Giant kelp, Macrocystis pyrifera, is the largest submarine plant in the world and is believed to be one of the most productive organisms on earth. The fast growth and high turnover of biomass in kelp forests results in very high productivity, rivaling that recorded for many tropical rain forests. A single individual kelp plant can reach lengths of more than 30 m, and consist of over 200 fronds that grow at rates approaching 50 cm in length per day. Kelp plants typically may live up to four to six years (some individuals have been known to live as long as seven years), whereas individual fronds live about 6 months.
Gas-filled bladders at the base of each blade of giant kelp float the fronds up to the surface where light is abundant. These floating fronds form a surface canopy that can be very dense, greatly reducing the light below the canopy. Each kelp plant is anchored to the bottom by a tough branching holdfast.
Giant kelp forests occur on shallow rock reefs along the cooler coasts of western North and South America, southern Africa, Australia and most of the subantarctic islands, including Tasmania and New Zealand. Populations of giant kelp even extend into the tropics where the cool waters of the Peruvian current extend northward along the western coast of South America. Giant kelp,Macrocystis pyrifera, is considered a good biological indicator of sea surface temperature change because the geographical limits of its distribution throughout the world are closely related to the maximum summer sea surface temperature. Kelp forests can also sometimes form and persist in sandy subtidal habitats in moderately protected waters, such as Goleta Bay, along the mainland coast of Santa Barbara.
The characteristic three-dimensional structure of giant kelp, coupled with its extremely high productivity, enable kelp forests to provide food and habitat for a rich diversity of algae, invertebrates, fishes, birds and marine mammals, many of which are ecologically and economically important.
Giant kelp itself is of great commerical value, with thousands of tons harvested annually in California and elsewhere in the world. Most of the kelp harvest is used for the extraction of algin, a hydrocolloid. Algin products are used as binders, stabilizers, emulsifiers, and molding materials in the pharmaceutical industry, in cosmetics and soaps, in dental and food technology and in a wide variety of food and industrial products. There is also a growing demand for kelp as feed for abalone in mariculture facilities.
Controls On Abundance
The shallow inshore edge of the forest is generally set by wave action, whereas the deeper offshore edge is typically limited by light. In the Santa Barbara region, terrestrial runoff and the resuspension of fine sediments greatly reduces water clarity and light penetration nearshore. Consequently, giant kelp forests along the mainland coast are largely restricted to depths of less than 20 m. Coarser sediments and lower runoff result in relatively clear waters surrounding the Channel Islands, and kelp forests there frequently extend down to 30 m in depth.
Seasonal and inter-annual variation in the distribution and size of giant kelp forests is substantial. In general, productivity and biomass of kelp in the Santa Barbara region are greatest in the the late spring and early summer and lowest in the winter.
The abundance, areal extent, and condition of giant kelp forests vary dramatically over time in the SBC-LTER. The factors considered most responsible for variation observed in the abundance of giant kelp forest in southern California are intrusions of warm nutrient-poor water, dislodgment during large storms, and excessive grazing by sea urchins. For example, low nutrients that occur seasonally and also are often associated with ENSO events reduce the growth and survival of kelp plants.
During storm and wave events, the surface canopy is removed and entire kelp plants and their holdfasts are dislodged from reefs and set adrift. It has been estimated that 39,000 to 348,000 drifting rafts of kelp can be present in the waters of the Southern California Bight at one time. The majority of these "rafts" are subsequently stranded on beaches. Very large amounts of kelp, including entire plants with holdfasts, accumulate on SBC beaches following storms.
The intense grazing of large numbers of sea urchins (Strongylocentrotus spp.) can rapidly reduce the abundance of giant kelp, transforming lush kelp forests into "sea urchin barrens". Urchin barrens may persist for years as remaining urchins graze on newly recruited kelp plants and prevent kelp from establishing on the reef.