Ocean Chemistry & Life
Salinity, pH, dissolved O₂, zones & life
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Reading: Ocean Chemistry & Life
Ocean Chemistry & Life
The ocean is not just salt water, it is a complex chemical system. Its salinity, pH, temperature, and dissolved oxygen levels determine where life can thrive.
Salinity & Dissolved Oxygen
Salinity is approximately 35 g of dissolved salts per kg of seawater (3.5% by mass), mostly sodium chloride. The salt comes from minerals dissolved by rivers over billions of years and from hydrothermal vents on the ocean floor. Salinity varies: higher in warm, enclosed seas with high evaporation (e.g., Red Sea ~40 g/kg) and lower where rivers or melting ice dilute the water (e.g., Baltic Sea ~7 g/kg).
Dissolved oxygen is essential for virtually all marine animals. It is highest near the surface, where the ocean contacts the atmosphere and phytoplankton photosynthesize. Cold water holds more dissolved oxygen than warm water. Between roughly 200–1,000 m lies the oxygen minimum zone, where bacteria decomposing sinking organic matter consume oxygen faster than it is replenished.
Ocean pH & Acidification
Ocean pH is currently about 8.1 on average, slightly basic. Pre-industrial ocean pH was approximately 8.2. The ocean absorbs roughly 25–30% of human-emitted CO₂. When CO₂ dissolves in seawater, it forms carbonic acid, which releases hydrogen ions and lowers pH, this is ocean acidification. The 0.1 unit decline since pre-industrial times represents about a 26% increase in hydrogen ion concentration, since the pH scale is logarithmic.
Important: the ocean is not becoming acidic (below pH 7), it is becoming less basic. Lower pH reduces the availability of carbonate ions that corals, shellfish, and many plankton species need to build calcium carbonate shells and skeletons, threatening organisms at the base of marine food webs.
Ocean Life Zones
Coral Reefs and Hydrothermal Vents
Coral reefs are built by coral animals that host symbiotic algae called zooxanthellae. The algae photosynthesize and provide corals with food and color. When stressed, primarily by warming, corals expel the algae and turn white, a process called coral bleaching. Prolonged or repeated bleaching can lead to starvation, disease, and coral death, though corals can recover if stress ends soon enough.
Hydrothermal vents are cracks in the ocean floor where superheated mineral-rich water emerges. Bacteria use chemosynthesis, chemical energy instead of sunlight, to produce food, supporting entire ecosystems in complete darkness, including tube worms and specialized shrimp.