Northern Prairie Wildlife Research Center
In many ephemeral (Class I), temporary (Class II), and seasonal (Class III) ponds that are situated in croplands, 50 percent or more of the central zone is occupied by emergent plant associations characteristic of tilled conditions. Whenever these associations can be identified as phases of wet meadow or shallow marsh, the disturbed land-use condition can be specified by adding a small letter "t" as a superscript after the class number, e.g., IIIt-B-2 or IIt-4. However, if the central zone is in the open-water phase, drawdown bare-soil phase, or cropland tillage phase, it may be difficult to determine which class the pond belongs to. For these ponds, a large letter "T" can be substituted for the class number. A pond of this type without emergent vegetation could be designated as T-4 (plate 30), while one located in a stubble field could be designated as T-2 (plate 31) or T-3.
In certain extensive types of wetland investigations, including aerial surveys and rapid ground reconnaissance, it may be difficult or infeasible to identify wetland subclasses. For these studies, the recognition of major classes or combinations of major classes and cover types should suffice. Wetland subclasses can also be more broadly interpreted by combining the five subclasses into three categories. Subclasses A and B are related, in that characteristic plant species of relatively fresh water predominate. In subclasses C and D the common occurrence of brackish-water species in combination with fresh-water and salt-water species represents the intermediate salinity range. Halophytes are predominant in subclass E, and this subclass should therefore be considered separately. These three categories could be designated as fresh (subclass AB), brackish (subclass CD), and subsaline (subclass E).
In order to distinguish permanent ponds and lakes (Class V) from semipermanent ponds and lakes (Class IV) of cover types 3 and 4, a close inspection of the submerged vegetation in the central deeper portions may be required. Occasionally, a determination can be made by examining windrows of vegetative debris cast ashore by wave action.