Unit 3: The hydrosphere                                                    1   2   3   4   5

3. Water in the oceans

a)  Properties of sea water

Sea water has special properties:

- It is salty. Each liter of sea water contains about 35 grams of dissolved salts.

- It contains dissolved gases: mainly nitrogen, oxygen and carbon dioxide.

   These gases are dissolved in the water by two processes:

     - the movement of the waves which mixes water with air.

      - the activity of aquatic living beings. Oxygen is produced by the

       photosynthesis of algae; living things breathe out carbon dioxide.

- The temperature of salt water varies with depth.

     - At the sea surface, the temperature is higher.

     - In the deepest zones, the temperature of the water is lower:

        between 4°C and -2°C.

b) Movements of the sea water

Ocean waters move in three ways: waves, currents and tides.

- Waves

   They are movements of the surface water caused by the wind.

       - Waves mix water with the air above the surface, dissolving gases

         from the atmosphere (a lot of oxygen and carbon dioxide).

       - Wave action causes coastal erosion and creates beaches.

         Waves transport materials (sand and mud) along the coast and

         out to sea.

- Ocean currents

   They are masses of water which move like rivers through the sea.

   The currents are produced by wind, differences in temperature and salinity.

   Salter and cold water is denser and falls down, while water less salty and warm

   is slight and rises up. This produce circulation of water masses from Equator

   to the Poles.

- Tides

   Tides are the periodic rise and fall of the sea level.

   They are caused by the gravitational attraction of the Moon and the Sun.

3.1. Answer the questions:

a. Why is there more oxygen dissolved in sea water near the surface

    than in the deep?

b. What gases are added to sea water by the action of living beings?

c. What are the different movements of sea water in oceans?