Chemical elements in sea water do not exist on their own but are attracted to preferential ions of opposite charge: sulphur will occur mainly as sulphate, sodium as sodium chloride, and so on.
Salinity and the main salt ions The salinity of sea water (usually 3.5%) is made up by all the dissolved salts shown in the above table.
Dissolved gases in seawater The gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature.
Note that the figures above differ slightly in differing publications.Also landlocked seas like the Black Sea and the Baltic Sea, have differing concentrations.Interestingly, their proportions are always the same, which can be understood if salinity differences are caused by either evaporating fresh water or adding fresh water from rivers. Salinity affects marine organisms because the process of osmosis transports water towards a higher concentration through cell walls.A fish with a cellular salinity of 1.8% will swell in fresh water and dehydrate in salt water.Plankton organisms (like diatoms) that make shells of silicon compounds furthermore need dissolved silicon salts (Si O2) which at 3ppm can be rather limiting.
The main salt ions that make up 99.9% are the following: By adding the µmol in last column up, multiplied by respective valences, like: -546 468 -56.2 106.6 ....This world map shows how the salinity of the oceans changes slightly from around 32ppt (3.2%) to 40ppt (4.0%).Low salinity is found in cold seas, particularly during the summer season when ice melts.Note that normal sea water is undersaturated with respect to all its salts, except for calcium carbonate which may occur in saturated or near-saturated state in surface waters.Density The density of fresh water is 1.00 (gram/ml or kg/litre) but added salts can increase this. When water warms, it expands and becomes less dense. So it is possible that warm salty water remains on top of cold, less salty water.High salinity is found in the ocean 'deserts' in a band coinciding with the continental deserts.