Notes on Seawater Description

Seawater Description

Marine organisms must adapt to the chemical and physical nature of their environment, especially the seawater.

One of the fascinating things about water is its special properties. These properties are very important to life on Earth.

Learn about the characteristics of pure water

Salinity and Temperature of Seawater Description

• Seawater contains pure water with dissolved material
• Salinity averages 3.5 percent for most oceans
• Salinity is the total amount of salt dissolved in sea water
• Although the seawater contains many different solutes, about 99% of the solutes (dissolved material) consists of only 6 ions
• Sodium (Na+) and Chloride (Cl-) consist of about 85% of solute concentration in seawater
• Salinity of water is very important to the lives of many organisms
• Marine organisms that live in seawater can die in freshwater
• Even though the total amount of salt will vary from place to place in the seawater, major ions in the seawater will remain constant
• Rule of constant proportions: relative amounts of the different types of ions in seawater will always be the same.
• This suggests that salinity will vary more likely due to the addition or removal of pure water than the addition/removal of solutes
• For example, addition of freshwater will decrease the salinity.
• The saltier the water, the denser water
• Therefore, density of seawater can increase by an increase in salinity, a decrease in temperature or both

• The temperature in seawater varies more than salinity
• The temperature in the seawater can vary from -2°C to 30°C
• Salt water can freeze at a colder temperature than freshwater
• This is why rivers and lakes are more likely to freeze than seawater

Dissolved Gases in Seawater Description

• Oxygen (O2), Carbon Dioxide (CO2), and Nitrogen (N2) are the most important gases in seawater for marine organisms
• The ocean and atmosphere participate in gas exchange
• Oxygen, carbon dioxide, and nitrogen are found in the atmosphere and dissolves onto the surface of the seawater
• The surface of the sea can also release gas onto the atmosphere
• Gases dissolve better in cold water than in warm water

• True or false? Dissolved gas concentrations are higher in polar water than in the tropics?  True!

• Oxygen is not very soluble in sea water
• More oxygen concentration is in the air than in seawater
• Seawater contains about 4 to 6 milliliters of dissolved gas per 1 liter of seawater
• However, the total volume of air contains about 21% (210 ml) of oxygen
• The oxygen produced in seawater is by photosynthesis of certain capable marine organisms
• This oxygen gets released into the atmosphere
• Cellular respiration removes dissolved oxygen from the seawater
• Carbon dioxide consists of more than 80% of the dissolved gas concentration in the ocean
• The ocean stores much more CO2 than the air
• Carbon dioxide is more soluble in the ocean than oxygen due to its chemical properties

Transparency in Seawater Description

• Sunlight can penetrate into the ocean, because the ocean is relatively transparent
• If the seawater was not transparent, then photosynthesis would hardly occur in the sea except on the surface
• Blue light penetrates the deepest in the ocean water
• Red light penetrates the least in the ocean water
• Red is the most absorbed
• Transparency of water is greatly affected by the substances that are in the seawater
• For example, a lot of plankton can reduce the transparency of seawater

Pressure in Seawater Description

• Pressure increases with depth because the amount of water above gets greater
• The deeper the you go, the greater the pressure
• This is because more water presses down on you from above
• As pressure increases, gases are becoming more compressed
• Because water is heavier than air, organisms that live in the sea experience more pressure than organisms living on land
• Many marine organisms have gas filled internal organs; therefore, they can only reside in certain depth range

Ocean circulation
Ocean circulation affects the Earth’s climate and all habitats on land and sea

Surface Circulation

• Surface currents and the wind are influenced by Coriolis Effect

• Major wind patterns are created by the rising of warm air and the sinking of cold air
• Most of the sun’s energy is absorbed near the Equator
• The heat from the solar energy warms the air.
• The air becomes dense and it rises
• Colder air from adjacent areas blows into the areas where warm air rises, creating wind
• Colder air replaces the rising warm air in the equator, which created wind called trade winds.

• Major surface currents are influenced by the wind and heat energy of the sun.
• Water column has layers in which the least dense and relatively warm waters are the surface and the densest and coldest waters are at the bottom at the ocean.

• The ocean has three major layers
• 1. Surface layer
• Also known as mixed layer due to the mixture of waves, currents and wind
• Surface layer consists of the top 100 to 200 meters of the ocean
• 2. Intermediate layer
• This is the layer below the surface layer, which covers about 1,000 to 1,500 meters in depth
• The intermediate layer has the main thermocline
• The main thermocline is where the warm surface water transitions to cold water
• 3. Deep layer
• The deep layer is the layer below about 1,500 meters in depth
• This layer contains cold water, around 4°C.

Stability of Seawater

• Water columns are very stable because of the big difference is density between the shallow and deep water.
• Because of the high stability, large amount of energy is required to mix the deep and shallow waters.
• Water columns are stable especially when the surface layer floats on top of the denser water layers.
• Water columns, however, can become unstable when surface layer is denser the water below it
• Overturn is when the surface water sinks and mixes with the deeper water
• When this higher density surface water descends through the water column this is called downwelling
• Overturn occurs because the surface water cools and becomes slightly denser than the layer below it
• Places in the temperate and polar regions usually experience overturn during the winter
• Thermohaline circulation is the movement of water driven by changes in density
• The global thermohaline circulation is called the great ocean conveyor
• This is when surface water sinks (due to density) deep into the ocean and near the bottom, spreads around and flows back to the surface
• The great ocean conveyor plays an important role in the earth’s climate
• A slight variation is the conveyor circulation can bring about major changes in weather pattern around the globe
• The great ocean conveyor circulation also brings dissolved oxygen into the deeper parts of the sea

This ends the notes on seawater description, but we encourage you to learn more about waves and tides.

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