Seawater is unique in its physico-chemical properties. This module explains their characteristics, in a nutshell.

1. By
Prof. A. Balasubramanian

2. Introduction:
• All water existing at or near the surface of the
Earth belongs to the hydrosphere.
• It includes atmospheric water vapor, rainwater,
river water, groundwater, lake water, polar
icecaps, sea and the ocean waters.
• Water has its own physical, chemical and
biological properties.
• Water is classified into Freshwater, Brackish
water, saline water, Brine are the types.

3. Water quality
• Water quality is an important aspect on the earth.
Occurs in three states.
• Water is a universal solvent.
• Water can dissolve ions present in soils and rocks.
These are called as dissolvable salts.
• While undergoing evaporation, water accumulates
these salts and water becomes salty.
• Ions are expressed in two units. One is milliequivalents
per litre meq/L ort mg/L(ppm).
• Total dissolved solids (TDS) is sum of all ions(ppm).

4. Water types
• Rainwater has negligible amounts of salts (
TDS<20 to 30ppm)
• River water gets little more salts(100-300ppm)
• Lake water( 500-1500ppm)
• Groundwater ( 750 to 7500 ppm)
• Seawater (23000 to 35000 ppm)
• Brine ( 35000 to 100,000 ppm)

5. Properties of water
Physical Properties
• Color, odour, Temperature, MP, BP, FP, turbidity, density,
acoustics ( sound trasmission), light penetration
(Transparency)
Chemical composition
• Major ions-Ca,Mg,Na,K, HCO3,CO3, Cl, NO3, SO4, I
• Minor trace elements(many)
• Chemical Properties
• pH, TDS, Ec, Salinity, Alkalinity, solubility, thermal expansion
Biological Properties
• Dissolved Oxygen, Biological Oxygen Demand, Chemical
Oxygen Demand, Plankton Biomass, Coli form bacteria

6. High Heat Capacity :
• Seawater has several unique properties like high
heat capacity, latent heat of fusion (LHF), latent
heat of vaporization, latent heat of melting,
thermal expansion, density, viscosity and
turbidity.
• Water has the capacity to store heat, conduct
heat and release heat.
• The latent heat of fusion (LHF) is also the highest
in seawater except ammonia.
• Hence, it acts as a thermostat at freezing point
owing to uptake or release of latent heat.

7. Important properties
Latent Heat of Evaporation, thermal expansion are
important properties of seawater.
• Heat capacity of water (heat required to raise 1
deg C in water)= 1 calorie per gm of water.
Latent heat of water(for melting): (latent means
Hidden) from ice
• for vaporization, 540 calories per gm of water. of
heat to produce 1 gm of water vapour.
Latent heat for condensation: pure water cools at 4
deg C, but its density increases. From 4 to 0, it
decreases.

8. Physical Properties of Seawater:
• The waters of the seas and oceans has formed over
millions of years.
• It is complex nature of water.
• In fact seawater cannot be duplicated in any lab.
• Seawater has its own physical, chemical and biological
properties.
• Due to its huge volume and thickness, it has certain
unique characteristics in the distribution of
temperature, pressure and density.
• Most of these properties vary horizontally and
vertically.

9. Seawater is an Efficient Solvent:
• Water, in general, is a good solvent.
• Seawater is an effective solvent.
• Seawater is also at the receiving end to
dissolve all the sediments derived from land
and air.

10. Chemical Composition of Seawater:
• Seawater is a complex mixture of water, salts and
many other organic and inorganic substances.
• Seawater contains more dissolved ions.
• It contains 96.5 percent water, 2.5 percent salts.
• The chemical constituents of seawater include
major ions and minor trace elements.
• In addition, seawater contains the suspended
solids, organic substances, and dissolved gases.

11. Elemental Composition:
• Seawater chemistry shows 96 percent water
and only 4 percent other elemental
composition.

12. Overall Chemistry :
• When we analyze seawater, the major ion composition of
seawater will be invariably showing the following composition
in mg/L.
Ionic Concentration in seawater in mg/L
• Chloride (Cl-)=18980 mg/L.
• Sodium (Na+)=10556 mg/L
• Sulfate (SO4
2-)=2649 mg/L.
• Magnesium (Mg2+)=1262 mg/L.
• Calcium (Ca2+)=400 mg/L.
• Potassium (K+) 380 mg/L
• Bicarbonate (HCO3
-) 140 mg/L.
• Fluoride (F-).=1 mg/L.

13. • Strontium (Sr2+) 13 mg/L
• Bromide (Br-)=65 mg/L .
• Borate (BO3
3-) 26 mg/L
• Silicate (SiO3
2-).
• Iodide (I-)= <1 mg/L
• Total dissolved solids (TDS) of seawater is
34483 mg/L.
• It is also expressed in parts per million (ppm).

14. Minor Elements :
With reference to the minor elements,
• Bromine 67.3 mg/l.
• Inorganic carbon is 28 mg/L
• Nitrogen 15.5 mg/L.
• Fluoride is 13 mg/L
• Strontium 8.1 mg/L.
• Boron 4.45 mg/l
• Silicon 2.9 mg/L .
• Iodine 0.064mg/L
• Rubidium 0.12 mg/L.
• Barium 0.021 mg/L
• And Uranium is 0.0033 mg/L in seawater.

15. Addition Of Gases :
• At the ocean-atmosphere interface, gases are
added to seawater by diffusion and wave
action.
• Hence, Seawater contains various dissolved
atmospheric gases, chiefly nitrogen, oxygen,
argon, and carbon dioxide.

16. Solubility of Gases in Seawater :
• The solubility of gases in seawater is
controlled by the temperature, salinity and
pressure.
• All gases are less soluble as temperature
increases.
• When water is warmed, most of the gas
bubbles leave the liquid.

17. Sea Salt :
• Sea salt is produced by evaporating sea water.
• The salt content in seawater is indicated by
salinity (S), which is defined as the amount of
salt in grams dissolved in one kilogram of
seawater and expressed in parts per thousand
(ppt)

18. Salinity and Density of Oceans:
• Ionic Concentration is the amount (by weight) of salt
present in water and can be expressed in parts per million
(ppm).
• The classification of water based on total salt content is:
• Fresh water - less than 1,000 ppm
• Slightly saline water - From 1,000 ppm to 3,000 ppm
• Moderately saline water - From 3,000 ppm to 10,000 ppm
• Highly saline water - From 10,000 ppm to 35,000 ppm
• Ocean water has a salinity that is approximately 35,000
ppm.

19. Salinity is the Saltiness:
• In oceanography, salinity is expressed as
percent, but as parts per thousand (‰).
• It is approximately grams of salt per kilogram
of solution.
• All over the globe and from the top of the
ocean all the way to the bottom of the ocean,
salinity is between 33-37 parts per trillion.

20. Changes in Salinity :
• Salinity affects ocean organisms because the
process of osmosis transports water towards a
higher concentration through cell walls.
• Saltwater fish drink water copiously while
excreting excess salts through their gills.
• Marine plant life (seaweeds) and many lower
organisms have no mechanism to control
osmosis, which makes them very sensitive to
the salinity of the water in which they live.

21. High Salinity :
• Lowest salinity is found in the upper reaches
of the Baltic Sea (0.5%).
• The Dead Sea is 24% saline, containing mainly
magnesium chloride MgCl2.

22. Zones of Salinity :
• Based on the salinity, the oceanic water masses
are classified into various zones:
Thalassic series
• >300 =hyperhaline
• 60 - 80 =metahaline
• 40 =mixoeuhaline
• 30 =polyhaline
• 18 =mesohaline
• 5 =oligohaline

23. Types of Seas :
• Marine waters are those of the ocean, another
term for which is euhaline seas. The salinity of
euhaline seas is 30 to 35.
• Brackish seas or waters have salinity in the range
of 0.5 to 29 and metahaline seas from 36 to 40.
• These waters are all regarded as thalassic
because their salinity is derived from the ocean
and defined as homohaline if salinity does not
vary much over time (essentially constant).

24. Salinity is an Ecological Factor:
• The ocean salinity at the surface is high and
then salinity decreases until a depth of about
1,000 meters.
• Salinity then increases again slightly with
increasing depth.
• The halocline is a layer of water where the
salinity changes rapidly with depth. Salinity is
an ecological factor influencing the types of
organisms that live in a body of water.

25. Depth
v/s
salinity

26. Temperature v/s Salinity

27. Salinity v/s depth

28. Sea surface salinity

29. Sea surface salinity

30. Salinity influences the kinds of plants
• Salinity influences the kinds of plants that will
grow in seawater.
• A plant adapted to saline conditions is called a
halophyte.
• Organisms (mostly bacteria) that can live in
very salty conditions are classified as
extremophiles, halophiles specifically.
• An organism that can withstand a wide range
of salinities is euryhaline.

31. Residence Time :
• All dissolved materials have residence times
varying from hours to millions of years.
• Nitrate has the lowest residence time and
sodium has the highest residence time in
seawater.

32. pH of Seawater :
• pH is another important property of seawater.
It is reflected in the form of acidity and
alkalinity.
• Normally pH ranges from 0 to 14.
• Seawater pH is typically limited to a range
between 7.5 and 8.4.

33. Sea surface pH

34. Particles Dispersed in Seawater:
• Particles dispersed in seawater also influence the
chemical and biological behavior of seawater.
• Biological particles are relatively large ranging from 1
micrometer to 1 mm.
• These constitute upto 70% of the particulate matter in
the ocean.
• Particles are also destroyed either mechanically or
chemically.
• These dissolved particles release nutrients, silica and
metals.
• Depositions of sediments are also due to this.

35. Oxygen Dissolved In Seawater:
• Oxygen dissolved in seawater participates in both
biological and chemical processes of oceans.
• It comes from the atmosphere.
• When nutrients concentrations are high,
dissolved oxygen concentrations are low.
• 4-5 parts per million (ppm) of Dissolved Oxygen is
the minimum amount that will support a large,
diverse fish population.
• When an organism dies and decomposes, most of
its organic molecules end up in solution as
Dissolved Organic Carbon (DOC).

36. Carbon Reservoirs:
• Oceans are considered to be the global carbon
reservoirs.

37. Density :
• The density of pure water is 1000 kg/m3.
• Ocean water is more dense because of the salt
in it.
• Density of ocean water at the sea surface is
about 1027 kg/m3.
• The density of fresh water is 1.00 (gram/ml or
kg/liter) but added salts can increase this.
• The saltier the water, the higher its density.

38. Density v/s temperature
• When water warms, it expands and becomes
less dense.
• The colder the water, the denser it becomes.
• The density of 35ppt saline seawater at 15ºC
is about 1.0255, or s (sigma)= 25.5.
• Cold sea water is denser than warm sea water.
• At 4°C and with the salinity of 35, the density
σ of sea water is 1.02781 gram per cubic
centimeter.

39. Depth –temp- density

40. Density v/s NaCl

41. Sea surface density

42. Pycnocline
• Pycnocline, in oceanography, boundary
separating two liquid layers of different densities.
• In oceans a large density difference between
surface waters (or upper 100 metres [330 feet])
and deep ocean water effectively prevents
vertical currents;
• the one exception is in polar regions where
pycnocline is absent.
• Formation of pycnocline may result from changes
in salinity or temperature.

43. Density v/s depth

44. Temperature Distribution in Oceans:
• Ocean is the major recipient of the sun’s radiant
energy.
• It has the capacity to store heat.
• Many physical processes depend on temperature of
waters.
• Changes in temperature and salinity can increase or
decrease the density of ocean waters at the surface
which can lead to convection.
• The temperature of the world's ocean is highly variable
over the surface of the ocean, ranging from less than
0°C near the poles to more than 29°C in the tropics.

45. Depth v/s
temperature

46. Sea surface temperature

47. Depth v/s temperature distribution

48. Sea surface temperature

49. • Thermoclines are defined as an area in the
ocean where warm water on the surface is
separated from deep, colder water.
• The maximum surface temperature of course
depends on many factors, like latitude and
season.
• It is heated from the surface downward by
sunlight, but at depth most of the ocean is
very cold.
• The average temperature of all ocean water is
about 3.5° C.

50. Thermocline

51. The Temperature fluctuation
• The Temperature fluctuation is minimum in
oceanic waters.
• Arctic waters are colder.
• Tropical waters are warmer.
• The variation is seasonal and diurnal.

52. Ocean animals
• Ocean animals show a varied response to the
temperatures.
• There are two kinds of animals.
• 1. Stenothermal animals – the ones which live within 20°C.
• 2. Eurythermal animals – the animals which can withstand
the wide range of temperature variations.
• The cold water forms also show an increase their in sizes.
• The surface temperature of the oceanic waters vary from
about -2 °C near the North and South poles to about 30 °C
near the equator.
• In the polar regions, the surface sea water freezes.

53. Ocean temperature also varies with
depth.
• In general, the temperature falls as the depth
increases.
• The warm surface waters extend to depths of
about 150 metres in the tropics, and about
300 metres in the subtropics.
• Below the surface waters, the temperature
drops rapidly, forming a layer called the
thermocline.

54. Thermocline

55. The thermocline varies in thickness
• The thermocline varies in thickness, from
about 300 metres to 910 metres.
• Below the thermocline, the water cools more
slowly.
• Close to the deep-sea floor, the temperature
of the ocean ranges between 1° and 4 °C.

56. Life in Sea Water :
• Oceans are the homes for large amount of
marine population.
• How many species are there in the sea? Some
230,000 recorded so far.
• Over 17,000 species thrive in the deep sea
where no light penetrates the ocean waves.
• There are 38,000 different kinds of microbes
in a liter of seawater.

57. Biological Conditions of Seawater
• These include processes that occur at
molecular scales, such as photosynthesis,
respiration, and cycling of essential nutrients,
to large scale processes such as effects of
ocean currents on marine productivity.

58. Primary Productivity :
• Primary productivity is the process by which
inorganic forms of carbon are synthesized by
living organisms into simple organic
compounds.
• Both photosynthesis and chemosynthesis
contribute to the primary production of the
oceans.

59. Nitrogen and Phosphorous :
• Phosphorous and nitrogen compounds are
necessary for phytoplankton growth.
• These two compounds are more abundant in
deep waters than the near surface waters, as
they are removed by plant growth near the
surface.

60. • Dissolved Organic Matter: Seawater is
dominated by much amount of dissolved organic
matter. Dissolved organic matter remains in the
ocean for very long periods of time. May be many
thousands of years. It is roughly equal to the
abundance of living matter on earth. There are
many sources of organic matter. Decomposition
of dead plant and animal is one source. Secretion
of organic compounds by living plants is also
another source.

61. Forms of Organic Matter:
• The relative abundance of various forms of
organic matter in seawater are:
• Dissolved organic matter 95%.
• Particulate organic matter 5 %
• Phytoplankton 0.1%.
• Zooplankton 0.01 %
• Fishes 0.0001 %.
• Dissolved organic matter enters food webs
primarily through tiny bacteria.

62. Water Circulation :
• The variation in temp, density, salinity in oceans
is a driver of the world's ocean circulation.
• Seawater is a rich source of various commercially
important chemical elements.
• Much of the world’s magnesium is recovered
from seawater.
• In certain parts of the world, sodium chloride
(table salt) is still obtained by evaporating
seawater.
• In fact, the seawater cannot be easily duplicated
in any lab in any manner in the world.

63. Transmitting Sound Waves :
• It is about 1500 m per second.

64. Light penetration in seawater: Photic
zone
No light

65. Pressure with depth

66. Depth v/s sound velocity

67. Carbon-di-oxide exchange

68. Thank you