Kelp and Kelp Forests

Kelp and kelp forests

Brown algae, also known as kelps, are a group of seaweeds in the order Phaeophyta. They attach to rocks on the sea bottom by a tissue known as their holdfast, from which their flexible stems (known as a stipe) and leaf-like tissue (or fronds) grow into the water column. In some species, the fronds are kept buoyant by gas-filled bladders. Kelp tissues are extremely tough only the strongest storms are capable of tearing their fronds or ripping their holdfasts from the rocky bottom. When this happens, however, large masses of kelp biomass can float around as debris known as "paddies," or wash onto the shore as "wrack."

In some temperate marine habitats, large species of brown algae can be extremely abundant. These ecosystems are known as kelp forests. Because they are extremely productive ecosystems, and have a great deal of physical structure associated with their seaweed biomass, kelp forests provide habitat for a wide range of marine organisms. These include a diversity of species of smaller algae, invertebrates, fish, marine mammals, and birds. The kelp forests of the Pacific coast of North America are estimated to support more than 1,000 species of marine plants and animals.

Kelp forests occur in many parts of the world, including the Atlantic and Pacific coasts of North America. However, the tallest, best-developed kelp forests are in waters 20–210 ft (6–70 m) deep over rocky bottoms off the coast of California. This ecosystem is dominated by the giant kelp (Macrocystis pyrifera ), which ranges from central California to Baja California (the genus also occurs on the west coast of South America, and off South Africa, southern Australia, and New Zealand). This enormous seaweed is also known as the giant bladder kelp because of the flotation structures attached to its fronds. The giant kelp begins its life as a microscopic spore, but can grow as immensely long as 200 ft (60 m) and live for 4–7 years. Most of its photosynthetic activity occurs in the upper part of its tall canopy, because the lower areas are intensely shaded and do not receive much sunlight.

Other, somewhat smaller species of Macrocystis occur more widely along the Pacific coast, as far north as southern Alaska. Other giant seaweeds of kelp forests of the Pacific coast include the bull kelp (Nereocystis leutkeana ), the elk horn kelp (Pelagophycus porra ), the feather boa kelp (Egregia menziesii ), and the Fucalean alga (Cystoseira osmundacea ).

Sea urchins are marine invertebrates that feed voraciously on kelp biomass (they are herbivores meaning that plants are their primary source of food). Periodically, sea urchins of the genus Strongylocentrotus can become extremely abundant and cause an intense disturbance to the kelp-forest ecosystem. They do this by feeding on the hold-fasts and causing the kelp to detach from their rocky anchors, resulting in an ecosystem known as an "urchin barren" because it sustains so little biomass of seaweeds or other species. This sort of natural ecological damage has been observed numerous times, in various parts of the world.

Off the coast of western North America, however, sea otters (Enhydra lutris ) feed on the urchins and can prevent them from becoming too abundant, thereby keeping the kelp forests intact. This ecological balance among sea urchins, sea otters, and kelps became upset during the nineteenth century, when the populations of the otters were virtually wiped out by excessive hunting for the fur trade. Because of the collapse of otter populations, the urchins became more abundant. Their excessive feeding on kelps greatly reduced the extent and luxuriance of the kelp forests. Fortunately, this balance has since been restored by the cessation of the hunting of sea otters, allowing them to again control the abundance of the urchins. In turn, the productive kelp forests have been able to redevelop.

Seaweed biomass contains a number of useful chemicals, such as alginates used as thickeners and gelling agents in a wide variety of manufactured products. A minor use is as a food supplement. In some regions, kelps are being harvested as an economic resource to supply these industrial chemicals. Off the coast of California, for example, kelp harvesting amounts to as much as 176,000 tons (160,000 metric tons) per year. If the harvesting method takes care to not damage the holdfasts and other deeper tissues of the kelps, then the forest can regenerate quite well from the disturbance. In California, for example, kelp harvesters are only allowed to cut in the top 4 ft (1.4 m) of the water column, leaving the deeper parts of the forest intact. The kelp harvesting is done using a large barge-like apparatus, which can collect as much as 605 tons (550 metric tons) of kelp per day.

Kelp forests also have an extremely large indirect value to the economy, by serving as the critical habitat for many species of fish and shellfish that are harvested in the coastal fishery. The forests are also critical habitat for many species of indigenous biodiversity. This has an indirect benefit to the coastal economy, through recreational activities associated with ecotourism.