.png)
What the benthic zone?
The benthic zone is the ecological region at the
lowest level of a body of water such as an ocean or a lake, including the
sediment surface and some sub-surface layers. Organisms living in this zone are
called benthos,
e.g. the benthic invertebrate community, including crustaceans
and polychaetes.
The organisms generally live in close relationship with the substrate bottom
and many are permanently attached to the bottom. The superficial layer of the
soil lining the given body of water, the benthic boundary layer, is an integral part
of the benthic zone, as it greatly influences the biological activity which
takes place there. Examples of contact soil layers include sand bottoms, rocky
outcrops, coral,
and bay mud.
The benthic
region of the ocean begins at the shore line (intertidal
or eulittoral
zone) and extends downward along the surface of the continental
shelf out to sea. The continental shelf is a gently sloping benthic region
that extends away from the land mass. At the continental shelf edge, usually
about 200 meters deep, the gradient greatly increases and is known as the
continental slope. The continental slope drops down to the deep sea floor. The
deep-sea floor is called the abyssal
plain and is usually about 4,000 meters deep. The ocean floor is not all
flat but has submarine ridges and deep ocean
trenches known as the hadal zone.
For
comparison, the pelagic zone is the descriptive term for the
ecological region above the benthos, including the water-column up to the
surface. Depending on the water-body, the benthic zone may include areas which
are only a few inches below water, such as a stream or shallow pond; at the
other end of the spectrum, benthos of the deep ocean includes the bottom levels
of the oceanic abyssal zone.
For
information on animals that live in the deeper areas of the oceans see aphotic
zone. Generally, these include life forms that tolerate cool temperatures
and low oxygen
levels, but this depends on the depth of the water.
Benthos are
the organisms which live in the benthic zone, and are different from those
elsewhere in the water column. Many are adapted to live on the
substrate (bottom). In their habitats they can be considered as dominant
creatures, but they are often a source of prey for Carcharhinidae
such as the lemon shark. Many organisms adapted to deep-water
pressure cannot survive in the upper parts of the water column. The pressure
difference can be very significant (approximately one atmosphere for each 10 meters of water depth).
Because
light does not penetrate very deep into ocean-water, the energy source for the
benthic ecosystem is often organic matter from higher up in the water column
which drifts down to the depths. This dead and decaying
matter sustains the benthic food chain;
most organisms in the benthic zone are scavengers or
detritivores.
Some microorganisms
use chemosynthesis
to produce biomass.
Benthic
organisms can be divided into two categories based on whether they make their
home on the ocean floor or an inch or two into the ocean floor. Those living on
the surface of the ocean floor are known as epifauna. Those
who live burrowed into the ocean floor are known as infauna.
Extremophiles like the piezophile which can survive in high pressures may also
live there.
Sources of food for benthic communities can derive from the
water column above these habitats in the form of aggregations of detritus,
inorganic matter, and living organisms. These aggregations are commonly
referred to as marine snow, and are important for the deposition of
organic matter, and bacterial communities. The amount of material sinking to
the ocean floor can average 307,000 aggregates per m2 per day. This
amount will vary on the depth of the benthos, and the degree of benthic-pelagic
coupling. The benthos in a shallow region will have more available food than
the benthos in the deep sea. Because of their reliance on it, microbes may
become spatially dependent on detritus in the benthic zone. The microbes found
in the benthic zone, specifically dinoflagellates
and foraminifera,
colonize quite rapidly on detritus matter while forming a symbiotic
relationship with each other.
In oceanic environments,
benthic habitats
can be further zoned by depth. From the shallowest to the deepest are: the epipelagic
(less than 200 meters), the mesopelagic (200–1,000 metres), the bathyal
(1,000–4,000 meters), the abyssal (4,000–6,000 meters) and the deepest, the hadal (below 6,000
meters).
The lower
zones are in deep, pressurized areas of the ocean. Because of the high
pressures and seclusion neither tidal changes nor human impacts have had much
of an effect on these areas, and the habitats have not changed much over the
years. Many benthic organisms have retained their historic evolutionary
characteristics. Some organisms are significantly larger than their relatives
living in shallower zones, largely because of higher oxygen concentration in
deep water.
It is not
easy to map or observe these organisms and their habitats, and most observation
has been done through remote controlled submarines.
Benthic macroinvertebrates
have many important ecological functions, such as regulating the flow of
materials and energy in river ecosystems through their food web
linkages. Because of this correlation between flow of energy and nutrients,
benthic macroinvertebrates have the ability to influence food resources on fish
and other organisms in aquatic ecosystems. For example, the addition of
a moderate amount of nutrients to a river over the course of several years
resulted in increases in invertebrate richness, abundance, and biomass. These in turn resulted in increased food
resources for native species of fish with insignificant alteration of the macroinvertebrate
community structure and trophic pathways. In addition, because benthic
zones are influenced by the flow of dead organic
material, there have been studies conducted on the relationship between
stream and river water flows and the resulting effects on the benthic zone. Low
flow events show a restriction in nutrient transport from benthic substrates to food webs, and caused a
decrease in benthic macroinvertebrate biomass, which lead to the disappearance
of food sources into the substrate.
Because the
benthic system regulates energy in aquatic ecosystems, studies have been made
of the mechanisms of the benthic zone in order to better understand the
ecosystem. Benthic diatoms
have been used by the European Union’s Water Framework Directive (WFD) to
establish ecological quality ratios that determined the ecological status of
lakes in the UK. Beginning research is being made on benthic assemblages to see
if they can be used as indicators of healthy aquatic ecosystems. Benthic
assemblages in urbanized coastal regions are not functionally equivalent to
benthic assemblages in untouched regions.
Ecologists
are attempting to understand the relationship between heterogeneity
and maintaining biodiversity in aquatic ecosystems. Benthic algae has been used
as an inherently good subject for studying short term changes and community
responses to heterogeneous conditions in streams. Understanding the potential
mechanisms involving benthic periphyton and the effects on heterogeneity within a stream
may provide a better understanding of the structure and function of stream
ecosystems. Benthic gross primary production (GPP) may be
important in maintaining biodiversity hotspots in littoral
zones in large lake ecosystems. However, the relative contributions
of benthic habitats within specific ecosystems are poorly explored and more
research is needed.
