Inland water ecosystem

Physical and chemical properties of water

Water has several unique physical and chemical properties that have influenced life as it has evolved. Indeed, the very concept of the Earth as biosphere is dependent on the special physicochemical properties of water. These characteristics have significantly influenced the structure of inland aquatic ecosystems.

At prevailing global temperatures most inland waters exist in liquid form. As a liquid, water has special thermal features that minimize temperature fluctuations. First among these features is its high specific heat—i.e., a relatively large amount of heat is required to raise the temperature of water. The quantity of heat required to convert water from a liquid to a gaseous state (latent heat of evaporation) or from a solid to a liquid state (latent heat of fusion) is also high. This capacity to absorb heat has several important consequences for the biosphere, including the ability of inland waters to moderate seasonal and diurnal (daily) temperature differences both within aquatic ecosystems and, to a lesser extent, beyond them. Most of the heat input to inland waters is in the form of solar energy. The amount of this energy that actually reaches inland waters at any given time depends on several factors, including time of day, season, latitude, altitude, and amount of cloud cover. A significant fraction of the solar radiation that reaches the water surface is lost through reflection and backscattering. The remaining fraction enters the water column where its energy rapidly diminishes with depth as it is absorbed and converted either to heat by physical processes or to chemical energy by the biological process of photosynthesis. In large, deep lakes most of the energy required by the biota is derived from this biological conversion. In other sorts of inland waters, however, a large proportion of the energy required by biological communities may come from emergent and nearby terrestrial vegetation. In any event, the amount and nature of solar energy entering inland waters is a principal determinant of the structure and function of the ecosystem.

The conversion of light energy into heat in inland waters has several significant physical consequences. Of special note are the changes that occur to water density as temperature varies. This relationship is illustrated in Figure 1, in which the density of pure water is plotted against temperature as a measure of heat content. Note that water has the greatest density at 4° C. Although this relationship is that of pure water, it closely approximates that of fresh water. Thus, ice, which forms at 0° C, develops first at the surface of freshwater lakes, above slightly warmer, denser water, and prevents lakes from freezing solid. Were this not the case, the biology of inland waters would be quite different. In saline waters, however, the relationship is somewhat different because greater concentrations of dissolved salts lower both the freezing point and the temperature of maximum density.

One of the most significant chemical properties of water is its function as a solvent. In this regard it has an unrivaled capacity to hold in solution an exceptionally wide range of substances, including electrolytes (salts, which dissociate into ions in aqueous solution), colloids (particulate matter small enough to remain suspended in solution), and nonelectrolytes (substances such as glucose that retain their molecular structure and do not dissociate into ions). A great variety of combinations of dissolved substances can occur in inland waters. Nevertheless, it is possible to discern some major trends in the amounts and types of solutes. The major inorganic solutes are the cations (positive ions) sodium, potassium, calcium, and magnesium and the anions (negative ions) chloride, sulphate, and bicarbonate/carbonate. When the total concentration of all these ions (i.e., the salinity, or salt content) is less than 3 grams per litre (i.e., 3 grams per kilogram, or 3 parts per thousand [ 0/00]), inland waters are conventionally regarded as fresh. Most fresh waters have salinities less than 0.5 gram per litre and are dominated by calcium, magnesium, and bicarbonate or carbonate ions. Conventionally, saline waters are defined as those that have salinities greater than 3 grams per litre, with maximum values determined by the dominant type of ions present. Sodium and chloride ions are dominant in most but not all salt lakes, and maximum salinities are therefore about 350 grams per litre. In addition to these major ions, all inland waters contain smaller quantities of other ions, of which phosphate and nitrate—essential plant nutrients—are particularly significant. Also of biological significance are certain dissolved gases, especially oxygen, carbon dioxide, and nitrogen, whose solubilities are inversely correlated with temperature, altitude, and salinity. Hydrion concentrations (pH) and concentrations of a variety of dissolved organic compounds of undetermined significance affect the biota as well.

Physicochemical phenomena affect every body of inland water, creating unique relationships among and within the biotic and abiotic components of the ecosystem. Of particular interest are the pathways or biogeochemical cycles that are traveled by the chemical elements essential to life—nitrogen, phosphorus, carbon, and a variety of micronutrients such as iron, sulfur, and silica (see biosphere: The organism and the environment: Resources of the biosphere: Nutrient cycling). The degree to which output of a particular element balances input within a given aquatic ecosystem varies according to the type of inland water involved. However, all essential elements follow pathways in inland waters that are numerous, complex, well-defined, and often interdependent on other biogeochemical cycles. In fact, a defining characteristic of all inland aquatic ecosystems, including the most simple temporary bodies of highly saline water, is the occurrence of well-defined biogeochemical cycles.

Some of the most salient general physicochemical features of inland waters having been indicated, it is important to emphasize that these features are expressed differently in various types of inland waters.