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UNIT 3 PHYSICOCHEMICAL PROPERTIES OF MARINE
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loss (evaporation) and gain (runoff and precipitation). Typically, seawater has a salinity of 35 parts per thousand.
Freezing: Seawater freezes at a temperature slightly lower than for fresh water (0.0° Celsius). The freezing temperature of seawater changes with the concentration of salts with more salts lower the initial freezing temperature.
At a salinity of 35 parts per thousand, seawater freezes at a temperature of -1.9° Celsius.
Sea ice contains less salt than seawater because most of the salts in liquid seawater are forced out during freezing. Because of the density difference between ice and seawater, ice floats on the surface of the ocean.
Dissolved gases: Sea water also contains gases dissolved from atmosphere.
Dissolution of gases in seawater depends on temperature and salinity.
Increasing temperatures and salinities decrease the amounts of dissolved gases in seawater. The gases dissolved in seawater include nitrogen, oxygen, and carbon dioxide (majorly in the form of bicarbonate). The dissolved carbon dioxide is usually more than other gases. Some gases are involved in oceanic organic and inorganic processes e.g. oxygen and carbon dioxide may be temporarily generated or depleted by such processes at varying concentrations at specific locations in the ocean.
The density of seawater increases with decreasing temperature, increasing salinity, and increasing depth in the ocean. Highest densities are achieved with depth because of the overlying weight of water. In the ocean, seawater density values vary between 1.022-1.030g/cm3 depending on salinity.
Density has important effects on ocean water. Denser bodies of water sink below less dense water bodies.
Factors that affect the density of seawater
1. Temperature: As temperature increases, density decreases.
Temperature has the greatest effect on density of seawater. A layer of rapidly changing temperature is called the thermocline.
2. Salinity: As salinity increases, density increases (addition of more dissolved materials).
3. Pressure: As pressure increases, density increases (pressure compresses materials). A layer of rapidly changing density is called the pycnocline. Above the pycnocline, the layers of water are thoroughly mixed by currents, tides and waves. In the high latitudes (temperate and polar regions), pycnoclines and thermoclines rarely
45 form and the water column is described as isothermal and isopycnal i.e. same temperature and density.
Salinity: This is the total amount of solids dissolved in water including gases. The salinity of seawater is 3.5% i.e. 96.5% is pure water. But salinity is measured in parts per thousand i.e. 35parts per thousand or 35g/kg or 35‰ or 35ppt.The major constituents of seawater are chloride, sodium, sulphate, magnesium; calcium and potassium (contribute about 99% of sea salts). These elements are relatively constant regardless of which ocean is involved. Chlorine makes up about 55%of the salt in seawater. Minor constituents include bromine, carbon, nitrogen gas, carbon dioxide. Trace compounds are phosphorus, iodine, manganese and iron. Salinity varies in the ocean between35-38ppt. In coastal areas, salinity can be decreased e.g.
10 ppt in river estuaries (brackish water- freshwater and sea water mixing).
Salinity in Red sea is 42 ppt (hyper-saline).
Hyper-salinity occurs in areas with limited circulation or connection with the open sea. Dead Sea has a salinity of 330ppt. Salinity varies with season and processes that increase salinity include removal of water through evaporation, formation of sea ice, loss of water through sea spray, addition of minerals from dead and decayed organisms form part of the sediments.
Processes that decrease salinity include addition of water from melting if ice (iceberg, glacier), river/ stream flow and rainfall runoff.
Alkalinity and acidity: Oceans should be acidic but this is prevented by the carbonate buffering system. In deep cold water, more carbon dioxide dissolves in seawater, this should make the deep sea more acidic but when marine organisms that contain calcium carbonate in their skeleton or shells die, the calcium carbonate they contain is released and helps to buffer the acidic conditions.
3.2 Adaptations of Living Organisms to Marine Environment Water is essential for life and constitutes 80% or more by weight of active protoplasm. It is the most efficient of all solvents and carries in solution gases, oxygen and carbon dioxide; the mineral substances necessary to the growth of plants and animals. It is an essential raw material in the manufacture of foods by plants.
Organisms living in the terrestrial environment have devised means for survival, such as impervious integuments, to conserve water. Plants on land have roots and vascular systems to transport water to all growing parts. In the marine environment there is freedom from desiccation, except at
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tide levels, and there are no highly specialised means for conservation of water or for its transport in plants.
The high heat capacity of water and its high latent heat of evaporation prevent the danger that might result from rapid change of temperature in the environmental medium. Owing to the high degree of transparency of water it is possible for the sea to sustain plant life throughout a relatively deep layer, and in animals the development of organs of vision and of orientation has progressed to a marked degree.
Sea water is a buffered solution implying that changes from acid to alkaline condition, or vice versa, are resisted. This property is of vital importance to the marine organisms, mainly for two reasons:
i. Abundant supply of carbon can be available in the form of carbon dioxide for the use of plants to synthesize carbohydrates without disturbance to the animal life that may be sensitive to small changes in pH.
ii. In the slightly alkaline habitat, organisms that construct shells of calcium carbonate or other calcium salts can carry on this function more efficiently than in a neutral solution.
Sea water supports the bodies of marine organisms reducing the need for special supporting skeletal structures e.g. jelly fishes, unarmored molluscs, unarmored dinoflagellates, and large marine mammals.
Sea water contains all the chemical elements essential to the growth and maintenance of plant and animal protoplasm. The ratios of the major salts to each other, and their total concentration, are similar in sea water and in the body fluids of marine invertebrates. The similarity of composition is also found in modified form in both terrestrial and fresh-water animals.
The teleost (bony) fishes in marine waters are hypotonic (the pressure of external medium is less than the internal medium) and, in order to keep their body fluids down to the required osmotic pressure, they secrete chloride through the “chloride cells” of the gills. This function is a regulation toward a low osmotic pressure of the blood, as opposed to regulation toward a high one as performed by the kidneys of animals in fresh-water environments. The elasmo branchs (sharks and rays) are isotonic with sea water
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4.0 CONCLUSION
Sea water differs from freshwater and brackish water in having higher concentrations of salts from rock weathering and runoff. The presence of dissolved salts, gases etc affect the water quality.
5.0 SUMMARY
In this unit, we discussed the characteristics of sea water and the adaptation of sea organisms for life in this environment.
6.0 TUTOR-MARKED ASSIGNMENT
1. Explain the characteristics of seawater.
2. Discuss the adaptation of living organisms to marine environment.
7.0 REFERENCES/FURTHER READING
Fundamentals of Physical Geography. (2nd ed.). .Accessed on 9th February, 2013.http://physical geography.net/fundamentals/8p.html Gutierrez, J. J. (2012). Estuaries and Coastal wetlands: where freshwater
and salt water mix.www.helium.com/item2347345estuaries-and-coastal-wetlands-where-freshwater- and- saltwater- mix.html www.marinepolicy.net (accessed 9th February, 2013). Oceanography 5-Properties of Water.pdf
Pidwirny, M. (2006). Physical and Chemical Characteristics of Seawater.
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MODULE 3
Unit 1 Major Water Compartments Unit 2 Hydrology and Water Cycle Unit 3 Transport and Exchange Processes
UNIT 1 MAJOR WATER COMPARTMENTS