A schematic representation of 6 types of freshwater marsh environments and their hydrologic regime.

1. Raised-convex (ombrogenous bogs, Gorham, 1957)

2. Meadow (blanket bogs, DuRietz, 1949; lacustrine bogs)

3. Sunken minerotrophic (fen, DuRietz, 1949; carr, Gorham, 1957)

4. Lotic (fen, carr; reed swamp, Gorham, 1957)

5. Tidal

6. Lentic (riverine).

These considerations of the importance of hydrology in wetlands suggest that the classification of wetlands along a hydrodynamic energy gradient could be a useful approach to understanding functional relationships. The above figure divides freshwater wetlands into 6 classes based on source and velocity of water flow. In general flow rate, or other indications of hydrologic energy such as renewal time or frequency of flooding, increases from raised convex wetlands to lentic and tidal wetlands. If a classification of wetlands on a hydrodynamic energy gradient is of functional value, it should correspond with gradients in ecosystem attributes.

During classical successional processes, hydrologic flows are diverted and reduced by the developing biota. According to ecosystem theory, diversity should increase with maturity. This would mean that diversity should be highest in perched bogs and lowest in high-energy wetlands. Comparing perched bogs (raised-convex wetland), to those in contact with the groundwater supply (sunken wetland) reveals much higher vegetation diversity on the latter habitat, which he related to a richer nutrient supply. These examples suggest that the opportunities for niche differentiation increase with the hydrologic energy of the wetland. Whether this relationship holds true for extremely high-energy freshwater tidal and lentic wetlands is not clear. For example freshwater marshes at the mouth of the Mississippi River are often monotypic compared to the diverse freshwater marshes found in less energetic situations elsewhere along the Louisiana coast.

Productivity is so strongly influenced by many factors that generalizations may be impossible to make. Low-energy wetlands tend to be concentrated in northern areas, whereas southern wetlands are usually lentic, lotic, or tidal. Thus, comparisons of different wetland types are confounded by different temperatures and growing seasons. In addition, productivity measurements have not been standardized, and large fluctuations in estimates result from use of different techniques. Comparisons of single species growing in different hydrologic regimes within a limited geographical area may provide some insight into the role of hydrology in production. Whigham and Simpson (1977) compared production of Zizania aquatica from tidal and nontidal sites. Considering only those in New Jersey, tidal sites were more productive than nontidal ones. This correlation between the energy of the flooding regime and productivity is reinforced by a comparison of productivity of cypress swamps in the southeastern United States. Productivity appears to be positively correlated with the water-flow regime.

Accumulation of organic matter and peat development are characteristic of low-energy wetlands. Raised sphagnum bogs and lotic freshwater marshes are alike in this respect. The rate of peat accumulation depends on the production rates as well as on the duration and depth of surface flooding, because oxidation of the substrate occurs rapidly upon exposure. The proportion of primary production that is deposited as peat in different wetlands is difficult to determine. It is expected that as flooding energy increases, the proportion of production exported also increases. Thus deposition and export arc inversely related to one another and to flooding energy.

On a continuum from low-energy, omhrotrophic hogs to lentic and tidal wetlands internal cycling of nutrients shoiuld decrease and dependence on external sources increase. Nutrient budgets have not been calculated for many marsh systems. Nevertheless, much information can be pieced together. Perched bogs are limited to precipitation for nutrient input,' so are usually very nutrient- poor. Most nutrients in these wetlands are bound in organic form. It has been reported that the nutrients in interstitial waters of forested bogs in contact with the regional water table (sunken wetland?)are two to four times higher than in a perched bog. At the other extreme in tidal salt marshes, nitrogen fluxes across salt marsh boundaries in coastal Louisiana are 75% of that recycled within the marsh. This is a very open system.