When you think of seawater, you probably think of ocean waves rolling up onto a beach and the taste of salt you get when you swim in the sea. Seawater can contain almost anything, since water runs off the land into the ocean and carries with it traces of everything it encounters along the way. However, there are some main characteristics that are fairly consistent in ocean water around the globe.
The most obvious thing about seawater is the salt content. This is normally measured at about 35 parts per thousand. It is mostly ordinary salt, sodium chloride, but there are also trace amounts of other salts containing elements such as calcium, magnesium, potassium, sulfur, bromine, and fluorine. 3.5% of the ocean’s weight is contained in these salts.
Salinity can be affected by many things. High levels of rain can reduce salinity. The flow of freshwater coming from the mouth of large rivers can create an area of minimal salinity spreading out a long way from the river mouth. Periods of drought and higher evaporation can cause salinity levels to rise because the salt doesn’t evaporate but remains in the ocean, thus increasing its proportion as compared to the remaining volume of water.
Seawater contains dissolved gases including oxygen and carbon dioxide. In the air, nitrogen is the most prevalent element (about 78%), followed by oxygen (21%), and very small amounts of other gases. In the ocean, there is more dissolved carbon dioxide than any other gas, followed by nitrogen and then oxygen. In a parallel to life on land, sea creatures with gills are able to separate the oxygen from the water to support breathing and marine plants use the carbon dioxide to fuel photosynthesis.
The balance of dissolved oxygen depends on the numbers of plants and animals that are producing and using the oxygen. Oxygen is produced only during daylight hours when photosynthesis is taking place. Some gases can be dissolved into the water when the surface is rough and being churned by the wind. Dissolved oxygen in seawater can range from zero to 20 parts per million, with higher levels normally found in the daytime and when there are large numbers of marine plants in an area.
Ocean water tends to be slightly alkaline, in the range of 7.5 to 8.5. A completely neutral pH is 7 and the acidic range is below 7. The dissolved carbon dioxide in seawater, converted to bicarbonate, helps to buffer the seawater and maintain this slight alkalinity.
Nutrients are found in seawater in the form of excretions from animals and from the decomposition of dead plants and creatures. Run off from agricultural lands carrying man-made fertilizers can also add to ocean nutrients. These nutrients tend to sink to the sea floor and reduce the nutrient levels near the surface. Plants must live near the surface because this is where the sunlight can penetrate to permit photosynthesis. If the nutrients at the surface are used up, plant species may die off and sea creatures move to other parts of the ocean to look for food.
A particular type of ocean current called an upwelling brings dense, cooler water full of nutrients up from the bottom into shallower waters. These nutrient sources provide a stimulus for the growth of phytoplankton, which in turn feeds the whole food chain leading up to humans.
Beginning with the basic molecule of hydrogen and oxygen that is water, the content of seawater also includes salts, dissolved gases, and nutrients, all necessary to support life in the oceans. Despite huge distances and temperature variations, the content of seawater is incredibly consistent throughout the oceans.