The two key characteristics employed in this system are salinity and hydrogen-ion concentration (pH). All habitats are classified according to salinity, and freshwater habitats are further subdivided by pH levels.
Estuarine and Marine waters are a complex solution of salts, dominated by sodium chloride (NaCl). The term haline is used to indicate the dominance of ocean salt. The relative proportions of the various major ions are usually similar to those found in sea water, even if the water is diluted below sea water strength. Dilution of sea water with fresh water and concentration of sea water by evaporation result in a wide range of recorded salinities in both surface water and interstitial (soil) water.
We have modified the Venice System, suggested at a "Symposium on the Classification of Brackish Waters" in 1958, for use in the Marine and Estuarine Systems (Table 2). The System has been widely used during recent years (Macan 1961, 1963; Burbank 1967; Carriker 1967; Reid and Wood 1976), although there has been some criticism of its applicability (den Hartog 1960; Price and Gunter 1964).
The salinity of inland water is dominated by four major cations, calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K); and three major anions, carbonate (C03), sulfate (SO4), and chloride (Cl) (Wetzel 1975). Salinity is governed by the interactions between precipitation, surface runoff, groundwater flow, evaporation, and sometimes evapotranspiration by plants. The ionic ratios of inland waters usually differ appreciably from those in the sea, although there are exceptions (Bayly 1967). The great chemical diversity of these waters, the wide variation in physical conditions such as temperature, and often the relative impermanence of surface water, make it extremely difficult to subdivide the inland salinity range in a meaningful way. Bayly (l967) attempted a subdivision on the basis of animal life; Moyle (1945) and Stewart and Kantrud (1971) have suggested two very different divisions on the basis of plant life. We employ a subdivision that is identical to that used in the Estuarine and Marine Systems (Table 2).
The term saline is used to indicate that any of a number of ions may be dominant or codominant. The term brackish has been applied to inland waters of intermediate salinity (Remane and Schlieper 1971; Stewart and Kantrud 1971), but is not universally accepted (see Bayly 1967:84); therefore, mixosaline is used here. In some inland wetlands, high soil salinities control the invasion or establishment of many plants. These salinities are expressed in units of specific conductance as well as percent salt (Ungar 1974) and they are also covered by the salinity classes in Table 2.
There seems to be little doubt that, where a peat layer isolates plant roots from the underlyng mineral substrate, the availability of minerals in the root zone strongly influences the types of plants that occupy the site. For this reason, many authors subdivide freshwater, organic wetlands into mineral-rich and mineral-poor categories (Sjörs 1950; Heinselman 1970; Jeglum 1971; Moore and Bellamy 1974). We have instituted pH modifiers for freshwater wetlands (Table 3) because pH has been widely used to indicate the difference between mineral-rich and mineral-poor sites, and because it is relatively easy to determine. The ranges presented here are similar to those of Jeglum (1971), except that the upper limit of the circumneutral level (Jeglum's mesotrophic) was raised to bring it into agreement with usage of the term in the United States. The ranges given apply to the pH of water. They were converted from Jeglum's moist-peat equivalents by adding 0.5 pH units.