20.
Salinity
Core concepts
At the end of this topic, you should be able to:
Define the term ‘salinity’.
Describe how salt enters the ocean.
Describe the chemical composition of the water molecule and the process
of salt diluting in water.
Explain the terms relating to the dissolution of solids in water.
Understand the terms ‘ocean density’ and ‘buoyancy’.
Explain the concept of thermohaline circulation.
Describe the layers of the ocean that relate to salinity, temperature and
density.
Describe the changes in ocean salinity in relation to latitude and explain
the reason for these changes at particular latitudes.
Explain how hydrometers, refractometers and salinometers are used to
measure salinity.
Discuss the range of saline waters from fresh to brine.
Fig 20.1: Two views of the Velddrift Salt Works. The pristine sea brine is extracted from a
unique aquifer, on the shores of St.Helena Bay.
Fig 20.2: Walvis Bay salt pans Fig 20.3: Namibian salt mine
20. Salinity 1
Introduction
All ocean basins contain salt at varying levels of concentration. The level of salt held
in suspension in seawater is known as salinity. It is expressed in (grams of salt) per
kilogram of water in the form of parts per thousand (ppt), for example 35‰.
The source of salt in seawater
Millions of years ago, and while Earth’s surface was finding its current form, a high
concentration of Carbon dioxide (CO2) and Sulphur dioxide (SO2) formed and fell to
the earth as acid rain, which landed on continental rock and dissolved the minerals
from its surface. These salts ended up in the oceans where it accumulated.
Salts enter the ocean through various ways:
Rivers that drain the salts and minerals from the ground into the ocean
Volcanic clouds that carry minerals and salts through the atmosphere to
the sea
Hydrothermal vents that carry seawater (which has percolated into the
ground beneath the seabed where it absorbs dissolved salts) back into
the ocean
Decaying organisms that leach salts into the ocean
Fig 20.4: How salts enter the ocean
Source: Trujillo & Thurman: Essentials of Geography
2 Marine Sciences Grade 10
A salinity equilibrium or balance is reached as the salts exit the oceans via:
wave splash
salt blown from the sea surface into the atmosphere
sea spray
uptake by living organisms
uplifted sediment in continental plates
Properties of seawater
Proportions
On average, 1 kg of seawater is made up of 35 g of salt and 965 g of water. The
amount of salt present in seawater is expressed as 35 ppt or 35‰.
Of these 35 grams of salt, 99% is made up of six ions listed below by their weight
percentages. Ions are atoms with either a positive or negative charge.
Chloride (Cl-) 55.00%
Sodium (Na) 30.60%
Sulphate (SO42-) 7.70%
Magnesium (Mg) 3.69%
Calcium (Ca) 1.16%
Potassium (K) 1.10%
The remaining 0.25% is made up of dissolved gases, nutrient substances and some
organic molecules.
The largest component of the salt in seawater (86%) is the combination of Sodium
and Chloride ions (NaCl), which forms our basic table salt.
Constant proportions
Despite all the salts entering it, the salinity of the ocean does not increase. Marine
scientists have studied water samples from around the globe and found that
regardless of where the sample comes from and regardless of the level of salinity,
the proportion of the different salts in seawater remains consistent.
The Principle of Constant Proportions states that no matter how much the
salinity varies in different samples of seawater, the ratio of the inorganic
elements and compounds is constant. This is also called Forchhammer's
Principle.
20. Salinity 3
Solubility
Sea salt, or Sodium chloride, dissolves easily into water because of the water
molecule’s polarity. Water is made up of two Hydrogen atoms and one Oxygen atom.
These three atoms are bonded in an angular shape of 105 degrees, making it a polar
molecule with the Oxygen end negatively charged and the Hydrogen end positively
charged. The water molecule thus has a positive end and a negative end.
The Sodium chloride or salt crystal (NaCl) is made up of positively charged Sodium
ions and negatively charged Chloride ions. When a NaCl salt crystal comes into
contact with water, the NaCl bonds break up and the salt crystal dissociates. The
Sodium ions are surrounded by the negatively charged Oxygen (O 2) end of the water
molecule and the Chloride ions are surrounded by the positively charged Hydrogen
(H2) end of the water molecule.
Fig 20.5: Chemistry of salt crystals
Hence the opposite charges of water and salt ions attract each other, and the water
molecules pull the salt crystal’s Chloride and Sodium ions away from the crystal
lattice, thus dissolving the salt.
Fig 20.6: Dissociation of Sodium chloride in water
4 Marine Sciences Grade 10
Water in solutions and mixtures
The polarity of water makes it the liquid into which more salts and substances
dissolve than into any other liquid. This gives water the reputation of being the
universal solvent.
When a solid such as sugar, coffee granules or salt dissolves in water, that solid is
the solute; the water is the solvent and combined they produce a solution. A true
solution is found when molecules of the solute are homogenously dispersed
throughout the solvent. When solids are ‘mixed’ and the material does not dissolve
into the solution, a mixture is produced, for example vegetables and noodles in a
soup is a mixture, as the vegetable pieces do not dissolve into the solution. They
remain present in their solid state. They are not uniformly dispersed and therefore it
is not a homogeneous solution.
Water is not a solvent for all substances
When oil is mixed with water the oil does not
dissolve into the water. When left to stand the
oil will float on top of the water column. This is
because the molecules of oil are non-polar.
Non-polar molecules do not have a charge
and are attracted to other oil molecules and
not attracted to the water molecules.
Fig 20.7: Non-polar oil cells do not dissolve in water
Buoyancy
More buoyant fresh water floats on less buoyant salt water. Similarly different
liquids ‘float upon’ or ‘sink below’ each other in layers. The diagram shows how
ethanol, vegetable oil; water, dishwashing liquid, glycerine and honey all form
separate layers floating or sinking due to
a variance of buoyancy. More buoyant
liquids float and less buoyant liquids sink.
Liquids that are less dense therefore float
on liquids that are relatively more dense.
Floating or sinking of material whether
solid or liquid relates to its relative
gravity or specific gravity. The specific
gravity of a substance refers to its mass
per unit of volume in relation to water.
Fig 20.8: Specific gravity causes different liquids to form separate layers
The specific gravity of a given volume of sea water varies according to its
temperature and its salinity. The higher the salinity, the less buoyant and the more
20. Salinity 5
dense the water. Warmer water is more buoyant and floats on colder water. On a
larger scale these dynamics apply in the ocean, in which there are regions or bodies
of water with increased salinity. There are other regions where the relative water
temperature is higher than it is in another region. Each body of water has differing
buoyancies. Each volume of water forms different layers of warmer water above
layers of relatively cooler water. Similarly, there are layers of highly saline water
which lie beneath layers of less saline water. These layers are dynamic and
constantly change.
Density and temperature
The density of a material is stated in units of mass per unit of volume; thus the
density of seawater is expressed in grams per cubic centimetre. For example, the
density of water is 1 gram per cubic centimetre.
The formula for density is d = M/V, where d is density, M is mass,
and V is volume. Density relates to buoyancy and is influenced by
temperature and pressure.
Since salt ions are heavier than water molecules,
seawater is more dense than fresh water. The
density of seawater ranges from 1 020
to 1 030 kg/m3 while the density of fresh water is
about 1 000 kg/m3.
When volumes of water with differing properties
(different salinities or temperatures) come into
contact, they will not mix easily. Division layers
occur between these layers of water.
Fig 20.9: Salinity variation with depth
Source: Trujillo & Thurman: Essentials of Oceanography, 10th
ed., Prentice Hall 2011 (Fig 5.25)
A thermocline divides warmer layers from colder layers below.
A halocline divides a layer of highly saline water from a layer of lower salinity.
A pycnocline is the mid-ocean layer of water where the salinity and density
increase, and the temperature decreases with depth. It separates the surface layer
from the denser layer below. This zone increases in density with depth, due to rapid
changes in both the temperature and the salinity.
Thermohaline circulation
The different layers of water in the ocean interact with each other. Their varying
temperature, salinity and density result in a dynamically moving system on a global
6 Marine Sciences Grade 10
scale. This large-scale interaction results in water movement called thermohaline
circulation which is discussed in more detail in Grade 11 in Topic 42: Currents.
Ocean layers
Scientists classify the ocean into three main layers according to depth:
The surface layer – surface to 200 m deep. It is exposed to the atmosphere
and contains the least dense water. It is warmer, being close to the sun. It
has a higher level of mixing of surface waters owing to currents, wind and
wave action. The more saline (saltier) water sinks.
The mid-ocean layer – pycnocline – 200 m to 1 000 m. (See pycnocline
above.)
The deep layer – below 1 000 m. Contains 80% of all ocean water. This is
the region below the pycnocline and has no additional change in density
with depth.
Salinity variations
Salinity differs from one region of the ocean to another. Conditions change from
season to season. Some salinity levels can be extremely high as conditions change
from time to time, but in nature one will not find a water sample of 0‰. In regions
where there are inland seas such as the Red Sea and the Dead Sea, the salinity can
be extremely high. In the Dead Sea the salinity is high enough to change the
buoyancy so that people can easily float
on the water.
Fig 20.10: High buoyancy of the Dead Sea
Global salinity variances
The range of salinity found in various bodies of waters around the globe is shown
below:
The Baltic Sea – 7‰
Mediterranean Sea off North Africa – 39‰
The Red Sea – 41‰
Caspian Sea – 155‰
20. Salinity 7
Great Salt Lake – 220‰
The Dead Sea – 332‰
Don Juan Pond in Antarctica – 400‰
Salinity levels are dynamic
The levels of ocean salinities are dynamic and the variance in different regions is
due to many factors:
A change in the amount and rate of evaporation increases salinity.
Evaporation is affected by wind and sunlight. Increased wind or sunlight
increases evaporation and vice versa.
The supply of fresh water from rivers, melted ice and rainfall all decrease
salinity levels.
Ocean currents mixing in the water column will result in a variance in the
salinity level.
Fig 20.11: Surface water-
salinity variation
Source: Trujillo & Thurman: Essentials
of Oceanography, 10th ed. (Fig 5.23)
Bands of salinity
The changes in salinity levels result in bands of similar salinity levels tending to
occur at different latitude belts.
Fig 20.12: Satellite images of
regions of ocean salinity levels
around the planet
Source:
https://science.nasa.gov/files/science-
red/s3fs-public/styles/large/public/download-
files/images/content/730117main_591159m
ain_pia14786-43_946-
710.jpg?itok=8l1QnPqj
8 Marine Sciences Grade 10
There are four bands or regions of salinity on Earth:
Polar zone (60 to 90 degrees latitude)
Temperate zone (35 to 60 degrees latitude)
Tropical zone (5 to 35 degrees latitude)
Equatorial zone (0 to 15 degrees latitude)
Fig 20.13: The four bands
of oceanic salinity
Polar zone
The ice shelf off both Antarctica and the Arctic increases in size during winter as the
ocean freezes. Here the ocean’s
water is taken up, leaving salt
behind. Only the water freezes,
the salts are left in the ocean
solution. This salt water, called
brine, sinks to the floor of the
continental shelf because brine
has a higher density than
seawater. In the Arctic, where
there is no continental shelf, the
brine water sinks straight to the
ocean floor. In some instances,
brine pools can be found.
Fig 20.14: Mussels on the edge of a brine pool
Source: Photo by Stephane Hourdez and courtesy of NOAA's Office of Ocean Exploration and Research
The reverse occurs during Polar summers when ice melts, reducing the ocean’s
salinity in the region. The higher level of precipitation in the polar regions (rain and
snow) adds to the water which dilutes the polar waters. On average, polar regions
have lower levels of salinity as the brine sinks to the ocean floor and the ice melt,
20. Salinity 9
together with the precipitation, dilutes polar sea water. The average salinity in the
polar regions is 31 ppt.
Temperate zone
The temperate regions receive low sunlight and a medium amount of seasonal
rainfall. The average salinity of the temperate regions is 33 ppt.
Tropical zone
The tropical regions receive medium sunlight, low rain in desert areas and fewer
rivers drain into the ocean. The average salinity in the tropical regions is 37 ppt.
Equatorial zone
The equatorial regions receive high sunlight and heavy rainfall, which causes runoffs
and a decrease in salinity because of the added water. The average salinity in the
equatorial region is 37 ppt.
Isohalines
To better understand salinity
graphically, oceanographers
measure the salinity of water
and join points of equal salinity
readings. Lines joining equal
salinity readings are called
isohalines.
Fig 20.15: Isohaline lines
Measuring salinity
In groups, fill 4 cups with water. Add ½ teaspoon of salt to the first cup, 1 teaspoon
to the 2nd cup, 1½ spoons to the 3rd cup and 2 spoons to the 4th cup. In your group,
take turns to do blindfolded tests and try to distinguish between the different
concentrations of salt in the different cups by tasting.
Measuring the salinity of seawater would seem straight forward by weighing for
example a kilogram of seawater, allowing the water to evaporate from the container
and weighing the mass of salt that remains. One would then state that for that
kilogram of water X grams of salt are found and the salinity would be X‰. However,
this would not be an accurate method because some salts leave the water as the
water evaporates.
10 Marine Sciences Grade 10
As stated earlier, salinity is measured by the amount of salt per kilogram of water,
expressed as g/kg or parts per thousand (‰), therefore 1 000 g (or 1 kg) of salt
water consists of 956 g of water and 35 g of salt.
Water’s salinity can be accurately measured using one of these three meters:
Hydrometer
Refractometer
Salinometer
Hydrometer
Hydrometers measure the bouyancy of a liquid.
They come in two forms and are available at local
pet shops:
A vertical glass floating device,
that looks like a floating thermometer
A version that looks like a scale
Both work on the principle that the more
salt in a solution, or the higher the salinity
of a liquid, the higher the density.
Fig 20.16: Hydrometers
Buoyancy changes with temperature, thus when measuring salinity using a
hydrometer, two readings are needed: First the salinity reading and second the
temperature reading. The conversion scale is used to interpret the salinity reading at
the indicated temperature.
Refractometer
Refractometers measure the degree to which light entering a liquid changes
direction, called the angle of refraction. A sample drop of water is placed onto a
glass plate which is on a hinge. That is closed and the measurement is observed by
looking through the eyepiece. A white and blue shading appears on the scale. The
line between the white and blue section gives the salinity reading.
Fig 20.17:
Refractometer and
reading
20. Salinity 11
Salinometer
The more salt that is present in water, the
higher its capacity to conduct electricity. A
salinometer is an electronic device used to
measure the amount of electricity passing
through water and this reading is converted to
a salinity reading. It has two probes which are
placed in water.
The higher the salinity the easier it is for an
electrical current to pass through the water
between the two probes. The lower the
salinity the less easy it is for the current to
pass through the water. Hence another name
for this apparatus is a ‘conductivity meter’.
Fig 20.18: Lawrence Thorn at the Two Oceans Aquarium Water Quality lab holding a water
quality testing device which includes a salinometer
Salinity scale
Different levels of salinity are ranked
according to a practical salinity
scale:
Fresh water = 0–0.5 ppt
Brackish water= 0.5–30 ppt
Saline water = 30–50 ppt
Brine water = 50–and greater
Fig 20.19: Scale of salinity levels
A saline solution is used by medical personnel to
clean wounds, clear sinuses and feed patients
intravenously to prevent dehydration. It is a mixture of
salt and water. Sea water has a higher salinity than a
normal saline solution, which is 9‰, the same salinity
as found in human blood or tears.
Fig 20.20: Intravenous saline solution
12 Marine Sciences Grade 10
Effect of salinity on marine life
Salinity has a significant influence on marine animals and plants. Therefore, Marine
biologists study the variation of salinity from species to species, from within the
animal to outside and from outside the animal inwards.
Each species regulates its own level of salinity within the body. Most fish are
adapted to either living in fresh or salt water. Fish, like all living animals, require
minerals to live.
Freshwater fish absorb water from
their surrounds through osmosis and
take in the minerals they require by
absorbing them from the water
through their gills or from their diet.
Excess water is excreted by their
kidneys as dilute urine into the
digestive tract.
Fig 20.21: Freshwater fish
In the sea, fish lose water through
their skin through osmosis and
therefore drink water actively. These
fish excrete excess salt via their gills
and through their kidneys into their
digestive system.
Fig 20.22: Seawater fish
Some fish can live in both fresh and salt water. For example, some fish enter an
estuary containing less saline water for the purpose of laying or fertilising eggs. This
area of less saline water becomes a nursery area for juvenile fish. These fish need to
have kidneys which are adapted to deal with both fresh and salt water.
20. Salinity 13
Additional resources
You may find the following video links useful.
Chemical oceanography: https://www.youtube.com/watch?v=xs9ey_eqr7E
The chemistry of water: https://www.youtube.com/watch?v=pNN7VyfIX0U
Brine pool and salinity: https://www.youtube.com/watch?v=jzTBR2APU-k
How salt dissolves in water: https://www.youtube.com/watch?v=xdedxfhcpWo
14 Marine Sciences Grade 10
Test your knowledge
Use these sample questions to test whether you have understood the material in this
topic and to help with preparing for your exams.
1. How is salinity expressed and defined?
2. Describe the process in which the sea has become salty. Explain
whether the ocean is expected to experience a change in salinity.
Give reasons for your answer.
3. What are the six main elements found in sea water? Give their
proportions by weight?
4. Explain Forchhammer's Principle. How does this inform us about the
salinity of a sample of water which we take from the sea?
5. Describe the chemical composition of the water molecule and the
process of salt diluting in water.
6. Explain the terms used to describe the process of dissolving solids
in water.
7. Why is water called the universal solvent if not all substances, e.g. oil, dissolve
in water?
8. Describe the layers of the ocean that relate to salinity, temperature and density.
8. Explain thermohaline circulation and how the drivers of current movement come
into being.
9. Describe the changes in ocean salinity in relation to latitude. Give reasons
for the changes experienced.
10. Discuss the range of saline waters from fresh to brine.
20. Salinity 15
Marine Sciences definitions
These definitions are examinable and integral to your understanding of the topic.
Chloride an ion of the chemical element Chlorine with the symbol Cl-; it bonds with
Sodium to form Sodium chloride (salt)
density the mass of a substance in relation to its volume, expressed in grams per
cubic centimetre (g/cm3); also, the degree of compactness of a substance
dissociate in chemistry, the separation of an ionically bonded molecule into two or
more ions in solution
halocline a zone in the ocean where there is a large change in salinity with depth
hydrometer an instrument that measures the buoyancy of a liquid
ion an electrically charged atom or a group of atoms that results in an
unbalanced electrical charge because it has either lost or gained one or
more electrons
isohaline lines joining equal salinity readings
mixture a combination of two or more substances which have retained their identity
pycnocline the middle zone of the ocean that separates the surface layer from the
denser layer below; increases in density with depth owing to rapid changes
in both the temperature and the salinity
refractometer an instrument that measures the degree to which light entering a liquid
changes direction
salinity the concentration of dissolved salt in a given volume of water, expressed in
(grams of salt) per kilogram of water (parts per thousand, ‰)
salinometer an electronic device used to measure the amount of electricity passing
through water
Sodium a chemical element with the symbol Na; the most common alkali metal and
the sixth most abundant element on Earth (2.8% of Earth’s crust and 80%
of salt water)
Sodium chloride commonly known as salt (though sea salt also contains other chemical
salts); an ionic compound with the chemical formula NaCl
solute a smaller component by mass in a solution, which is dissolved into a solvent
to make up a solution
solution a mixture in liquid form in which a solute (the minor component) is uniformly
mixed with the solvent (the major component) to make up a homogenous
substance referred to as a solution
solvent a fluid in which other molecules, known as solutes, can dissolve to form
solutions
specific gravity also known as relative density; a ratio of density of a particular substance to
that of water
thermocline a thin but noticeable layer in a body of water in which temperature changes
more rapidly than it does in the layers below or the layers above
thermohaline the circular movement of ocean water on a global scale through different
circulation ocean basins driven by the density gradients such as salinity and
temperature changes in the water
universal solvent a liquid substance that is capable of dissolving most chemicals; water is
regarded as a universal solvent because it is capable of dissolving more
materials than any other liquid
16 Marine Sciences Grade 10
General terminology
These words are terms commonly used in science and should be part of your
general vocabulary.
buoyant having the power to float or be held up in water
dissolve to become or cause to become incorporated into a liquid to form a solution
leach the movement of water down a soil rock or mineral profile out of which
minerals are dissolved and transported
percolate to trickle or ooze through a porous substance
20. Salinity 17
