2.1 Composition and properties of seawater

Seawater's unique chemical makeup shapes marine life and ecosystems. Its complex mixture of salts, gases, and organic compounds creates a delicate balance crucial for ocean inhabitants. Understanding these components is key to grasping how marine organisms thrive.

Salinity, temperature, and dissolved gases play vital roles in seawater properties. These factors influence density, circulation patterns, and the distribution of marine life. By examining these elements, we gain insight into the ocean's dynamic nature and its impact on global climate.

Chemical Composition and Properties of Seawater

Chemical components of seawater

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• Seawater is a complex mixture of dissolved salts, gases, and organic compounds
  • Salinity total concentration of dissolved salts in seawater, typically around 35 parts per thousand (ppt) or 3.5%
• Major ions in seawater:
  • Chloride (Cl⁻) 55% of total dissolved salts
  • Sodium (Na⁺) 30.6% of total dissolved salts
  • Sulfate (SO₄²⁻) 7.7% of total dissolved salts
  • Magnesium (Mg²⁺) 3.7% of total dissolved salts
  • Calcium (Ca²⁺) 1.2% of total dissolved salts
  • Potassium (K⁺) 1.1% of total dissolved salts
• Minor components include bicarbonate (HCO₃⁻), bromide (Br⁻), borate (BO₃³⁻), and fluoride (F⁻) which contribute to the unique properties of seawater (buffering capacity, osmotic balance)
• Trace elements essential for marine life but present in very low concentrations (iron, zinc, copper) play crucial roles in biological processes (photosynthesis, enzyme function)

Salinity measurement and concept

• Salinity is the total concentration of dissolved salts in seawater, expressed in parts per thousand (ppt) or practical salinity units (PSU)
• Average ocean salinity is approximately 35 ppt or 35 PSU but can vary regionally (Baltic Sea, Red Sea)
• Salinity is measured using:
  • Conductivity electrical conductivity of seawater increases with higher salt content
    • Measured using a conductivity meter or CTD (Conductivity, Temperature, Depth) sensor
  • Density seawater density increases with higher salinity
    • Measured using a hydrometer or calculated from conductivity and temperature measurements
  • Refractive index the way light bends when passing through seawater changes with salinity
    • Measured using a refractometer
• Salinity varies depending on location and depth due to factors such as evaporation, precipitation, river input, and ice formation (coastal regions, polar seas)

Factors influencing seawater properties

• Density of seawater is primarily influenced by:
  • Temperature seawater density decreases as temperature increases
    • Thermal expansion water expands as it warms, reducing its density
  • Salinity seawater density increases with higher salinity
    • Dissolved salts increase the mass of seawater without significantly increasing its volume
  • Pressure seawater density increases with increasing pressure (depth)
    • Compression water molecules are forced closer together under higher pressure
• Temperature of seawater is influenced by:
  • Solar radiation the primary source of heat for the ocean surface
    • Varies with latitude (equator vs poles), season (summer vs winter), and time of day (noon vs midnight)
  • Atmosphere-ocean heat exchange heat is transferred between the atmosphere and ocean surface through conduction, convection, and evaporation/condensation (sea breezes, hurricanes)
  • Ocean circulation currents redistribute heat from warmer to cooler regions (Gulf Stream, Antarctic Circumpolar Current)
  • Mixing wind-driven and tidal mixing can bring cooler, deeper water to the surface or warmer surface water to greater depths (upwelling, downwelling)

Dissolved gases in marine environments

• Major dissolved gases in seawater:
  • Oxygen (O₂) essential for aerobic respiration in marine organisms
    • Enters seawater through photosynthesis by marine plants and algae, and through gas exchange at the air-sea interface
    • Solubility decreases with increasing temperature and salinity (warmer, saltier water holds less oxygen)
  • Carbon dioxide (CO₂) used by marine plants and algae for photosynthesis
    • Enters seawater through gas exchange at the air-sea interface and respiration by marine organisms
    • Solubility increases with decreasing temperature and increasing pressure (colder, deeper water holds more CO₂)
    • Dissolved CO₂ forms carbonic acid (H₂CO₃), which can lower seawater pH (ocean acidification)
  • Nitrogen (N₂) used by some marine microorganisms for nitrogen fixation
    • Enters seawater through gas exchange at the air-sea interface
• Dissolved gas concentrations are influenced by:
  • Temperature gas solubility generally decreases with increasing temperature (warm water holds less gas)
  • Salinity gas solubility decreases with increasing salinity (saltier water holds less gas)
  • Pressure gas solubility increases with increasing pressure (depth) (deeper water holds more gas)
  • Biological activity photosynthesis, respiration, and decomposition can affect local dissolved gas concentrations (oxygen depletion in eutrophic waters)