23 



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 (C0 3 ), 

 sulfate (S0 4 ), and chloride (CI) (Wetzel 1975). Salinity is 

 governed by the interactions between precipitation, sur- 

 face runoff, groundwater flow, evaporation, and some- 

 times 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 extreme- 

 ly difficult to subdivide the inland salinity range in a mean- 

 ingful way. Bayly (1967) attempted a subdivision on the 

 basis of animal life; Moyle (1945) and Stewart and Kan- 

 trud (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 num- 

 ber 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 Kan- 

 trud 1971), but is not universally accepted (see Bayly 

 1967:84); therefore, mixosaline is used here. In some in- 

 land wetlands, high soil salinities control the invasion or 

 establishment of many plants. These salinities are ex- 

 pressed in units of specific conductance as well as percent 

 salt (Ungar 1974) and they are also covered by the salin- 

 ity classes in Table 2. 



pH Modifiers 



Acid waters are, almost by definition, poor in calcium 

 and often generally low in other ions, but some very soft 

 waters may have a neutral pH (Hynes 1970). It is difficult 

 to separate the effects of high concentrations of hydrogen 

 ions from low base content, and many studies suggest that 

 acidity may never be the major factor controlling the pre- 

 sence or absence of particular plants and animals. Never- 

 theless, some researchers have demonstrated a good 

 correlation between pH levels and plant distribution (Sjors 

 1950; Jeglum 1971). Jeglum (1971) showed that plants can 

 be used to predict the pH of moist peat. 



There seems to be little doubt that, where a peat layer 

 isolates plant roots from the underlying mineral substrate, 

 the availability of minerals in the root zone strongly in- 

 fluences the types of plants that occupy the site. For this 

 reason, many authors subdivide freshwater, organic wet- 



lands into mineral-rich and mineral-poor categories (Sjors 

 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. 



Soil Modifiers 



Soil is one of the most important physical components 

 of wetlands. Through its depth, mineral composition, 

 organic matter content, moisture regime, temperature 

 regime, and chemistry, it exercises a strong influence over 

 the types of plants that live on its surface and the kinds 

 of organisms that dwell within it. In addition, the nature 

 of soil in a wetland, particularly the thickness of organic 

 soil, is of critical importance to engineers planning con- 

 struction of highways or buildings. For these and other 

 reasons, it is essential that soil be considered in the 

 classification of wetlands. 



According to the U.S. Soil Conservation Service, Soil 

 Survey Staff (1975:1-2), soil is limited to terrestrial situa- 

 tions and shallow waters; however, "areas are not con- 

 sidered to have soil if the surface is permanently covered 

 by water deep enough that only floating plants are 

 present. ..." Since emergent plants do not grow beyond 

 a depth of about 2 m in inland waters, the waterward limit 

 of soil is virtually equivalent to the waterward limit of 

 wetland, according to our definition. Wetlands can then 

 be regarded as having soil in most cases, whereas deep- 

 water habitats are never considered to have soil. 



The most basic distinction in soil classification in the 

 United States is between mineral soil and organic soil (U.S. 

 Soil Conservation Service, Soil Survey Staff 1975). The 

 Soil Conservation Service recognizes nine orders of 

 mineral soils and one order of organic soils (Histosols) in 

 its taxonomy. Their classification is hierarchical and per- 

 mits the description of soils at several levels of detail. For 

 example, suborders of Histosols are recognized according 

 to the degree of decomposition of the organic matter. 



Table 3. pH Modifiers used in this 

 classification system. 



Modifier 



pH of Water 



Acid 



Circumneutral 



Alkaline 



<5.5 



5.5-7.4 



>7.4 



