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    • controls rates of chemical reactions
    • controls rates of biological processes (Q10: ratio of metabolic processes at 10°C difference; mostly close to 2)
    • with salinity determine water density (which controls vertical water move- ment, currents, stability/mixing)
    • determines concentration of dissolved gases in water (decrease of gas solubility with increasing temperature)
    • sets limits to species’ distribution:
      • eurytherm: wide temp. range
      • stenotherm: narrow temp. range
      • poikilotherm: body temperature = water temperature
      • stenotherm: body temperature constant and controlled

      Cirral activity of upper (solid) & lower (dashed) intertidal barnacle species; note that the upper intertidal species has a much wider temperature range it tolerates because it has to cope with high air temperature fluctuations when exposed during low tide.
    Sea Surface Temperature
    • Tropical: 25 °C
    • Subtropical: 15 °C
    • Temperate: 5 °C (northern limit) 2 °C (southern limit)
    • Polar: <0-2 °C or 5 °C

    Temperature Profiles: Seasonal Development of the Thermocline


    Potential Temperature

    • When water sinks to greater depth, it becomes warmer
    • As pressure increases with depth, water is compressed
    • Compression exerts work (in physical meaning) on water
    • This “work” or energy is converted to heat, which increases the water temperature – adiabatic process = no exchange of heat 
    • Heating is small but noticeable (ca. 0.1 °C / 1000 m depth)
    • Potential Temperature is written QT     (“Theta T”)
    • Only important at depths >1000 m