25 



oration, and sometimes evapotranspiration by plants. 

 The ionic ratios of inland waters usually differ appre- 

 ciably from those in the sea, although there are excep- 

 tions (Bayly 1967). The great chemical diversity of 

 these waters, the wide variation in physical conditions 

 such as temperature, and often the relative imperma- 

 nence of surface water, make it extremely difficult to 

 subdivide the inland salinity range in a meaningful 

 way. Bayly (1967) attempted a subdivision on the 

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

 Kantrud (1971) have suggested two very different divi- 

 sions on the basis of plant life. We employ a sub- 

 division that is identical with that used in the Estua- 

 rine 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 con- 

 ductance as well as percent salt (Ungar 1974) and they 

 are also covered by the salinity 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 concen- 

 trations of hydrogen ions from low base content, and 

 many studies suggest that acidity may never be the 

 major factor controlling the presence or absence of 

 particular plants and animals. Nevertheless, some re- 

 searchers have demonstrated a good correlation be- 

 tween 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 influences the types of plants that occupy the 

 site. For this reason, many authors subdivide fresh- 

 water, organic wetlands 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 dif- 

 ference 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 

 (197 1 ), 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. 



Table 3. pH modifiers used in this 

 classification system. 



Soil is one of the most important physical com- 

 ponents of wetlands. Through its depth, mineral com- 

 position, 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 construction 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 terres- 

 trial situations and shallow waters; however, "areas 

 are not considered to have soil if the surface is perma- 

 nently covered by water deep enough that only float- 

 ing 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 equi- 

 valent to the waterward limit of wetland, according to 

 our definition. Wetlands can then be regarded as 

 having soil in most cases, whereas deepwater 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 their taxonomy. Their classification is 

 hierarchical and permits the description of soils at sev- 

 eral levels of detail. For example, suborders of Histo- 

 sols are recognized according to the degree of decom- 

 position of the organic matter. 



We use the modifiers mineral and organic in this 

 classification. Mineral soils and organic soils are dif- 

 ferentiated on the basis of specific criteria that are 

 enumerated in soil taxonomy (U. S. Soil Conser- 

 vation Service, Soil Survey Staff 1975:13-14, 65). 

 These criteria are restated in our Appendix D for ready 

 reference. If a more detailed classification is desired, 

 the U. S. Soil Conservation Service classification 

 system should be used. 



Special Modifiers 

 Many wetlands and deepwater habitats are man- 



