The benthic environment includes the ocean floor,
and the benthos are those organisms that dwell on or near
the seafloor. Bottom-dwelling benthos organisms include
large plants that grow in shallow water, as well as animals
that dwell on the seafloor at all depths.
Many of the sediments on the deep seafloor are derived
from erosion of the continents and carried to the deep sea by
turbidity currents, carried by wind (e.g., volcanic ash), or
released from floating ice. Other sediments, known as deepsea
oozes, include pelagic sediments derived from marine
organic activity. When small organisms die, such as diatoms
in the ocean, their shells sink to the bottom and over time can
make significant accumulations. Calcareous ooze occurs at
low to middle latitudes where warm water favors the growth
of carbonate-secreting organisms. Calcareous oozes are not
found in water that is more than 2.5–3 miles (4–5 km) deep
because this water is under such high pressure that it contains
a lot of dissolved CO2, which dissolves carbonate shells.
Siliceous ooze is produced by organisms that use silicon to
make their shell structure.
The benthic world is amazingly diverse yet parts of the
deep seafloor are less explored than the surface of the Moon.
Organisms that live in the benthic community generally use
one or more of three main strategies for living. Some attach
themselves to anchored surfaces and get food by filtering it
from the seawater. Other organisms move freely about on the
ocean bottom and get their food by predation. Still others
burrow or bury themselves in the ocean bottom sediments
and get nourishment by digesting and extracting nutrients
from the benthic sediments. All the benthic organisms must
compete for living space and food, with other factors including
light levels, temperature, salinity, and the nature of the
bottom controlling the distribution and diversity of some
organisms. Species diversification is related to the stability of
the benthic environment. Areas that experience large fluctuations
in temperature, salinity, and water agitation tend to
have low species diversification, but they may have large
numbers of a few different types of organisms. In contrast,
stable environments tend to show much greater diversity with
a larger number of species present.
There are a large number of different benthic environments.
Rocky shore environments in the intertidal zone have
a wide range of conditions from alternately wet and dry to
always submerged, with wave agitation and predation being
important factors. These rocky shore environments tend to
show a distinct zonation in benthos, with some organisms
inhabiting one narrow niche, and other organisms in others.
Barnacles and other organisms that can firmly attach themselves
to the bottom do well in wave-agitated environments,
whereas certain types of algae prefer areas from slightly
above the low tide line to about 33 feet (10 m) deep. The
area around the low tide mark tends to be inhabited by abundant
organisms, including snails, starfish, crabs, mussels, sea
anemones, urchins, and hydroids. Tide pools are highly variable
environments that host specialized plants and animals
including crustaceans, worms, starfish, snails, and seaweed.
The subtidal environment may host lobster, worms, mollusks,
and even octopus. Kelp, which are a brown benthic algae,
inhabit the subtidal zone in subtropical to subpolar waters.
Kelp can grow down to a depth of about 130 feet (40 m),
often forming thick underwater forests that may extend along
a coast for many kilometers.
Sandy and muddy bottom benthic environments often
form at the edges of deltas, sandy beaches, marshes, and estuaries.
Many of the world’s temperate to tropical coastlines
have salt marshes in the intertidal zone, and beds of sea
grasses growing just below the low-tide line. Surface dwelling
organisms in these environments are known as epifauna,
whereas organisms that bury themselves in the bottom sand
and mud are called infauna. Many of these organisms obtain
nourishment either by filtering seawater that they pump
though their digestive system, or by selecting edible particles
from the seafloor. Deposit-feeding bivalves such as clams
inhabit the area below the low tide mark, whereas other
deposit feeders may inhabit the intertidal zone. Other organisms
that inhabit these environments include shrimp, snails,
oysters, tube-building crustaceans, and hydroids.
Coral reefs are special benthic environments that require
warm water greater than 64.4°F (18°C) to survive. Colonial
animals secrete calcareous skeletons, placing new active layers
on top of the skeletons of dead organisms, and thus build
the reef structure. Encrusting red algae, as well as green and
red algae, produces the calcareous cement of the coral reefs.
The reef hosts a huge variety and number of other organisms,
some growing in symbiotic relationships with the reef
builders, others seeking shelter or food among the complex
reef. Nutrients are brought to the reef by upwelling waters
and currents. The currents release more nutrients produced
by the reef organisms. Some of the world’s most spectacular
coral reefs include the Great Barrier Reef off the northeast
coast of Australia, reefs along the Red Sea, Indonesia, and in
the Caribbean and South Florida.
Unique forms of life were recently discovered deep in the
ocean near hot vents located along the mid-ocean ridge system.
The organisms that live in these benthic environments are
unusual in that they get their energy from chemosynthesis of
sulfides exhaled by hot hydrothermal vents, and not from photosynthesis
and sunlight. The organisms that live around these
vents include tube worms, sulfate-reducing chemosynthetic
bacteria, crabs, giant clams, mussels, and fish. The tube worms
grow to enormous sizes, some being 10 feet in length and
0.8–1.2 inches wide (3 m long and 2–3 cm wide). Some of the
bacteria that live near these vents include the most heat-tolerant
(thermophyllic) organisms recognized on the planet, living
at temperatures of up to 235°F (113°C). They are thought to
be some of the most primitive organisms known, being both
chemosynthetic and thermophyllic, and may be related to
some of the oldest life-forms that inhabited the Earth.
The deep seafloor away from the mid-ocean ridges and
hot vents is also inhabited by many of the main groups of
animals that inhabit the shallower continental shelves. However,
the number of organisms on the deep seafloor is few,
and the size of the animals tends to be much smaller than
those found at shallower levels. Some deep-water benthos
similar to the hot-vent communities have recently been discovered
living near cold vents above accretionary prisms at
subduction zones, near hydrocarbon vents on continental
shelves, and around decaying whale carcasses.
benthos See BENTHIC.
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