Topic 2: Seawater
The physical and chemical properties of the ocean.
🗺️ Topic 2 Mind Map
2.1 Composition and Salinity
Seawater is a complex solution containing dissolved salts, gases, and other substances. The total dissolved salt content is called salinity.
Composition of Seawater
| Ion | % of Dissolved Salts |
|---|---|
| Chloride (Cl⁻) | 55.0% |
| Sodium (Na⁺) | 30.6% |
| Sulfate (SO₄²⁻) | 7.7% |
| Magnesium (Mg²⁺) | 3.7% |
| Calcium (Ca²⁺) | 1.2% |
| Potassium (K⁺) | 1.1% |
| Others | 0.7% |
Factors Affecting Salinity
| Factor | Effect on Salinity | Example |
|---|---|---|
| Evaporation | Increases salinity (water leaves, salt stays) | Red Sea (~40 ppt) — hot, arid climate |
| Freshwater input | Decreases salinity (dilution) | Baltic Sea (~10 ppt) — many rivers |
| Precipitation | Decreases salinity (dilution) | Equatorial regions with high rainfall |
| Ice formation | Increases local salinity (salt excluded from ice) | Polar regions in winter |
| Ice melting | Decreases local salinity (freshwater added) | Polar regions in spring/summer |
2.2 Temperature and Density Relationships
The density of seawater is determined by three factors:
- Temperature: Colder water = denser (molecules closer together)
- Salinity: Saltier water = denser (more dissolved material)
- Pressure: Greater pressure = slightly denser (at depth)
Diagram: Ocean Depth Profile
Ocean Layering (Stratification)
| Layer | What Changes | Description |
|---|---|---|
| Thermocline | Temperature | Rapid temperature decrease with depth (~200-1000m) |
| Halocline | Salinity | Rapid salinity change with depth |
| Pycnocline | Density | Rapid density change with depth |
2.3 pH and Dissolved Gases
pH of Seawater
The pH scale measures how acidic or alkaline a solution is (0-14). Ocean surface water is naturally slightly alkaline at approximately pH 8.1.
Ocean Acidification
Ocean acidification is the decrease in ocean pH caused by the absorption of excess CO₂ from the atmosphere:
Dissolved Gases
| Gas | Sources | Importance |
|---|---|---|
| Oxygen (O₂) | Photosynthesis, atmosphere | Essential for respiration by marine organisms |
| Carbon Dioxide (CO₂) | Atmosphere, respiration, decomposition | Used in photosynthesis; causes acidification when excess |
Dissolved oxygen is highest at the surface (contact with air + phytoplankton) and decreases with depth to an oxygen minimum zone before slightly increasing in cold deep water.
2.4 Effects of Depth on Seawater Properties
As ocean depth increases, several properties change dramatically:
| Property | Change with Depth | Detail |
|---|---|---|
| Temperature | Decreases | Rapid drop at thermocline; deep water ~2-4°C |
| Light | Decreases | Photic zone (0-200m); aphotic zone below 200m |
| Pressure | Increases | +1 atm per 10m depth |
| Dissolved O₂ | Decreases then stabilises | Minimum at ~500-1000m; slightly more in cold deep water |
Measuring Water Clarity
A Secchi disc is a simple tool used to measure how far light penetrates water. The disc is lowered until it disappears from view — this depth is the Secchi depth. Greater Secchi depth = clearer water.
Factors reducing clarity: sediment, algal blooms, pollution, river runoff.
2.5 Water Cycle and Ocean Interactions
The water cycle (hydrological cycle) describes the continuous movement of water between the oceans, atmosphere, and land.
Key Processes
| Process | Description | Effect on Ocean Salinity |
|---|---|---|
| Evaporation | Sun heats ocean surface; water turns to vapour | Increases salinity (salt left behind) |
| Condensation | Water vapour cools and forms clouds | No direct effect |
| Precipitation | Rain/snow falls on ocean or land | Decreases salinity (adds fresh water) |
| Runoff | Water flows from land to ocean via rivers | Decreases coastal salinity |
Diagram: The Water Cycle and the Ocean
- The ocean holds ~97% of all water on Earth
- Evaporation removes water but leaves salt behind, increasing salinity
- River input and rain dilute seawater, decreasing salinity near coasts
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