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Unlock This CourseSeawater isn't just water and salt - it's also full of dissolved gases that are absolutely vital for marine life. The two most important gases are oxygen (O₂) and carbon dioxide (CO₂). These gases don't just float around randomly; they follow specific patterns based on temperature, pressure and salinity. Understanding how these gases behave helps us understand everything from why fish live where they do, to how climate change affects our oceans.
Key Definitions:
Oxygen enters seawater mainly at the surface through contact with air and photosynthesis by marine plants. Cold water holds more oxygen than warm water - that's why polar seas are often rich in marine life. Oxygen levels decrease with depth as organisms use it up for respiration.
CO₂ behaves differently from oxygen. It's more soluble in cold water and can form carbonic acid, making seawater slightly acidic. Unlike oxygen, CO₂ levels often increase with depth due to decomposition of organic matter and less photosynthesis.
Temperature is the biggest factor controlling how much gas can dissolve in seawater. Think of it like fizzy drinks - cold cola holds more bubbles than warm cola. The same principle applies to gases in seawater, but oxygen and carbon dioxide respond differently.
As water temperature increases, gas solubility decreases. This happens because warmer water molecules move faster and have more energy, making it harder for gas molecules to stay dissolved. This relationship is crucial for understanding marine ecosystems and ocean circulation.
High oxygen levels (up to 14-15 mg/L). Supports abundant marine life. Found in polar regions and deep ocean waters.
Moderate oxygen levels (8-12 mg/L). Seasonal variations affect gas concentrations. Common in mid-latitude oceans.
Lower oxygen levels (6-8 mg/L). Warmer temperatures reduce gas solubility. May create oxygen minimum zones.
In the North Atlantic, cold surface waters can hold up to 15 mg/L of dissolved oxygen, whilst tropical surface waters typically hold only 6-7 mg/L. This difference drives massive ocean circulation patterns as dense, oxygen-rich cold water sinks and flows towards the equator, whilst warm, less dense water flows poleward at the surface.
Salinity (salt content) also affects gas solubility, though not as dramatically as temperature. Higher salinity means lower gas solubility because salt ions take up space that gas molecules would otherwise occupy. Pressure increases gas solubility, which is why deep ocean waters can hold more dissolved gases despite being cold.
Seawater typically contains about 35 grams of salt per kilogram of water (35 ppt). As salinity increases, the ability to hold dissolved gases decreases. This "salting out" effect is more pronounced for oxygen than carbon dioxide.
River mouths, polar regions with melting ice and areas with high rainfall have lower salinity. These waters can hold more dissolved oxygen, supporting diverse marine ecosystems.
Enclosed seas like the Mediterranean and areas with high evaporation have higher salinity. Reduced gas solubility can create challenging conditions for marine life.
Water density increases with lower temperature, higher salinity and higher pressure. Dissolved gases also affect density, though to a smaller extent. Dense water sinks, carrying dissolved gases to deeper layers. This process is fundamental to ocean circulation and the global distribution of marine life.
Cold, salty water is denser than warm, fresh water. When surface water becomes dense enough, it sinks, taking dissolved oxygen with it. This process, called thermohaline circulation, distributes oxygen throughout the ocean depths and brings nutrient-rich water to the surface.
Around Antarctica, extremely cold surface water (often below 0°C) becomes very dense and sinks to the ocean floor. This water is rich in dissolved oxygen and flows northward along the sea floor, providing oxygen to deep ocean basins worldwide. This process takes hundreds of years, making deep ocean oxygen levels vulnerable to surface changes.
Marine organisms depend entirely on dissolved oxygen for survival, just like we depend on oxygen in air. Different species have different oxygen requirements, which explains why we find different communities at different depths and locations.
High oxygen levels support active fish, marine mammals and photosynthetic plankton. Constant gas exchange with atmosphere maintains saturation.
Oxygen minimum zones occur where consumption exceeds supply. Specialised organisms with low oxygen requirements dominate these areas.
Cold temperatures allow higher oxygen storage, but limited supply creates unique deep-sea communities adapted to stable, low-oxygen conditions.
Carbon dioxide doesn't just dissolve in seawater - it reacts with it to form carbonic acid. This makes the ocean slightly acidic and affects shell-forming organisms like corals, molluscs and some plankton. As atmospheric CO₂ increases, oceans become more acidic, a process called ocean acidification.
When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which then breaks down into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions. This buffering system helps regulate ocean pH, but it has limits. Too much CO₂ overwhelms the system, leading to acidification.
The Great Barrier Reef faces dual threats from dissolved gases. Rising temperatures reduce oxygen solubility and cause coral bleaching, whilst increasing CO₂ makes seawater more acidic, making it harder for corals to build their calcium carbonate skeletons. These combined effects threaten one of the world's most diverse marine ecosystems.
Scientists use various methods to measure dissolved gases in seawater. Understanding these measurements helps us track ocean health and predict changes in marine ecosystems.
Measured in milligrams per litre (mg/L) or as percentage saturation. Healthy surface waters are typically 90-100% saturated. Values below 2 mg/L create hypoxic conditions harmful to most marine life.
Measured as partial pressure (pCO₂) or pH levels. Surface ocean pCO₂ has increased from about 280 to over 410 parts per million, tracking atmospheric increases.
Climate change significantly affects dissolved gas patterns in our oceans. Warmer temperatures reduce gas solubility, whilst changing weather patterns affect mixing and circulation. These changes have profound implications for marine ecosystems and global ocean health.
As oceans warm, they'll hold less dissolved oxygen, creating larger oxygen minimum zones. Simultaneously, continued CO₂ absorption will increase acidification. These changes will reshape marine ecosystems, potentially reducing biodiversity and altering food webs that humans depend upon.