Giant Kelp

Introduction

Giant kelp (Macrocystis pyrifera) is a species of marine alga found along the Pacific coast of North America from central California to Baja California. Although it begins life as a microscopic spore at the ocean floor, this species may grow to lengths of 60 m (200 ft) with its upper fronds forming a dense canopy at the surface. Giant kelp prefers depths less than 40 m (120 ft), temperatures less than 20 C (72 F), hard substrate such as rocky bottoms, and bottom light intensities above 1% that of the surface.

The genus name Macrocystis means "large bladder" and it contains at least two recognized species. Macrocystis pyrifera, or giant bladder kelp, is sometimes referred to as the sequoia of the sea. Macrocystis integrifolia is the small perennial kelp.

Geographic Distribution

In the Northern hemisphere it occurs only along the Pacific coasts of Canada, the United States and Baja California. Populations of Macrocystis in the North Pacific extend from Alaska to localities of cool, upwelled water in Baja California. The kelp beds along the Pacific coast are the most extensive and elaborate submarine forests in the world. The genus is best developed as the species Macrocystis pyrifera from the southern California Channel Islands to northwestern Baja California.



Kelp Forests and Associated Species

There are five species of large brown kelps that may form canopies: Macrocystis or giant bladder kelp, Nereocystis or bull kelp, Pelagophycus or elk horn kelp, Egregia or feather boa kelp and Cystoseira which is a form of sargassum. South of Point Conception Egregia dominates the inshore waters and Macrocystis the intermediate depths with Pelagophycus offshore south of Point La Jolla.

The term kelp forest refers to the dense growths of Nereocystis, Macrocystis and Pelagophycus while the term kelp "beds" is used to refer to the smaller laminariales or brown algae which have limited vertical structure and canopies. A giant kelp forest may vary from several hundred feet to one mile wide and several miles long.

Macrocystis plays an important role in the marine environment by providing food and habitat for a wide range of marine invertebrates and fishes in southern California. Forests of giant kelp may support millions of individual organisms and more than 1,000 species of marine plants and animals.

The presence or absence of Macrocystis is not essential for the spawning of any sport fish species. However, kelp beds do provide shelter for the larvae and juveniles of several species such as the kelp topsmelt. The abundance and diversity of life associated with the structurally complex and high productive Macrocystis kelp populations are obvious. The giant kelp holdfast alone may contain over 150 species. Another report states 178 species were found living in the kelp holdfasts. 114 species of invertebrates were associated with the Macrocystis fronds in one study.

Factors Affecting Kelp

Macrocystis pyrifera usually does not occur shallower than about 5 meters or deeper than about 20 meters. From the holdfast to the tip of the longest frond these plants may reach lengths of 200 feet with 100 feet growing from the holdfast to the surface and another 100 feet stretched out in the canopy. Macrocystis is a perennial kelp with the holdfast surviving 4 to 10 years and the individual fronds 6 to 12 months.


Giant kelp prefers areas with ocean temperatures above 5 degrees C (the lethal temperature for the gametophytes) and below 20 degrees C. The upper temperature limit may actually be a result of decreased nutrients, especially nitrogen, noted in warmer waters.

Nutrient levels are low in the summer and fall in southern California, especially above the thermocline and during periods when warm water masses move into the region from the south. The reduced summer growth of Macrocystis may be due to nitrate depletion. In southern California the giant kelp canopies commonly deteriorate during the summer when inorganic nitrogen is low.

Macrocystis and the other large algae are inhabited by a diverse range of pathogenic parasites, bacteria and fungi. Kelp plants may be found with tumors and galls, although few of these have been observed to cause severe damage to individual kelp plants. High water temperatures are often associated with a condition known as "black rot," which can result in considerable kelp mortality.

Kelp Growth and Life Cycle

Kelp begins life as a microscopic