50 • Wetlands: Their Use and Regulation 



20-day period.* Another fraction of the nutrients 

 in the plant is exported from the wetland as detritus; 

 this fraction is probably highly variable, depending 

 largely on the hydrology of the wetland. The dead 

 plant tissue remaining in the wedand is rapidly col- 

 onized by bacteria and the byproducts of the de- 

 composition process, including inorganic nutrients, 

 are released into the water column. Nitrogen stored 

 in the plant, for example, is converted by these de- 

 composers to ammonia. Plant material remaining 

 in the wedand is eventually reincorporated into the 

 sediment. It has been hypothesized that a signifi- 

 cant amount of the nitrogen and phosphorus avail- 

 able from the sediment for plant uptake is recycled 

 from the plant growth of the previous year (42). 



Water Quality Considerations 



Aggregate Effect. — Present understanding of the 

 processes described above is not sophisticated 

 enough to predict their aggregate effect on water 

 quality. Nitrogen fixation, for instance, the opposite 

 process of denitrification (atmospheric nitrogen is 

 fixed by certain bacteria and algae), can contribute 

 significant amounts of nitrogen to the wetland ni- 

 trogen budget and therefore cancel the effects of 

 denitrification. Some wetland studies have 

 measured the quantity of all pollutants entering the 

 wetland from all sources — ground water, surface 

 water, precipitation, and so forth — and the amount 

 leaving the wetland. The aggregate effect of all 

 wetland processes on water quality is reflected by 

 the difference between the amount of pollutant 

 entering and leaving the wetland. In this manner, 

 it can be determined whether wetlands act as a sink 

 or a source of pollutants. 



Thirty-nine input-output studies, focusing for the 

 most part on nitrogen and phosphorus, were re- 

 viewed. These studies were screened carefully to 

 meet a number of stringent criteria. First, since the 

 behavior of the wetland varies greatly during dif- 



*The fate of nitrogen is more complicated than that of other pol- 

 lutants thus far discussed. Nitrogen occurs in several forms in natural 

 water; nitrite, nitrate ammonia, and organic nitrogen (proteins and 

 other large molecules). In addition, the air contains over 78 percent 

 nitrogen gas, which is exchanged continuously through the surface 

 waters. Relatively large populations of micro-organisms in wetlands, 

 under the right circurnstances, can convert nitrogen from one form 

 to another. Thus, nitrogen can be removed ultimately from water by 

 microbial conversion to gas through the process of denitrification, or 

 conversely, fixed from the atmosphere and converted to inorganic ni- 

 trogen. 



ferent seasons, only those studies sampling month- 

 ly for at least a year were selected. Second, all chem- 

 ical forms of nitrogen and phosphorus had to be 

 measured: measurement of both organic and in- 

 organic forms is necessary since the various forms 

 are interconvertible. For nitrogen, total nitrogen 

 (Kjeldahl) must have been measured in unfiltered 

 samples and in nitrate and nitrite. For phosphorus, 

 measurement of total phosphorus from unfiltered 

 samples was required. Third, for studies of undis- 

 turbed wetlands, all reasonable input and output 

 sources had to be measured, including intermittent 

 or temporary sources of surface runoff, ground 

 water, and precipitation. In the case of an artificial 

 pollution source, such as a sewage outfall, the 

 failure to measure natural sources of nutrients was 

 overlooked on the assumption that such sources 

 were comparatively trivial. Measurement of all sig- 

 nificant sources and sinks of water, however, was 

 required, even if the quantity of naturally occur- 

 ring nutrients was overlooked. 



Freshwater Systems. — Of 30 freshwater input- 

 output studies reviewed, only seven (12,23,27,52, 

 62,98,99), met all the criteria listed above. A ma- 

 jor drawback of these studies is that large quan- 

 tities of pollutants doubtlessly flow into and out of 

 wetlands during storms or floods. The chance of 

 getting a good sample of nutrients flowing into a 

 wetland during a major flood is small if outflow is 

 sampled only monthly. One study (52), for in- 

 stance, found that 99 percent of the nutrient flow 

 into a flood plain swamp occurred during a single 

 flood. The swamp floods approximately once every 

 1.13 years. 



Although Crisp (23) found a net export of nitro- 

 gen and phosphorus in an eroding British peadand, 

 all other authors found net reductions of nutrients 

 in freshwater wetlands. Large percentage reduc- 

 tions generally were observed where sewage was 

 applied (12,27,98) and small percentage reductions 

 were observed where nutrient sources were natural 

 (52,62). One study (99) was unusual in that sewage 

 and natural water were applied to artificially enclos- 

 ed marsh plants so that surface outflow was pre- 

 vented. Water that had filtered through the marsh 

 sediments was sampled in outside wells. Since the 

 natural hydrology of the marshes had been altered, 

 the large percentage reductions in both the natural 

 and sewage-treated marshes may not be represent- 

 ative of activity of natural marshes. 



