Kartikeya Tiwari

08BT01209

What is salinity?

What are its units?

Does it vary in oceans and if it does then how and why?

Why is salinity important?

Defined as the mass of dissolved ions (<0.5 mm) in a kg of seawater.

Absolute (or ideal) salinity is the mass fraction of salts in seawater

In practical terms, salinity is expressed as PSU (practical salinity units) or PPM (Parts Per Mil) which

are based on water temperature and conductivity measurements.

Salinity is generally expressed in parts per thousand (ppm).

For oceanic seawater, ppm and PSU are very close.

Average salinity of SW is 35: means 3.5 ‰‰ of salt and 96.5 ‰‰ of pure water.

Salinity, Temperature and Salinity, Temperature and DensityDensity

Salinity - the total quantity of dissolved salts in Salinity - the total quantity of dissolved salts in seawater.seawater.

Salinity of seawater is reported in parts per Salinity of seawater is reported in parts per thousand – psu, thousand – psu, ‰ ‰ or ppmor ppm

Salinity has no units- It is a ratio. As a ratio of conductivities has no units, the

calculated salinity is also dimensionless. Salinity and Temperature jointly define density.Salinity and Temperature jointly define density. Dense water sinks to a depth at which the Dense water sinks to a depth at which the

density of the surrounding water equals the density of the surrounding water equals the density of the introduced object.density of the introduced object.

Salinity, Temperature and Salinity, Temperature and Density for the Full Oceanic Density for the Full Oceanic

Range of ValuesRange of Values

Heat Heat Budget by Budget by LatitudeLatitude

Heat gain is compensated for by ocean currents and transfer of latent heat polewards.

This is the driving force for the entire ocean/ atmosphere system.

Stratified conditionsStratified coastal waterways are characterised by a distinct

increase in salinity with water depth (Figure 2A). Stratification occurs when riverine flow is sufficient to produce a plume of low-

density freshwater (1000 kg/m3 at 20oC) which can flow over higher-density seawater (1025 kg/m3 at 20oC), and where tidal

currents and waves are not strong enough to mix the water column (e.g. in wave-dominated estuaries). Such conditions can lead to anoxic and hypoxic events because bottom waters can become isolated from dissolved oxygen enriching processes,

including gas exchange across the water surface and photosynthesis by plants in shallow water.

EstuariesSalinity of estuaries usually increases away from a freshwater source such as a river, although evaporation sometimes causes the salinity at the head of the estuary to exceed seawater.

Partially mixed conditionsIn partially mixed coastal waterways, tidal currents generate

turbulence which promotes vertical mixing (Figure 2B). However, the tidal currents are of insufficient strength to fully mix the water

column, and salinity varies both vertically and horizontally.

Fully mixed conditionsFully mixed conditions occur in coastal waterways in cases where tide, river or wave energy produces enough turbulence to mix the water column (Figure 2C). In this case, salinity is uniform through

the water column, but varies between the riverine and oceanic ends of the estuary.

Inverse estuary High evaporation rates in the presence of low freshwater inflow can lead to

hyper-salinity in tidal embayments and wide shallow estuaries. Estuarine water can become denser than oceanic waters under these conditions (Figure 2D),

and thus sink forming a highly saline bottom layer that flows seaward.

Seawater of lower salinity flows into the estuary over the top of this layer.

What causes salinity regimes in coastal waters to change?

The salinity distribution within coastal waterways reflects the relative influx of fresh water supplied by rivers, and marine water supplied by exchange with the ocean (Figure 1). Salinity levels fluctuate with the penetration of tidal flows, and with mixing of

fresh water and marine water by wind and currents. Freshwater discharges to coastal waterways are primarily controlled by

conditions in the catchment including rainfall patterns, vegetation type and cover, topography, catchment area, and geology.

Climatic factors may vary seasonally and inter-annually (e.g. with El Nino Southern Oscillation events). Entrance size (and seasonal

closure in some areas) and sea level dictate marine water exchange, and the extent to which salinity can build up within the

coastal waterway due to evaporation during times of low river flow

Decreased freshwater inflows, due to the diversion of rivers and streams into impoundments, lead to the dissipation of salinity gradients and extended periods of elevated salinity in the landward sections of the estuary [4]. Both the vertical and longitudinal salinity distributions will occur with resulting changes in the circulation and mixing characteristics of the estuary. Large incursions of stormwater runoff can severely depress normal salinity levels in inshore areas. Engineering works such as trained entrances and dredged channels increase the water exchange between the estuary and the ocean (increased flushing). This has implications for salinity levels in a coastal waterway.

Salinity distribution in oceansOn sea surface, significantly controlled by processes of evaporation andprecipitation.Highest at sub-tropics (20 to 30 0N and S) because of more evaporation thanrainfallEvaporation and freezing Salts excluded from vapor and ice, respectively

World World Ocean salinity Ocean salinity

and and temperaturetemperature

Considerations for measurement and interpretationThe accepted method for determining the salinity of marine waters is by measuring the conductivity (EC) and temperature of the waters, then calculating the value of salinity using standard equations. A depth determination is also required for greater accuracy in calculation of salinities in deeper water. Salinity is calculated using the ratio of the conductivity of the marine water to the conductivity of a standard solution of pure water and potassium chloride (KCl) at 15oC and one standard atmosphere in pressure, and in which the mass fraction of the KCl is 32.4356x10-3. This ratio is called the K15 value, and is used to calculate salinity on the Practical Salinity Scale 1978 [20]. As a ratio of conductivities has no units, the calculated salinity is also dimensionless. Thus for practicality, salinity is assigned Practical Salinity Units (PSU). A salinity of 35.000 has a K15 of 1 and is the salinity of standardised North Atlantic marine water that is slightly diluted. This is the accepted standard for high accuracy calibration of salinity determining instrumentation, and is sold with the appropriate recognition of the International Association for the Physical Sciences of the Ocean (IAPSO) as the IAPSO Seawater Standard.

The main advantage of using PSU is that marine waters from around the world can be compared on a common scale. Strictly speaking, this comparison is only robust in open marine waters. There can be problems with the Practical Salinity Scale 1978 in estuaries because the ion composition of freshwater inflow is often different than that of seawater [9], and the conductivity measurement is influenced by ion composition. One should therefore exercise caution in how the acquired data and calculated salinities are used. For example, serious problems may occur if these data are used for highly accurate density calculations. When abundant freshwater inflow is apparent, one might consider measuring salinity from the sum of the weights of the suite of major, minor ions, and trace constituents. Suites of major ions (e.g. Na+, Mg2+, Ca2+, K+, Cl-, SO42- and HCO3-+CO32-) are commonly used on their own, but incur small charge-balance errors.

In practice the ratio of the waters and KCl solution are not measured. Instead, calibrated sondes and probe measure the conductivities, temperatures and depths. These values are used to calculate the salinity in accordance with the Practical Salinity Scale 1978. Some sondes do not provide a PSU calculation. A seawater equation of state calculator is available on the web, and can be used in such cases. Salinity is determined in-situ using 'environmental water quality' or 'oceanographic' sondes (or probes). These instruments are often referred to as a CTD which is short for conductivity, temperature and depth (depth is derived from measurement of pressure). The conductivity probes have two or more electrodes made of stainless steel, gold or other suitable conductors that have a precise alternating voltage applied which is used to determine the conductance of the marine water in siemens per metre (S/m).

Some points to consider when making salinity determinations with sondes: Ensure that salinity values reported by the sonde are calculated in accordance with the Practical Salinity Scale 1978. Ensure the conductivity electrodes are cleaned appropriately often and calibrated at regular intervals. The length of the period between calibration and cleaning is determined by the user, as they develop familiarity with instrument and the environment under investigation. The conductivity electrodes can corrode slightly and may also acquire coatings from dissolved and suspended matter in the water column. The coatings and corrosion can increase resistance on the surface of the electrodes and reduce the accuracy of the determinations. This can be a significant problem when sondes are left in-situ for long periods of time (i.e. when they are used for continuous monitoring).

Confirm the calibration of the sonde is stable across the range of temperatures in the environment in which the measurements will be taken because the electronics modules within the sondes may be sensitive to - and not corrected for temperature variation. Do not leave sondes in the sun prior to taking measurements because this will increase the time required for temperature equilibration. Most sondes report salinity to three decimal places, but the measure may only be accurate to one decimal place. It is best to determine the level of accuracy required prior to a survey. Always record the sondes report of temperature, conductivity and depth (if available) as these are the key values used to calculate salinity. Record reported salinity also. Be sure to routinely note the units of conductivity the sonde is reporting, as the sondes display can be toggled for different values. The last person to use the sonde may have been recording specific conductance (a value compensated usually to 25 C).    

The temperature response on most sondes is not instant and lags behind the conductance measurement. Thus equilibration time is required for an accurate measurement and determination of salinity. This requires the sonde to be stationary or very slow moving. Estuaries often have a significant thermal gradient which may be an issue if you are in a small boat rocking on the waves attempting to attain a stable measurement. In the open ocean, by comparison, the temperature response is usually rapid and thus this is less of an issue. Half a degree error in water temperature can cause a significant change in the salinity calculation. To determine the salinity value to a high degree of accuracy, a bench top precision salinometer, standardized seawater and a temperature controlled room away from strong electromagnetic fields is required.

How to measure Salinity?Salinity of a sample is using conductivity—determined as a ratio to a standardof KCl (at 15 deg cel and 1 atm).Practical salinity is calculated using a polynomial equationS=f (R, T and constants)S = ao + a1R0.5 + a2R + a3R1.5 + a4R2 + a5R2.5 ++ (t-15)/{1 + k(t-15)} [bo + b1R0.5 + b2R + b3R1.5 + b4R2 +b5R2.5]

3. Vertical distribution of water properties

Salinity in the surface ocean layer

3. Vertical distribution of water properties

Pressure p (kiloPascal, 10 kPa = 1 dbar = 1 m);Temperature T (degrees C); Salinity S (Practical Salinity Units - psu);

correspond to promille (g salt/kg sea water);Density (kg m-3) represented by t = - 1000.

IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

3. Vertical distribution of water properties

The water density is a function of temperature, salinity, and pressure.

IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

Importance of salinityTogether with T, it controls the density-and hence the ocean

circulation.Salinity can be used as tracer for origin and mixing of water masses

Salinity record physical processes (evaporation/precipitation) occurringwhen it was at the surface.

Salinity is important in coastal waterways for the following reasons:

Salinity is a dynamic indicator of the nature of the exchange system. The salinity of the water within the estuary tells us how much fresh water has mixed with sea water. Also, plots that show the relationship between salinity and other soluble substances (e.g. nutrients) can be used to demonstrate the dynamic or

conservative nature of those substances in 'mixing plots'; Salinity is an important determinant of the mixing regime because of the density variation associated with salinity variation, salinity stratification tends to inhibit

vertical mixing in an estuary; which can have important implications for dissolved oxygen concentrations.

The circulation with estuaries and coastal regions can derive from or be strongly influenced by the density variation associated with salinity. In effect,

dense saline water tends to flow under fresh water. Salinity is an important ecological parameter in its own right; and it is important

in some chemical processes (discussed below).

Ecological importanceMost aquatic organisms function optimally within a narrow range of salinity (see example in Figure 5). When salinity changes to above or below this

range, an organism may lose the ability to regulate its internal ion concentration. Indeed, 'osmoregulation' may become so energetically

expensive that the organism dies due to the direct physiological effects or it becomes more vulnerable to biotic pressures such as predation, competition, disease or parasitism. Consequently, shifting salinity

distributions can affect the distributions of macrobenthos [13] as well as those of rooted vegetation (e.g. seagrasses) and sessile organisms [14]. The nature of the longitudinal salinity gradient (and the position of certain

isohalines) is an important factor in the successful recruitment of larval and juvenile fish [6,16]. Salinity is also an important control on the types of pathogenic organisms and invasive species that can occur in a coastal

waterway, on the types of species that can occur in algal blooms [5,10], and on the activity of nitrifying and denitrifying bacteria [8]. As a general rule,

widely-varying salinity regimes tend to select for a low-abundance and low-diversity suite of species, which are adapted to a broad range of ionic

concentrations (e.g. euryhaline species).

Density= f( T, S)Higher T means lower D, at fixed SHigher S means higher D at fixed TWater mass of different density is stratified (means unmixed)—it takes longer timeto get mixed

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