The Chemistry of The Oceans

The ocean as a continuous body of salt water is only a part of the hydrosphere. The hydrosphere is comprised of the earth’s water portion and includes groundwater, water in lakes and rivers, and ice frozen in glaciers. Water moves from ocean in a continuous cycle.

Oceanographers, scientists who study the oceans believe that ocean water contains all of the natural elements on the earth. There are about 0 elements known to exist in nature. About 85 of these elements have already been found in ocean water.

Ocean water is about 96 percent pure water. The remaining 4 percent is dissolved elements. If you accidentally swallow a mouthful of ocean water while swimming in the beach, you have probably recognized the taste of sodium chloride. Sodium chloride is common table salt. Oceanographers use the term salinity to describe the amount of dissolved salts in ocean water. Salinity is the number of grams of dissolved salts in 1 kg of ocean water.

The salts dissolved in the sea make it saline. The salinity of the sea is about 3 ½ percent. That is, there are 3 ½ parts of salt for each 100 parts of seawater.

The salinity of the ocean water is not constant. Several factors cause it to vary. At the mouth of a river, where quantities of fresh water enter and mix with sea, the salinity drops. On the other hand, in warm climates where the evaporation is great and the rainfall is low, the salinity is high.

The salt content of the oceans is slowly but steadily increasing. Rivers continuously carry dissolved minerals into the sea. When the sun evaporates water, the minerals are left behind. The evaporated water eventually returns as rain that dissolves and carries additional minerals into the sea.

In addition to the dissolved solids, there are dissolved gasses in the ocean. The most important of these are oxygen and carbon dioxide. Dissolved oxygen in necessary for maintaining life in the sea. It is this dissolved oxygen that fishes extract from seawater with their gills.

Carbon dioxide that is dissolved in the water serves two purposes. It is needed by sea plants to carry on photosynthesis. This process is the basis for life in the sea. Carbon dioxide also reacts with the lime shells, and the minerals return to the sea to be used again by other organisms. There are about 60 times as much carbon dioxide as oxygen in the ocean.

In a delicate equilibrium, gases are exchanged between air and water. Thus the presence of a large amount of carbon dioxide in the ocean is important. As carbon dioxide is used plants (mainly phytoplankton) in the ocean it is replenished from the atmosphere. If plant life in the ocean were reduced, more carbon dioxide would stay in the atmosphere, and this could cause a disastrous alteration in climate.

Both oxygen for animals and carbon dioxide for plants are dissolved in seawater. Some animals get food simply by circulating seawater though their bodies and filtering out nutrients. Buoyancy or the upward lift of water makes movement in ocean water very easy. Little protection is needed is needed against either heat or cold because the temperature range of seawater is so small. Despite all the advantages of the sea, however, many marine animals don not live long. This is because different creatures feed on them.

Ocean plants can live only as deep as sunlight reaches them. Most plants belong to the floating plankton. Food becomes scarce as water depth increases.