Primary productivity may be limited at times by any of the critical substrate parameters listed above. Water stress is not usually a factor , but the lateral and vertical spread of ombrotrophic bogs seems to be controlled by the availability of water. The timing or seasonality of the rain input may also be critical.

The availability of dissolved nutrients for plant growth is a function of concentration (i.e., source) of the nutrients and of renewal. The total flux of nutrients into a wetland is more important than the instantaneous concentration, and the nutrients available in the water in the vicinity of the plant roots at any one time are < the amount taken up by the plant during the growing season. In the smooth cordgrass (Spartina alterniflora) marshes of coastal Massachusetts, N is limiting in inland marshes but not in streamside marshes, presumably because the frequent flushing of the latter results in a larger total N supply.

Dissolved nutrient availability is also influenced indirectly by the degree of anoxia of the substrate, since many nutrients, especially micronutrients and PO4, become soluble under anaerobic conditions.

The availability of nutrients bound to particulate materials is also strongly influenced by hydrodynamic considerations, particularly the velocity of flooding waters. Perhaps the best example of this is that the end-of-season biomass of smooth cordgrass is directly related to the density of the substrate. Substrate density is a reflection of the amount of inorganic sediment input compared to organic content. The higher the flooding velocity, the greater the sediment input, and the more vigorous the plant growth. If one assumes that streamside locations sustain higher velocities than inland ones, then the same relationship between velocity and productivity is shown also in the well-known "edge effect," the stimulation of production along stream banks.

In addition to nutrients, the source of water is also a source of toxins. Probably the most ubiquitous of these are salts, usually associated with sea water. There is an extensive literature to show that salt inhibits growth of all hut a few true halophytes. The importance of the renewal rate is illustrated clearly with this chemical. In many coastal ;eras subject to regular flooding, saline marshes support a vigorous flora. However, at sites subject to infrequent tidal flooding (for instance, at the spring tide elevation) salt is concentrated as a result of evapotranspiration, and all vegetation is killed. The regular renewal of the water at lower elevations prevents salt accumulation.

Other toxins, such as herbicides and pesticides, are also carried into wetlands by flooding waters. In these cases also the frequency and velocity of flooding are important in determining the total amount available to wetland flora and fauna.

Under saturated conditions, with low renewal rates, the depletion of 02 in the substrate leads to a number of chemical changes, which together have a significant effect on plant productivity. In the first place, apparently only those plants flourish that have evolved specialized tissues to allow diffusion of 02 from the leaves through the stem into the roots. Even so, the anaerobic environment of the roots seems a significant stress. The effect may be indirect, as through accumulation of toxic sulfides, or solubilization and absorption of high concentrations of micronutrients. The results of highly anaerobic conditions have been described without the exact mechanism being known. For example, tupelo seedlings grow best under aerobic conditions, moderately well when continuously flooded with moving water, and poorest with stagnant flooding. This effect is related to depth and duration of flooding (renewal rate) and also to water velocity directly. In one of the few careful quantitative studies of hydrodynamics of wetlands, the 02 content of bog waters was directly related to its velocity. At velocities >1 cm/s, the waters remained agitated and saturated with 02. Below 0.4 cm/s, 02 replenishment was equal to that of diffusion into stationary water; because of sediment and root respiration, 02 then tends to be depleted.