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Major Constituents
As you might guess, the most important component of seawater is water, H2O, which makes up 96.5 percent of the total by mass. Abundant ions include chloride with 1.9 percent of total mass, sodium with 1.1 percent of total mass, sulfate with 0.3 percent of total mass, and magnesium, calcium, potassium and bicarbonate with even smaller percentages. Note that in solid form, sodium chloride or magnesium sulfate are salts that exist as crystals; when they dissolve in water, however, the ions dissociate and are no longer held together.
Minor Components
Some other elements are found in seawater at concentrations measured in parts per million. These include bromide ions with 65 parts per million (ppm), strontium with 8 ppm, boron with 4 ppm, silicon with 3 ppm and fluoride with 1 ppm. Silicon's presence in seawater stems from minerals such as the quartz that makes up beach sand; quartz and sand are made from silicon dioxide. In seawater, tiny plankton called diatoms use these dissolved silicates to make their transparent cell walls.
Trace Components
Many other elements are present in even more minute amounts; some of these elements, however, are absolutely essential for life in the oceans. Nitrogen in biologically available form, for example, constitutes a mere 280 parts per billion in seawater--yet neither plankton nor anything else in the ocean could survive without it. The same is true for phosphorus with 30 pars per billion; another important nutrient is iron with 6 parts per billion. Among the many other elements it contains, seawater also has iodine with 60 parts per billion, lithium at 125 parts per billion, lead at 0.04 parts per billion and mercury at 0.03 parts per billion.
Dissolved Gases
Most solids dissolve more readily in hot water than in cold; gases, by contrast, are exactly the reverse because gas molecules are better able to escape when they are moving more quickly. The most abundant dissolved gas in seawater is nitrogen. Although all living organisms require nitrogen, this dissolved gas is useless to them; it must be converted into biologically useful forms such as nitrates before any other organisms can use it. Thirty-six percent of the gas dissolved in seawater is oxygen; this equates to a mere 6 parts per million, a very small amount yet sufficient to supply organisms such as fish with the oxygen they need to breathe. Finally, dissolved CO2 constitutes about 15 percent of the total dissolved gases; this CO2 is important as the source of all the carbon in the marine food chain since organisms like plankton and cyanobacteria use it in photosynthesis.