The biological health of a lake is crucially dependent on its chemical characteristics. Limnologists and hydrobiologists are attentive to the dissolved oxygen content of the water because it is a primary indicator of water quality. Well-oxygenated water is considered to be of good quality. Low dissolved oxygen content results in anaerobic fermentation, which releases such gases as toxic hydrogen sulfide into the water, with a drastic effect on biological processes.

Another major concern of limnologists and hydrobiologists is the cycling of basic nutrients within a lake system, particularly carbon, nitrogen, phosphorus, and sulfate. An excess of the latter in runoff waters entering a lake may result in high concentrations of hydrogen ions in the water. Such acid (low values of pH) waters are harmful to the lake biology. In particular, aluminium compounds are soluble in water at low pH and may cause fish to die because of the response induced in their gills.

The origin of the ocean basins

About 71 percent of Earth’s surface is covered by seawater, with the proportion of sea to land being greater in the Southern Hemisphere (4 to 1) than in the Northern Hemisphere (1.5 to 1). Current theories of plate tectonics explain the development of the present ocean basins in terms of a splitting of a large continental landmass brought about by convective circulation within Earth’s mantle. This circulation causes material to rise from the mantle, resulting in the formation of new lithosphere at the mid-ocean ridges. Continued ascent of material in these areas has resulted in the movement of older material away from the ridges, a process known as seafloor spreading. Clear evidence of the broad symmetry of these movements has been produced by studies of the residual magnetism of the rocks of the seafloor, which exhibit successive zones of magnetic reversals parallel to the mid-ocean ridges. Near the continental masses some of the oceanic material sinks into the mantle in zones of subduction, which are associated with oceanic troughs, deep-focus earthquakes, and volcanic activity.

The physical properties of seawater

The physical properties of seawater depend on the chemical constituents dissolved in it. The spatial variability of seawater composition is only partially known, since many areas of the oceans have not been fully sampled. It has been shown that while the salinity of seawater varies from place to place, the relative proportions of the major constituents remain fairly constant. Chlorine accounts for about 55 percent of dissolved solids, sodium 30.6 percent, sulfate 7.7 percent, magnesium 3.7 percent, and potassium 1.1 percent. The average salinity of seawater is about 35 grams of dissolved salts per kilogram of seawater. Higher values occur in areas where evaporation rates are high, such as the Red Sea (41 grams per kilogram), and in areas where relatively pure water freezes out as sea ice, thereby increasing the salinity of the water below. Variations in salinity may be measured indirectly by measurements of the electrical conductivity of seawater, which also yield an accurate estimate of density. With the electrical conductivity method, it is easy to obtain in situ estimates of density and salinity. Using temperature sensors, it is also possible to obtain in situ measurements of seawater temperature, the other very important physical characteristic of seawater. It is primarily variations in temperature and density that drive the circulation of water in the oceans.

The density of seawater in high latitudes tends to gradually increase with depth. In tropical areas there is usually a well-mixed layer of uniform density close to the surface above the pycnocline, a layer in which density increases extremely rapidly. Below the pycnocline density continues to increase but much more slowly. The pycnocline is a very stable layer that acts as a barrier to the transfer of water and energy between the surface and subsurface layers.

Seawater has a higher specific heat (about 0.95) than land (0.5 on average), so that changes in the temperature of the oceans are much slower than on land. In fact, the oceans represent a vast store of heat that exercises an important, but not fully understood, control on the variability of climate regimes observed on land. The temperature distribution at the surface of the open seas tends to follow lines of latitude, with warmer waters in tropical regions. Close to the landmasses, the lines of equal temperature (isotherms) become deflected, indicating warm and cold coastal currents. Below the surface there is generally a well-mixed layer of fairly uniform temperature about 50 to 200 metres thick below which lies the thermocline, which may extend to depths of 1,000 metres. Below the thermocline, temperature decreases slowly with depth. A typical temperature profile in equatorial regions would be 20 °C down to 200 metres, 8 °C at 500 metres, 5 °C at 1,000 metres, and 2 °C at 4,000 metres.

Plotting the temperature and salinity of a sample of seawater on a graph with linear axes (a T–S diagram) can be a powerful research tool. A mass of fully mixed water having a homogeneous distribution of temperature and salinity would plot as a single point on a T–S diagram. For actual water masses it is common to find that points plotted for samples taken from different depths plot as a curve (sometimes complex) on the diagram. Such curves provide an indication of the mixing between different water masses that is taking place in the profile. T–S curves also are useful for uncovering errors in data.