hysical 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.