(a) Salinity is the degree of concentration of salt solution in seas and oceans. This degree of saltiness of water is expressed in percentage or parts per thousand. The average salinity of ocean is 35.2 per thousand. Salinity varies across seas and oceans.
(b) Causes of variations in salinity of seas and oceans;
(i) The rates of evaporation
(1) The water around the high pressure belts of the Trade Wind Deserts between 20° and 30° N and S have high salinity because of the high rate of evaporation caused by high temperature and low humidity
(2) The temperature oceans have a lower rate of evaporation
(ii) The amount of fresh water added by precipitation, streams and icebergs:
(1) Salinity is lower in equatorial waters because of heavy rainfall and high relative humidity
(2) Oceans into which huge rivers like the Amazon, Congo and, Ganges flow have a lower salinity.
(3) Melting of icebergs add freshwater to Baltic, Arctic and Antarctic waters and thereby lower their salinity
(iii) The degree of water mixing by currents;
(1) Salinity is high in wholly or partially enclosed seas e.g. Caspian sea because the water does not mix free with the ocean water and is not penetrated by ocean currents
(2) in areas of inland drainage without links to the ocean continuous evaporation causes high salinity.
(3) In open oceans where currents freely flow salinity tends to be low.
(4) Free mixing of water by surface and subsurface currents reduces salinity of oceans.
(c) Importance of ocean to man;
(1) Oceans provide natural route by which goods from one continents reach another at lower possible cost
(2) Serve as sources of fish
(3) Serve as sources of minerals
(4) Employment opportunity
(5) Oceans are important recreation centres
(6) Oceans influence the climate of adjoining coastlands.
(7) Sea weeds for the making of cosmetic and fertilizers.
(8) Source of research in aquatic plants and animals.