What is Salinity?
- Salinity refers to the amount of dissolved salts in seawater, mainly sodium chloride (NaCl).
- It is measured in parts per thousand (ppt or ‰).
- Average ocean salinity = 35‰ (35 grams of salt per 1,000 grams of water).
📌 Fact: Dead Sea has one of the highest salinities (~330‰), allowing people to float easily!
Composition of Sea Water 🌊
| Component | Percentage (%) |
|---|---|
| Water (H₂O) | 96.5% |
| Sodium Chloride (NaCl) | 2.7% |
| Magnesium, Calcium, Potassium, Sulfates | 0.8% |
📌 Fact: Salt in oceans comes from weathering of rocks, river runoff, and volcanic eruptions.
Factors Affecting Ocean Salinity
1. Evaporation 🌞
✅ More evaporation = Higher salinity.
✅ Example: Red Sea (41‰) has high salinity due to high evaporation.
2. Precipitation (Rainfall) 🌧️
✅ More rainfall = Lower salinity (dilutes saltwater).
✅ Example: Equatorial regions have lower salinity due to heavy rainfall.
3. River Runoff 🏞️
✅ More freshwater from rivers = Lower salinity.
✅ Example: Amazon River reduces Atlantic Ocean salinity near its mouth.
4. Ocean Currents & Circulation 🌊
✅ Cold currents bring low salinity water.
✅ Warm currents increase evaporation → higher salinity.
✅ Example: Gulf Stream (North Atlantic) increases salinity, while Peru Current (Pacific) lowers salinity.
5. Ice Formation & Melting ❄️
✅ Ice formation increases salinity (removes freshwater).
✅ Ice melting decreases salinity (adds freshwater).
✅ Example: Arctic Ocean has lower salinity due to melting ice.
Distribution of Salinity in Oceans
| Region | Salinity Level | Example |
|---|---|---|
| Tropical Regions | High (>37‰) | Red Sea, Persian Gulf |
| Equatorial Regions | Moderate (~35‰) | Atlantic near Amazon |
| Polar Regions | Low (~32-34‰) | Arctic Ocean, Southern Ocean |
| Closed Seas & Lakes | Very High (>50‰) | Dead Sea, Great Salt Lake |
📌 Fact: The Dead Sea (~330‰) and Lake Van (Turkey, ~220‰) are among the saltiest water bodies.
Salinity Variation in Different Water Bodies
| Water Body | Salinity (‰) | Special Feature |
|---|---|---|
| Oceans (Average) | 35‰ | Normal seawater salinity |
| Red Sea | 41‰ | High evaporation, less river inflow |
| Baltic Sea | 6-18‰ | Low due to heavy river inflow |
| Dead Sea | 330‰ | Saltiest large water body |
| Great Lakes (Freshwater) | 0.5‰ | Almost no salt |
📌 Fact: The Baltic Sea has the lowest ocean salinity (6-18‰) due to high freshwater input.
Importance of Salinity
✔ Controls Ocean Density – Higher salinity = Higher density, affecting ocean currents.
✔ Influences Climate & Weather – Ocean salinity impacts monsoons and global climate.
✔ Affects Marine Life – Organisms adapt to specific salinity levels (e.g., corals need stable salinity).
✔ Drives Ocean Circulation – Saltwater movement creates deep ocean currents (Thermohaline Circulation).
📌 Example: The Atlantic Meridional Overturning Circulation (AMOC) is driven by salinity and temperature.
Threats to Ocean Salinity 🚨
❌ Climate Change – Melting ice caps dilute seawater, disrupting ocean currents.
❌ Pollution & River Damming – Less freshwater reaches oceans, altering salinity.
❌ Desalination Plants – Removing too much salt can harm marine life.
❌ Excessive Evaporation – Global warming increases ocean salinity in dry regions.
✅ Conservation Measures:
- Protect glaciers & ice caps to prevent excessive dilution.
- Reduce industrial pollution to maintain natural water balance.
- Monitor salinity changes for climate research & disaster prediction.
📌 Example: The Indian Ocean is becoming saltier due to high evaporation and reduced rainfall.
MCQs on Salinity
1. What is the average salinity of ocean water?
a) 25‰
b) 30‰
c) 35‰
d) 40‰
Answer: c) 35‰
2. Which sea has the highest salinity?
a) Red Sea
b) Baltic Sea
c) Dead Sea
d) Arabian Sea
Answer: c) Dead Sea
3. What happens when ocean salinity increases?
a) Water becomes less dense
b) Water becomes more dense
c) Ocean currents stop
d) Ice caps melt
Answer: b) Water becomes more dense
4. Which factor decreases ocean salinity?
a) Evaporation
b) Ice formation
c) River runoff
d) High temperature
Answer: c) River runoff
5. What is the salinity of the Baltic Sea?
a) 35‰
b) 41‰
c) 6-18‰
d) 50‰
Answer: c) 6-18‰
Short Answer Questions
1. What is salinity?
Salinity is the amount of dissolved salts in seawater, measured in parts per thousand (ppt or ‰).
2. What is the average salinity of oceans?
The average salinity of oceans is 35‰ (35 grams of salt per liter of seawater).
3. Name three factors that affect ocean salinity.
- Evaporation (increases salinity).
- Precipitation & River Runoff (decreases salinity).
- Ice Formation & Melting (affects local salinity).
4. Why is the Dead Sea so salty?
- High evaporation and low freshwater inflow cause salt concentration to rise.
- No rivers flow out, trapping the salt.
5. How does salinity affect ocean currents?
- Higher salinity = Denser water, which sinks and drives deep ocean currents.
- This process is called thermohaline circulation.
📌 Example: The Atlantic Ocean’s salinity controls the Gulf Stream and monsoons.
Conclusion
Salinity influences ocean currents, climate, and marine life. Understanding it helps in climate studies, oceanography, and weather forecasting.
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