The movement of water down concentration gradients in terrestrial and aquatic environments determines the availability of water to organisms. The most familiar relative measure of the water content of air is relative humidity, defined as water vapor density divided by saturation water vapor density multiplied by 100. On land, the tendency of water to move from organisms to the atmosphere can be approximated by the vapor pressure deficit of the air. Vapor pressure deficit is calculated as the difference between the actual water vapor pressure and the saturation water vapor pressure.
In the aquatic environment, water moves down its concentration gradient, from solutions of higher water concentration and lower salt content (hypoosmotic) to solutions of lower water concentration and higher salt content (hyperosmotic). This movement of water creates osmotic pressure. Larger osmotic differences, between organism and environment, generate higher osmotic pressures.
In the soil-plant system water flows from areas of higher water potential to areas of lower water potential. The water potential of pure water, which by convention is set at zero, is reduced by adding solute and by matric forces, the tendency of water to cling to soil particles and to plant cell walls. Typically, the water potential of plant fluids is determined by a combination of solute concentrations and matric forces, while the water potential of soils is determined mainly by matric forces. In saline soils, solutes may also influence soil water potential. Water potential, osmotic pressure, and vapor pressure deficit can all be measured in pascals (newtons/m2), a common currency for considering the water relations of diverse organisms in very different environments.Terrestrial plants and animals regulate their internal water by balancing water acquisition against water loss. Water regulation by terrestrial animals is summarized by Wia = Wd + Wf + Wa – We – Ws, where Wd = drinking, Wf = taken in with food, Wa = absorption from the air, We = evaporation, and Ws = secretions and excretions. Water regulation by terrestrial plants is summarized by Wip = Wr + Wa – Wt – Ws, where Wr = uptake by roots, Wa = absorption from the air, Wt = transpiration, and Ws = secretions and reproductive structures. Some very different terrestrial plants and animals, such as the camel and saguaro cactus, use similar mechanisms to survive in arid climates. Some organisms, such as scorpions and cicadas, use radically different mechanisms. Comparisons such as these suggest that natural selection is opportunistic. Marine and freshwater organisms use complementary mechanisms for water and salt regulation. Marine and freshwater organisms face exactly opposite osmotic challenges. Water regulation in aquatic environments is summarized by:
Wi = Wd – Ws ± Wo, where Wd = drinking, Ws = secretions and excretions, Wo = osmosis. An aquatic organism may either gain or lose water through osmosis, depending on the organism and the environment. Many marine invertebrates reduce their water regulation problems by being isosmotic with seawater. Some freshwater invertebrates also reduce the osmotic gradient between themselves and their environment. Sharks, skates, and rays elevate the urea and TMAO content of their body fluids to the point where they are slightly hyperosmotic to seawater. Marine bony fish and saltwater mosquito larvae are hypoosmotic relative to their environments, while freshwater bony fish and freshwater mosquito larvae are hyperosmotic.
While the strength of environmental challenge varies from one environment to another, and the details of water regulation vary from one organism to another, all organisms in all environments expend energy to maintain their internal pool of water and dissolved substances.
Stable isotope analysis, an important new tool in ecology, involves the analysis of the relative concentrations of stable isotopes in materials. Examples of stable isotopes include the stable isotopes of hydrogen 2H (which is usually symbolized by D, referring to deuterium) and 1H, and the stable isotopes of carbon, 13C and 12C. Stable isotope analysis has proved a very powerful tool in studies of water uptake by plants. For example deuterium: hydrogen (D:1H) ratios, or ðD, has been used to quantify the relative use of summer versus winter rainfall by various plant growth forms in the deserts of southern Utah.
