Bull Kelp (Nereocystis luetkeana) becomes established instead

In areas where wave exposure is extreme in California, beds of Giant Perennial Kelp can be found further offshore in deeper water, where they are protected from the breaking surf. During times when wave intensity is low for extended periods, however, this kelp can quickly become established in shallower waters, only to be removed again during the next severe storm. Lack of disturbance therefore favors the establishment of Giant Perennial Kelp.

In central California, rafts of torn blades and stipes drift with the prevailing winds and are blown onshore in about a week or less. The pneumatocysts lose their buoyancy in about a week at sea, so longer range transport is unlikely. However, after being deposited on the beach, much of this kelp can be picked up on the next high tide and carried offshore where it sinks and thus impacts the food chain in benthic communities.

This species also occurs in New Zealand, and a study there showed that rafts of Giant Perennial Kelp are colonized by talitrid amphipods (beach hoppers) within one day of being washed ashore. The amphipod population peaked on the third day and then declined; meanwhile, nematodes (roundworms), enchytraeids (segmented worms related to earthworms), fly maggots and mites increased. After 18 days, only about half of the algal mass remained, but despite the fact that all of these animals undoubtedly fed on the decaying kelp, the researchers concluded that such processes as bacterial decay, leaching and fragmentation were the major causes of kelp breakdown.

Reproduction:
The motile spores of Giant Perennial Kelp swim actively towards sources of nitrogen (nitrate and low ammonium levels), glycine, aspartate, low iron levels, boron, cobalt and manganese—presumably all nutrients needed for growth.

On the Monterey Peninsula, California, recent studies showed that the establishment of new sporophytes was inhibited in the presence of adult canopy cover. Researchers thought that sporophyte recruitment in Giant Perennial Kelp is therefore inhibited by low light levels. At exposed sites, where the adult canopy is often damaged or lost during storms, recruitment can be episodic and lead to large populations of juveniles.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Macrocystis integrifolia Bory

Giant Perennial Kelp is utilized in the United States to make kelp tablets used as a source of vitamins and minerals. It is a major source of alginate, a polysaccharide added to emulsions like salad dressing to make them smooth and creamy. Source: North Pacific Seaweeds.