Major constituents and salinity

It is uncertain to what extent the composition of seawater may have changed during geological time, but it is not thought to have varied very widely over the period that life has existed. At present, the principal cations are sodium, magnesium, calcium, potassium and strontium, and the chief anions are chloride, sulphate, bromide and bicarbonate. These make up over 99.9 per cent of the dissolved material, forming approximately a 3.5 per cent solution. The amount of inorganic material dissolved in seawater expressed as weight in grams per kilogram of seawater is termed the salinity (S), and usually amounts to about 35g/kg, i.e. S = 35 parts per thousand (generally written 35%o). The quantities of the major constituents of a typical sample of ocean water are shown in Table 4.1.

The relative proportions of the major ionic constituents in ocean water remain virtually constant despite some variation in total salinity. Estimation of the concentration of any of these ions therefore enables the total salinity to be calculated. Salinity determinations may be made by titrating seawater with silver nitrate solution. This precipitates the halides, mainly chloride with a trace of bromide, and their total weight in grams per kilogram of seawater is termed the chlorinity, Cl. The salinity is then determined from the empirical relationship known as Knudsen's formula:

A convenient method is to titrate 10 ml of seawater with silver nitrate solution containing 27.25 g/€, using a chromate or fluorescein indicator. The added volume of silver nitrate in millilitres is approximately equal to the salinity in grams per

Table 4.1 Major constituents of an ocean water. S = 35.00%o

Constituent g/kg

Sodium Magnesium

Calcium Potassium Strontium Chloride

Sulphate as SO, Bromide

Carbon, present as bicarbonate, carbonate and molecular carbon dioxide

kilogram, and a small correction is made from tables to allow for the slight differences in weight of unit volume of seawater at different salinities.

Greater accuracy is obtained by using 'standard seawater' for comparison. This is water of very accurately known chlorinity available from specialized suppliers. By comparing the titrations of silver nitrate against both sample and standard seawater the calculation of chlorinity becomes independent of the concentration of silver nitrate solution, and all measurements are made to the same standard.

There are various objections to using chlorinity measurements as a basis for all determinations of salinity. It assumes a constant ratio between chlorinity and total amount of dissolved material, which obviously cannot be true for all dilutions of seawater with other waters of differing compositions. Also, silver nitrate is an expensive reagent, and titration is a relatively time-consuming technique. Consequently, attention has turned to other methods of salinity measurement. Several physical properties of seawater vary with the amount of dissolved salts and can be used for salinity determination (see Section 3.1.2); for example, electrical conductivity, density, vapour pressure, freezing point, refractive index and sound conductivity (Grasshof, 1976; Johnston, 1969).

The electrical conductivity of seawater increases the more ions it contains. This makes it possible quickly and directly to measure salinity using an instrument called a salinometer. An electrical probe can be lowered into the water to the required depth and a direct reading taken. Since it is actually the conductivity that is being measured, the instrument must be calibrated for temperature if it is to read out directly in salinity units. Salinometers are of great value in areas such as enclosed sea lochs where there may be a halocline - a sudden change in salinity with depth. These instruments should be periodically tested and calibrated by comparing the readout with samples of the same water for which the salinity has been determined chemically.

The salinity of most ocean water is within the range 34-36%o. There are slight seasonal variations of salinity, and average positions for the surface isohalines during the northern summer are shown in Figure 4.7. High salinities are associated with low rainfall and rapid evaporation, especially where the circulation of the water is relatively poor. Such conditions are found in the Sargasso area of the North Atlantic and in the South Atlantic off the east coast of Brazil, where the surface salinities are about 37%o. In high latitudes, the melting of ice, heavy precipitation and land drainage together with low evaporation reduce the salinity of the surface water. In the Arctic, the surface salinity fluctuates between 28 and 33.5%o with alternate melting and freezing of ice.

In land-locked areas there are appreciable departures from the normal oceanic range of salinities. For instance, in the Baltic, dilution by fresh water reduces the salinity from 29%o in the Kattegat region to below 5%o in the Gulf of Bothnia. In the Black Sea, rainfall and the outflow of the Danube, Dnieper and Dniester lower the surface salinity to 18%o or below. This low-salinity water forms a low-density layer overlying the more saline, deep layers with little mixing between them, and cuts off the depths of the Black Sea from the air, producing the peculiar hydrographic conditions mentioned later (see page 114). In hot regions, high surface salinities are found in enclosed seas due to rapid evaporation. Throughout most of the Mediterranean surface salinities are above 37%o, increasing from west to east and reaching about 39%o in the eastern part. In the Red Sea, surface salinities may exceed 40%o. On the shore the salinity of evaporating pools is sometimes greater than 100%o. Peculiar salinities occur in deep-sea pits at tectonic plate boundaries (see page 98).

The salinity of neritic water is subject to fluctuation due to changes in the rate of dilution by fresh water from the land. River water often contains ions in very different proportions to those of normal seawater, and this may produce appreciable changes in the composition of seawater near a river mouth.

Except for the teleosts and higher vertebrates the majority of marine creatures are in osmotic equilibrium with the surrounding water. The ionic composition of their internal fluids has, in most cases, a close similarity to that of seawater, containing relatively high concentrations of sodium and chloride and considerably lower concentrations of potassium, magnesium and sulphate. There is commonly, though not invariably, a rather higher proportion of potassium to sodium in body fluids than that which occurs in seawater, and somewhat less magnesium and sulphate (see Table 4.2).

External salinity changes usually produce corresponding changes in the concentration of internal fluids by passage of water into, or out of, the body (osmotic adjustment) to preserve the osmotic equilibrium, and these changes are often accompanied by alterations in the proportions of the constituent ions of the internal fluids. Beyond limits, which differ for different species, departures from the normal concentration and composition of the internal medium cause metabolic disturbances and eventual death.

Major Ions Seawater
Table 4.2 Concentrations of Ions in Body Fluids of some Marine Invertebrates (g/kg)







Seawater (S%o = 34.3)







Aurelia aurita







Arenicola marina







Carcinus maenas







Mytilus edulis







Phallusia mammillata







The majority of organisms of the open sea have very limited tolerance of salinity change, i.e. they are stenohaline. Euryhaline forms which can withstand wider fluctuations of salinity are typical of the less stable conditions of coastal water (Kinne, 1963, 1964). Extreme euryhalinity characterizes estuarine species.

Organisms which remain in osmotic balance with their surroundings when the salinity varies are termed poikilosmotic, and these include some widely euryhaline creatures. The lugworm Arenicola marina is a familiar example from the British coastline, where it is widely distributed in marine, brackish and estuarine muddy sands and able to survive salinities down to about 18%o. In other parts of its range, for example the Baltic, it is found at even lower salinities. Other examples from the British fauna which are poikilosmotic and moderately euryhaline are the bivalves Mytilus edulis, Cerastoderma (Cardium) edule, and Mya areanaria, the barnacles Semibalanus balanoides and Balanus improvisus, the polychaete worms Hediste (Nereis) pelagica and Perinereis cultrifera and many other common shore forms.

Some animals are able to control within limits the concentration of their internal fluids independently of salinity changes in the water. This process is known as osmoregulation, and organisms which maintain this stability of internal environment are described as homoiosmotic. The shore crab Carcinus maenas is a very euryhaline osmoregulator which extends up estuaries to levels where it encounters immersion in fresh water. Some powers of osmoregulation are also present in the ragworm Nereis diversicolor, the prawn Palaemon serratus, and the amphipods Gammarus locusta, G. duebeni and Marinogammarus ( = Chaetogammarus) marinus. The ability to osmoregulate is influenced by temperature and fails above and below certain limiting temperatures.

In marine teleost fish the concentration of salts in their internal fluids is lower than in seawater, so water tends to pass out of their tissues by osmosis. To counteract this water loss and maintain a correct water balance the fish swallow seawater and absorb it through the gut. The excess salts, and much of their excretory nitrogenous products, are eliminated by special secretory cells in the gill membranes. The kidneys of many marine teleosts have a much reduced number of glomeruli, or glomeruli may be absent. Urine is produced in small quantity and is nearly isotonic with the blood. Waste nitrogen in the urine of teleosts is excreted mainly as trimethylamine oxide in substitution for ammonia, which is the chief nitrogenous end-product of the majority of aquatic organisms. Excretion of ammonia, which is highly toxic compared with trimethylamine oxide, requires a copious, very dilute urine. Replacement of ammonia by trimethylamine oxide in the urine of marine teleosts is a useful adaptation for conserving water.

Vascular plants growing on the seashore are exposed to a very different environment to that of other terrestrial plants. Compared with normal soil water, the concentration of salts is much higher and the ionic composition of the water quite different. Almost all halophytes have adapted to these conditions by increasing the intracellular concentration of their tissues sufficiently to be able to take in water by osmosis, and by selective control of ion absorption.

Change of salinity alters the specific gravity of the water, and this influences pelagic organisms indirectly through its effects on buoyancy.

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  • P
    Is palaemon poikilosmotic?
    8 years ago
  • ashlee
    What are the major and minor constituents of seawater and their appropriate ratios.?
    3 years ago
    Which is not a major constituent of ocean salinity?
    1 year ago
  • ethan graham
    What are the two major constituents of salinity?
    6 months ago

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