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KKelp and kelp forestsELP 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 associ-
ated with their seaweed biomass, kelp forests provide habitat
for a wide range of marine organisms. These include a diver-
sity 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 Macrocystisoccur
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 vora-
ciously 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 lux-
uriance 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 chemi-
cals, 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 thewomi_K 5/6/03 3:29 PM Page 323