Properties of Seawater
The ocean covers 71% of Earth's surface and contains 97% of its water. Seawater averages 35 parts per thousand (ppt) salinity—primarily sodium chloride, but also magnesium, sulfate, calcium, and potassium ions derived from rock weathering and volcanic activity. Salinity affects organisms through osmotic pressure: freshwater fish placed in seawater would lose body water through osmosis and dehydrate. Temperature ranges from -2°C under polar ice to 36°C in shallow tropical seas, with a permanent thermocline (rapid temperature change layer) between 200-1,000 meters depth separating warm surface waters from cold deep waters (1-4°C). Pressure increases by one atmosphere for every 10 meters of depth—organisms at 4,000 meters experience 400 atmospheres, requiring specialized biochemical adaptations to prevent protein denaturation.