Glossary
abyssopelagic [67]: the zone in the ocean ranging from about 4000 m down to 6000 m water depth.
acoustic Doppler techniques [28]: these techniques are used to make remote measurements of water currents. A pulse of sound transmitted into the water is scattered off suspended particles and the echo is received back at the transmitter. The time delay between transmission and echo gives the distance to the scattering material. The Doppler frequency shift between transmitted, and echo pulses, give the speed at which the suspended particles (and hence the water) are moving. By processing echoes from different distances, a profile of water current velocity can be constructed.
albedo [124]: the fraction of the incident shortwave radiation that is reflected from a surface. Snow and ice have high albedo and dark surfaces such as open ocean have a low albedo.
amphidromic system [44]: describes a wave created in a basin on a rotating Earth. The wave travels around the edge of the basin, in an anti-clockwise sense in the northern hemisphere, around a point in the centre called the amphidromic point. At the amphidromic point there is no vertical wave motion, but maximum current.
archaea [70]: one of the three major domains of life (the other two are the Bacteria and the Eukaryotes).
Archimedes’ Principle [11]: states that when an object is placed in water it experiences an upthrust (or upward force) equal to the weight of water it has displaced. A special case of Archimedes’ principle is that a floating body displaces its own weight of water.
aspect ratio [1]: the ratio of two dimensions of an object, for example the width and depth of an ocean.
azoic hypothesis [111]: this was the hypothesis proposed in the early to mid-1800s that nothing lived in the deep sea. This is clearly not true.
barycentre [00]: the centre of mass of an object, or series of objects. The barycentre of the Earth and Moon together is an important point in tidal studies. Both Earth and Moon revolve about their barycentre once a month.
bathypelagic [67]: the zone in the ocean ranging from about 1000 m down to 4000 m water depth.
benthic [67]: a term given to organisms living on the sea/ocean floor (the benthos).
buoyancy [11]: the upward force on an object placed in water. See Archimedes’ Principle.
caballing [14]: when two cold water masses mix, the density of the mixture can be greater than the density of the original water masses. This effect, called caballing, is produced by the non-linear dependency of the density of seawater on temperature and salinity and is thought to contribute to the formation of the densest and deepest water in the ocean.
chart datum [37]: the lowest level the tide will ever reach at a given location. Depths on UK Admiralty charts are expressed relative to chart datum.
ciliates [72]: a group of protozoans characterised by having numerous hair-like organelles (cilia) which they use for movement and collecting food particles.
compensation irradiance [95]: irradiance above which algae produce more oxygen by photosynthesis than they consume by respiration.
Coriolis effect [31]: this is the term given to the apparent deflection of moving objects on a rotating earth. Viewed from the turning earth, objects actually moving in a perfectly straight line when viewed from space appear to bend to the right in the northern hemisphere and to the left in the southern hemisphere.
cyanobacteria [71]: formerly called blue-green algae, the cyanobacteria are photosynthetic bacteria commonly found in marine systems.
cyclonic gyres [99]: regions of large water movement (anticlockwise in northern hemisphere and clockwise in southern hemisphere). They result in water being upwelled from below the thermocline into surface waters as the Coriolis effect produces slopes in the pycnocline.
decibar [16]: unit of pressure equal to one tenth of atmospheric pressure at sea level. Pressure in the ocean increases by approximately 1 decibar per metre increase in depth.
density [10]: the mass of a sample of seawater divided by the volume of the sample. Density is expressed in units of kg m–3. The density of seawater is typically a little over 1000 kg m–3. The extra density above 1000 kg m–3 is called sigma-t. For example, a seawater of density 1020 kg m–3. has a sigma-t value of 20 kg m–3.
density current [14]: a current that is produced by differences in density of seawater, with the denser water flowing underneath the less dense.
diapause [77]: a state of dormancy.
diffuse attenuation coefficient [52]: (symbol k in equation 7.2) controls the rate at which natural sunlight decays with depth in the sea. k has units of mm–1 At a depth equal to 1/k, solar irradiance has decreased to 1/e (about 37%) of its surface value.
dinoflagellates [71]: a group of unicellular organisms that have two flagella which they use to move. They can be ‘naked’ or ‘armoured’. The latter are covered in plate-like structures made of cellulose. Many dinoflagellate species are photosynthetic, but other dinoflagellates capture food, which they digest. Some species have both photosynthetic and non-photosynthetic modes of obtaining energy (mixotrophs).
diurnal tide [38]: a tide with just one high water per day.
entrainment [58]: in the context of turbulent layers in the sea, this is the process by which one turbulent layer can expand into another, less turbulent, layer. A good example is the deepening of the surface wind mixed layer after a storm.
epipelagic [67]: the zone in the ocean ranging from the surface down to about 200 m water depth.
eukaryote [67]: a cell or an organism comprised of cells that have a membrane-enclosed nucleus and usually other organelles (cf. prokaryote).
Eulerian observations [28, 133]: these are observations made at a fixed point in the sea. Changes in, for example, Eulerian observations of temperature, may be produced by local heating or cooling, or they may be produced by a current bringing warmer water to the observing site (see also Lagrangian observations).
eutrophication [126]: a process by which increased growth of primary producers (photosynthetic algae and bacteria) is induced by a change in the physical or chemical environment. Normally it is caused by increased nutrient concentrations in the water.
fetch [20]: the distance over which the wind acts upon the sea surface. Fetch is an important parameter controlling wave heights.
fix [89]: this term is often used to describe the assimilation of carbon by photosynthetic organisms, i.e. taking inorganic carbon and converting it to organically bound carbon.
flagellum [71]: (plural flagella). A filamentous organ of motility used by many groups of bacteria, protozoans and algae. An organism with a flagellum or flagella is sometimes described as being flagellated.
foraminifera [72]: commonly called Forams. A group of protozoans commonly found in the marine plankton. There are other formainfer species that are benthic. The tests of foraminifers are made of calcium carbonate and these can sink from the pelagic and accumulate on the sea floor in such concentrations they are referred to as foraminiferal oozes.
geostrophic balance [32]: this is the name given to the situation in which a pressure force acting on a parcel of water is exactly balanced by the Coriolis force arising from the Earth’s spin. A geostrophic balance is common, at least approximately, in many oceanic flows.
grease ice [113]: the earliest stage of sea ice formation, when ice crystals rise through the water column to accumulate in slicks of ice that are strongly influenced by wind and surface water movement.
group velocity [23]: the speed at which a group of waves with a range of wavelengths travels.
growth yield [102]: the efficiency by which the energy taken in is converted into growth is called the growth yield, and is normally somewhere between 10 and 30%.
hadalpelagic [67]: the deepest waters in the ocean, below 6000 m water depth.
halocline [14]: a region in the ocean where salinity changes with depth.
haloplankton [69]: organisms that live in the plankton for the whole of their life. (cf. meroplankton).
ice pancakes [114]: early stage of ice formed in turbulent water conditions. Predominantly circular pieces of ice form about 3 cm to 3 m in diameter, generally less than 30 cm in thickness and with raised edges.
internal waves [14]: waves that form in the body of the ocean on an interface between water of different densities.
intertidal [67]: the region of a coastal shoreline that is periodically exposed to the air at low tides. irradiance [56]: light energy falling on unit area of a horizontal surface in unit time, U.
Kelvin wave [39]: progressive waves affected by Earth rotation. In the northern hemisphere, the crest of the wave slopes up to the right (and the trough of the wave down to the right) looking in the direction of wave travel. A Kelvin wave travelling down a channel therefore has greater amplitude on the right-hand shore.
kinematic viscosity [79]: in everyday terms, the viscosity of a liquid is a measure of its stickiness. Kinematic viscosity is a particular measure of this stickiness that includes the fluid density. It has units of m2s–1. The kinematic viscosity of water at 20 °C is about 10–6 m2s–1.
Lagrangian observations [28]: these are observations made following the flow, for example from a drifting buoy. E.g. changes in Lagrangian observations of temperature must be produced by local heating or cooling (cf. Eulerian observations).
Lambert-Beer Law [60]: states that irradiance decreases exponentially with depth in the sea.
lock gate experiment [5]: a laboratory experiment in which waters of two different densities are separated by a lock gate. When the lock gate is removed, a density current is formed.
lunar hour [39]: one twelfth of a semi-diurnal tidal cycle, equal to one hour and two minutes.
meroplankton [69]: organisms that have part of their life history in the plankton and other stages on the sea floor or intertidal shores.
mesopelagic [67]: the zone in the ocean ranging from about 200 m down to 1000 m water depth.
microbial loop [104]: this is the pathway where organic matter (both dissolved and particulate) produced by all organisms in the whole food web is broken down by bacteria. Organic matter derived from terrestrial sources is also broken down in a similar manner. A consequence of the microbial loop is the regeneration of inorganic nutrients for new phytoplankton growth.
neap tides [38]: a period with relatively low tidal range that occurs twice each month.
nekton [67]: those animals that are strong enough swimmers to be able to move against a water current (e.g. large crustaceans, fish, squid and whales) (cf. plankton).
nodal line [42]: a line in a standing wave along which there is no vertical motion of the water surface. The first nodal line forms ¼ wavelength from the reflecting shore, the second ¾ wavelength, and so on. Although there is no vertical motion at the nodal line, the horizontal currents are faster than at other places in the wave.
orbitals [20]: the name given to the motion of water parcels beneath waves. For waves in deep water, the orbitals are circular and diminish in diameter with increasing depth. As waves move into shallower water, the orbitals become first elliptical and then flattened, just a backwards and forwards motion (called rectilinear motion).
pelagic [67]: this is the open sea and includes the whole water column. Pelagic organisms are those that live in the open waters, whereas benthic organisms live on the benthos.
phase speed (of waves) [20]: the speed at which the crest of a wave of a given wavelength travels.
photic zone [00]: the region of the ocean close to the surface where there is sufficient light for photosynthesis.
photosynthetically active radiation [56]: (or PAR) is the name given to the portion of the sun’s energy that can be used in photosynthesis. It spans the wavelength range 400–700 nanometres. This is also approximately the range over which the human eye is sensitive and so PAR is equivalent to ‘visible light’.
Phytoplankton [68]: photosynthetic organisms of the plankton.
plankton [67]: Pelagic organisms (many microscopic) that drift or float passively with the prevailing currents in an ocean, sea or lake. Plankton include many microscopic organisms, such as bacteria, algae, protozoans, animal larvae and larger animals that cannot swim against water currents. It is divided into zooplankton, phytoplankton and bacterioplankton.
potential energy [49]: the energy an object possesses by virtue of its height in the earth’s gravitational field. Potential energy can be calculated as mgh where m is the mass of an object, g is the acceleration due to gravity and h is the height of the object above or below a fixed reference. It therefore takes energy to lift an object (and increases its potential energy) and this energy is released when the object falls.
pressure [16]: force per unit area. The pressure in the ocean increases by approximately 1 decibar for every metre increase in depth.
pressure differences [32]: along a horizontal surface, are responsible for driving many oceanic flows. If the sea surface is sloping, for example, this produces pressure differences acting to produce a flow in the direction of the downwards slope. Pressure differences can also be caused by differences in water density.
prokaryote [67]: a cell or organism lacking a nucleus and other membrane-bound organelles.
pseudopod [71]: a pseudopod means ‘fake foot’ and refers to a temporary extension from a cell or unicellular organisms. Pseudopods can be used for catching prey, and for motility. In Foraminifers the pseudopodia are called reticulopodia, and these can form net-like structures to catch food items.
pycnocline [14]: a region in the ocean where density is changing with depth. To maintain vertical stability, the density must increase with increasing depth, otherwise, overturning will occur.
reduced gravity [11]: an object placed in water experiences buoyancy (see Archimedes’ Principle) and therefore weighs less than it does in air. This concept is expressed in terms of the reduced gravity, which is given the symbol g′.
refraction [23]: refraction of waves refers to a change in direction of the wave travel associated with a change in their speed. The speed of waves in shallow water, for example, depends on water depth. A part of a wave crest in shallow water will therefore travel more slowly than a part of the same crest in deeper water. This produces a bending of the wave crest and a change in the direction of wave travel (see also Snell’s Law).
resonance [43]: a body of water contained within shores has a natural period at which it will oscillate if set in motion. If a regular force is applied at this same period, large oscillations result, called resonance.
Reynolds number [80]: the product of the flow velocity and flow length scale divided by the kinematic viscosity of water. Turbulence becomes important at high Reynolds numbers and can be neglected compared to viscous forces (associated with the stickiness of the water) at low Reynolds numbers. In practice, turbulence is always important at scales above microscopic, but viscous forces dominate the motion of microscopic animals.
salinity [1]: the salt content of seawater. Salinity is properly expressed in Practical Salinity Units (PSU), which is a ratio and therefore dimensionless. In most cases, the salinity in PSU corresponds closely to the number of grams of salt in 1 kg of water (or parts per thousand).
salt fingers [17]: when warm salt water is placed above cold fresh water in a laboratory tank, rising ‘fingers’ of salt water can be observed to form on the interface. As the fingers rise, they gain heat from the surrounding water faster than they lose salt. This produces a reduction in density and an increase in the buoyancy of the salt finger, which therefore continues to rise.
Scombrids [104]: a group of fish including the mackerel and tuna. They are generally fast swimmers and include many of the commercially valuable fish species.
‘sea’ [19]: the name given to relatively short wavelength waves which have been produced locally by the wind.
sea ice biota [112]: organisms that are found living within or closely associated with sea ice for some, or part, of their seasonal life histories. Sometimes referred to as ‘sympagic biota’.
seasonal thermocline [47]: the boundary between the sun-warmed surface mixed layer and deeper, cooler water that forms in seas in temperate latitudes in the spring and lasts through the summer and early autumn. In late autumn and winter, the seasonal thermocline is destroyed by a combination of surface cooling and strong winds.
Secchi disc [61]: a (usually) white disc that is lowered on a marked rope into the sea to give a simple measure of water transparency. The depth at which the disc is no longer visible is called the Secchi depth.
semi-diurnal tide [36]: a tide with two high waters per day.
Snell’s Law [25]: is a law adapted from optics that describes the effect of refraction on the angle wave crests make with the shore. According to Snell’s Law, the sine of the angle the wave crests make with the shore divided by the wave speed remains constant. As the waves move into shallower water near the shore, their speed is reduced and so, according to this law, the crests become more parallel to the shore.
spring tide [38]: a period with relatively large tidal range, which occurs twice per month.
standing wave [42]: is a wave motion created by the interference of two equal progressive waves travelling in opposite directions. A standing wave will have one or more nodes where the vertical motion in the two waves cancels out.
Stokes’ Law [81]: governs the falling speed of objects in a viscous fluid. Objects sinking in the ocean quickly reach a terminal velocity in which their weight is balanced by the frictional force arising from their motion through water. The sinking speed increases with the square of the diameter of the object.
stratification [14]: the vertical layering of water of different density.
subtidal [67]: this is the area extending below the lowest low-tide point, and so is permanently submerged by water.
surface mixed layer [47]: is the name of the surface layer of the sea, warmed by the sun and stirred by the wind.
Sverdrup [28]: a unit for the volume of water transported by ocean currents, equal to one million cubic metres per second.
swell [19]: long wavelength, regular waves, with discernable long crests.
temperature–salinity (or T/S) diagram [6]: a graph on which the temperature of seawater at a given depth is plotted against the salinity at the same depth. T/S diagrams are useful for identifying water masses and their mixing in the ocean.
temperature staircase [17]: a series of ‘steps’ on a plot of water of water temperature against depth which has been observed in places in the ocean where it is thought that the difference in the rate of diffusion of salt and heat might be important. The steps are thought to form as a result of salt fingering.
test [72]: this is the shell-like structures produced by foraminifers. They are made up of chambers and constructed from calcium carbonate.
thermocline [5]: is the name given to a region in which temperature in the ocean is changing with depth. The main ocean thermocline separates sun-warmed surface water from water that is close to freezing near the ocean floor.
thermohaline circulation [5]: the relatively slow movement of the water in the deep ocean produced by differences in density of the water. The density differences, in turn, are produced by variations of temperature and salinity.
tidal mixing fronts [52]: tidal mixing fronts, found in shelf seas in temperate latitudes in the summer, separate thermally stratified water from vertically mixed water.
tidal range [37]: the vertical distance between low water and high water during a single tidal cycle.
tide generating force [35]: the tide generating force of the Moon on the Earth is the difference between the Moon’s gravitational force at a point on the Earth’s surface and the Moon’s gravitational force at the Earth’s centre.
transfer efficiency [102]: the efficiency (energy gain–energy loss) by which energy is passed between one trophic level and another in a food chain or web.
trophosome [116]: an organ in an animal that contains high densities of symbiotic bacteria which produce organic matter that the animal can use; e.g. in Riftia spp. found in hydrothermal vent systems.
tsunami [19]: the name given to a wave produced by an underwater disturbance, such as a submarine landslide or earthquake. Tsunamis have long wavelengths and so their speed, even in the deep ocean, is given by the shallow-water wave speed equation. Their wave height in the deep ocean is quite small, but grows dramatically as the tsunami moves into shallow water near a coast.
turbidity current [15]: an example of a density current in which the differences in density are produced by suspended particles.
volume transport [28]: the volume of water transported by an ocean current in unit time (see also Sverdrup).
wave dispersion [22]: the result of the speed of waves in deep water, depending on their period. Long period waves therefore outstrip shorter period waves, and waves of different period travelling away from a storm become separated.
wave height [20]: the vertical distance between the crest and the trough of a wave.
wave length [20]: the horizontal distance between one wave crest and the next.
wave period [20]: the time that elapses between one wave crest and the next passing a fixed point.
western intensification [27]: the name given to the formation of narrow, fast-moving currents on the western margins of the oceans, compared to the weaker, broader currents on the eastern margins. The effect is produced by the need to conserve angular momentum.
wind waves [19]: waves produced by the action of the wind on the sea.
zooplankton [69]: animals and protozoans in the plankton.