Bottom water
Encyclopedia
Bottom water is the lowermost water mass
in a water body, by its bottom, with distinct characteristics, in terms of physics
, chemistry
, and ecology
.
content, salinity
, bulk temperature
(characteristic temperature), and hence density
.
The Antarctic Bottom Water
is the source of most bottom water in southern parts of Pacific Ocean
, Indian Ocean
and Atlantic Ocean
, and even in parts of the North Atlantic. Bottom waters flow very slowly, driven mainly by slope topography and differences in temperature and salinity, especially compared to wind
-driven surface ocean current
s.
The bottom water of the Arctic Ocean
is more isolated, due to the topography of the Arctic ocean floor and the surrounding Arctic shelves.
of a river discharging into a saline body exhibits peculiar transport of mud
. Due to fresh/saline water intermixing by the estuary, a horizontal isohale gradient
is created, with lower salinity levels upstream, which generates the upstream flow of the bottom water. Mud particles carried by river begin settling down as the current and turbulence
decrease. When the particles nearly reach the floor, they are carried back to the head of estuary to accumulate at the point where the salinity of the surface and bottom waters become comparable and the bottom flow decreases. This process results is a distinguished pile of mud at this point.
Bottom water of lake
s may feature lower level of oxygen
, to the point of completely vanished dissolved oxygen (i.e., becoming anaerobic), and higher levels of chlorinity
and organic-induced acidity. In many lakes, especially in the zones of continental climate
, summer heating and winter cooling create strong vertical temperature gradients which oppose water intermixing, resulting in the periods of summer and winter thermal lake stratification
. They are intervened by bottom water overturning, which happens in autumn (autumn overturn) and in spring (spring overturn) due to equalizing of temperature gradients and the resulting easier intermixing by wind and other sources of turbulence.
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
in a water body, by its bottom, with distinct characteristics, in terms of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, and ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
.
Oceanology
In oceanology, bottom water is by the ocean floor. It has characteristics are markedly distinct from the above layer in terms of oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
content, salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
, bulk temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
(characteristic temperature), and hence density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
.
The Antarctic Bottom Water
Antarctic Bottom Water
The 'Antarctic Bottom Water' is a type of water mass in the seas surrounding Antarctica with temperatures ranging from 0 to -0.8◦ C, salinities from 34.6 to 34.7 psu, and a density near 27.88...
is the source of most bottom water in southern parts of Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
and Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, and even in parts of the North Atlantic. Bottom waters flow very slowly, driven mainly by slope topography and differences in temperature and salinity, especially compared to wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
-driven surface ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s.
The bottom water of the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...
is more isolated, due to the topography of the Arctic ocean floor and the surrounding Arctic shelves.
Estuaries
Bottom water by an estuaryEstuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
of a river discharging into a saline body exhibits peculiar transport of mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
. Due to fresh/saline water intermixing by the estuary, a horizontal isohale gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
is created, with lower salinity levels upstream, which generates the upstream flow of the bottom water. Mud particles carried by river begin settling down as the current and turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
decrease. When the particles nearly reach the floor, they are carried back to the head of estuary to accumulate at the point where the salinity of the surface and bottom waters become comparable and the bottom flow decreases. This process results is a distinguished pile of mud at this point.
Lake hydrography
Bottom water of lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s may feature lower level of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, to the point of completely vanished dissolved oxygen (i.e., becoming anaerobic), and higher levels of chlorinity
Chlorinity
The chlorinity of water is defined as the mass of chlorine equivalent to the total mass of halogen contained in 1 kg seawater. It is determined by the Mohr-Knudsen titration, which however neglects fluoride....
and organic-induced acidity. In many lakes, especially in the zones of continental climate
Continental climate
Continental climate is a climate characterized by important annual variation in temperature due to the lack of significant bodies of water nearby...
, summer heating and winter cooling create strong vertical temperature gradients which oppose water intermixing, resulting in the periods of summer and winter thermal lake stratification
Lake stratification
Lake stratification is the separation of lakes into three layers:# Epilimnion - top of the lake.# Metalimnion - middle layer that may change depth throughout the day.# Hypolimnion - the bottom layer....
. They are intervened by bottom water overturning, which happens in autumn (autumn overturn) and in spring (spring overturn) due to equalizing of temperature gradients and the resulting easier intermixing by wind and other sources of turbulence.