Ch. 3— Wetland Values and the Importance of Wetlands to Man • 49 



As the elevation of wetlands increases, accretion 

 of sediment will slow. In one study, for instance, 

 a Spartina marsh near the mean high-water level 

 annually accreted from 2.0 to 4.25 millimeters 

 (mm) of sediment. An area of colonizing Spartina 

 at a lower elevation, however, accreted sediment 

 at the rate of 9.5 to 37.0 mm/yr (10). Marshes tend 

 to trap sediment as long as they are inundated by 

 sediment-laden waters. 



Suspended organic and nonorganic material has 

 a strong tendency to adsorb other pollutants, in- 

 cluding nutrients, pathogens, and toxics, such as 

 heavy metals and chlorinated and petroleum hydro- 

 carbons, that then are deposited with the sediment 

 in wetlands (10). The ability of wetlands to "trap" 

 suspended material greatly influences the fate of 

 pollutants associated with the suspended material 

 and the potential ability of a particular wetland to 

 improve water quality. 



Removing Toxic Substances 



Heavy metals, chlorinated and petroleum hydro- 

 carbons, radionuclides, and other potentially harm- 

 ful toxic substances may persist for many years. 

 Because they tend to adsorb onto suspended ma- 

 terial, toxics can be trapped in wedands, either tem- 

 porarily or permanently. At the sediment surface, 

 these metals remain immobilized. Once buried and 

 exposed to the anaerobic conditions that typically 

 prevail in sediment, metals again can become mo- 

 bile; however, they will be trapped within the sedi- 

 ment by the oxygenated zone at the sediment sur- 

 face (54,55). Heavy-metal-removal efficiencies of 

 wetlands vary from 20 to 100 percent, depending 

 on the metals involved and the physical and bio- 

 logical variations that exist in wedand habitats (85). 



For compounds such as heptachlor, lindane, or 

 enderin, which degrade readily in soils, the trap- 

 ping of the sediment results in a very efficient and 

 permanent process for removing these contami- 

 nants from the water. (Natural or manmade altera- 

 tions of the wetland caused by lowering the water 

 table, dredging, and the like, however, could mo- 

 bilize large quantities of toxic materials.) However, 

 in general, it is not known yet to what extent wet- 

 lands processes are capable of removing toxic ma- 

 teriads over the long term. 



Some toxics may be tciken up from the sediment 

 by wetland plants and transferred through the food 

 chain to higher trophic levels when the plant ma- 

 terial is consumed, either directly by herbivores or 

 as detritus. Food chain transfer will depend on the 

 toxic chemical and its form as well as the charac- 

 teristics of the plant species and the chemical's loca- 

 tion in the plant. For example, food chain transfer 

 is known to occur with some metals, such as mer- 

 cury or cadmium, but may not occur with others, 

 such as lead. Synthetic materials, including chlor- 

 inated hydrocarbons, are taken up by wetland 

 plants, but food chain effects are not known. There 

 probably is some selectivity of uptake of toxics by 

 particular wetland plant species, but the available 

 data are insufficient to indicate any universal 

 trends. In summary, though wedands may remove 

 toxics from water, it is possible that such removal 

 of heavy metals eventually may lead to contamina- 

 tion of higher trophic levels by passage up the food 

 chain (42). 



Influencing Nitrogen and Phosphorus 



Nitrogen and phosphorus are two nutrients that 

 are necessary for the growth of algae. In excess, 

 however, they can cause "blooms" of algal growth 

 that can impart an unpleasant taste to drinking 

 water and can interfere with recreational uses of 

 water. In addition, the decomposition of algae can 

 reduce levels of dissolved oxygen in the water col- 

 umn to levels that may be harmful to other orga- 

 nisms that need oxygen for survival. 



Nutrients are retained in wetland by similar 

 mechanisms as other pollutants (85). Both nitrogen 

 and phosphorus readily adsorb to sediment and 

 thereby tend to become trapped in the anaerobic 

 sediment of wetlands. As with other toxics, how- 

 ever, nutrients are not necessarily permanently 

 trapped; they may, for instance, be rapidly assim- 

 ilated by rooted wetland plants. In fact, the bulk 

 of the nitrogen and phosphorus for plant growth 

 apparently comes from the sediment. At the end 

 of the growing season, much of the assimilated nu- 

 trients may be leached from the plants. Boyd, for 

 instance found that about 50 percent of the phos- 

 phorus in dead cattail tissue was leached over a 



