Determinants of Salinity

Quick Summary

• Salinity measures salt concentration in ocean water (average: 35 ppt)
• Four main factors control salinity levels worldwide
• Evaporation increases salinity, precipitation decreases it
• Red Sea has highest salinity (40 ppt), Baltic Sea has lowest (7 ppt)
• Understanding salinity is crucial for marine life and ocean currents

What Determines Ocean Salinity?

Ocean salinity does not stay the same everywhere. Some seas are saltier than others. The Dead Sea in the Middle East is so salty that you can float on it without trying. The Baltic Sea in Europe has very low salt because many rivers flow into it.

Four main factors control how salty ocean water becomes. These factors work together. Sometimes they increase salinity. Other times they decrease it.

1. Rate of Evaporation

Evaporation is the biggest factor that increases salinity. When the sun heats ocean water, water molecules escape into the air as vapor. But salt cannot evaporate. The salt stays behind in the ocean.

Hot areas near the equator have high evaporation rates. Places like the Red Sea and Persian Gulf lose lots of water to evaporation. This makes them very salty. The Red Sea has 40 ppt salinity because it is hot and surrounded by desert.

Cold polar regions have low evaporation. The Arctic Ocean stays less salty because water does not evaporate quickly in cold temperatures.

2. Fresh Water Supply

Fresh water from rivers, rain, and melting ice dilutes ocean salt. This lowers salinity. The more fresh water that enters the ocean, the less salty it becomes.

Precipitation (Rain and Snow): Heavy rainfall areas have lower salinity. The Gulf of Guinea receives heavy rain throughout the year. This keeps salinity around 30 ppt, which is below average.

River Discharge: Big rivers like River Niger, River Amazon, and River Congo carry fresh water to the ocean. Near river mouths, salinity drops sharply. Lagos lagoon has low salinity because River Ogun flows into it.

Ice Melt: When polar ice melts in summer, it adds fresh water to the ocean. This lowers salinity near Greenland and Antarctica. But when sea water freezes in winter, salt is left behind, increasing salinity.

3. Ocean Currents and Water Mixing

Ocean currents move water around the globe. They mix high-salinity water with low-salinity water. This creates more balanced salt levels.

The Gulf Stream current carries warm, salty water from the Caribbean to the North Atlantic. The cold Labrador Current brings fresh water south. Where they meet, salinity levels balance out.

In enclosed seas with poor water mixing, salinity can become extreme. The Dead Sea has no outlet and high evaporation. Its salinity reaches 340 ppt – ten times higher than normal ocean water.

4. Air Temperature Above the Ocean

Air temperature affects evaporation speed. Hot air holds more water vapor than cold air. This means hot regions lose more water to evaporation.

Trade wind zones between 20°N and 20°S have the highest salinity. Here, dry winds and hot temperatures cause rapid evaporation. Salinity often reaches 37 ppt in these zones.

Coastal areas with hot, dry climates also have high salinity. The coast of Mauritania in West Africa has higher salinity than humid coastal regions like Liberia.

Salinity Patterns Across the World

Why Salinity Matters

Ocean salinity affects many things. It controls ocean currents because salty water is denser and sinks. This creates deep ocean currents that move water around the world.

Marine life depends on stable salinity. Most ocean fish can only survive in water with 30-37 ppt salinity. If salinity changes too much, fish die or migrate away.

Salinity also affects climate. Salty water in the North Atlantic sinks and creates currents that carry heat from the equator to Europe. Without this, Europe would be much colder.

Common Exam Mistakes

WAEC Chief Examiners report that students often make these errors:

1. Confusing evaporation with precipitation: Remember – evaporation INCREASES salinity (removes water, leaves salt), precipitation DECREASES salinity (adds fresh water).
2. Only mentioning one factor: Questions asking for determinants require at least four factors. State all main factors for full marks.
3. Not explaining HOW factors work: Don’t just list “evaporation” – explain that it removes water while leaving salt behind, thus increasing concentration.
4. Mixing up cause and effect: High temperature causes high evaporation, which then causes high salinity. State the chain correctly.
5. Giving examples without African context: Use African examples like Red Sea, Gulf of Guinea, or River Niger mouth.

Practice Questions

Multiple Choice Questions

1. Which ocean has the highest average salinity?
a) Pacific Ocean
b) Indian Ocean
c) Atlantic Ocean ✓
d) Arctic Ocean

2. The Red Sea has very high salinity because:
a) Many rivers flow into it
b) It has high evaporation and low rainfall ✓
c) Cold currents mix the water
d) Ice melts into it during summer

3. Which of these DECREASES ocean salinity?
a) High air temperature
b) Strong sunshine
c) Heavy rainfall ✓
d) Dry trade winds

4. Ocean currents affect salinity by:
a) Adding more salt to the water
b) Mixing water of different salt levels ✓
c) Removing fresh water from the ocean
d) Stopping evaporation completely

Essay/Theory Questions

1. Explain four factors that determine ocean salinity. (8 marks)

Examiner’s tip: Use one paragraph per factor. Explain HOW each factor affects salinity, don’t just list names. Give examples from West Africa where possible.

2. Why does the Gulf of Guinea have lower salinity than the Red Sea? Give three reasons. (6 marks)

Examiner’s tip: Compare rainfall, evaporation rates, and river discharge between the two regions. Use specific examples like River Niger.

3. Describe how evaporation affects ocean salinity. (4 marks)

Examiner’s tip: Explain the process step by step – sun heats water, water evaporates, salt remains, concentration increases.

Memory Aids

Mnemonic for salinity determinants: EFTE

• E – Evaporation (increases salinity)
• F – Fresh water supply (decreases salinity)
• T – Temperature of air (affects evaporation)
• E – Exchange/mixing by ocean currents

Simple rule: Anything that removes fresh water = higher salinity. Anything that adds fresh water = lower salinity.

Related Topics

• Ocean currents and their formation
• Climate patterns in coastal regions
• Marine ecosystems and adaptation
• Water cycle and precipitation
• Temperature zones of the Earth