cance of such alterations at the ecosystem level of consideration, 

 however, involves not the levee ecosystem itself, but its relationship to 

 and regulatory influence on adjacent marsh ecosystems. As was pointed 

 out in the treatment of each of the marsh systems, alteration of the 

 hydrologic regime is considered to be the key ecosystem attribute altera- 

 tion. The role played by levees in diverting or impounding surface-water 

 flows is generally acknowledged to be highly important in determining 

 ecosystem characteristics. Levees may be constructed for specific manage- 

 ment purposes or they may result as a consequence of other coastal 

 activities. Regardless of the reason behind its construction, an estab- 

 lished levee can alter waterflow patterns. Pipeline construction can 

 effectively alter established flows by (1) creating permanent new cuts 

 (the flotation method without backfilling) in continuous levees or (2) 

 creating weak or "soft" areas in a levee that can eventually fail as a 

 result of continual water erosion. Such breaches may be considered either 

 beneficial or detrimental, depending upon the intended function of the 

 levee, the land use objectives, and the type of ecosystem changes that 

 may result. If a series of levees were constructed with specific manage- 

 ment objectives in mind, which is frequently the case on wildlife refuges, 

 then unplugged cuts would certainly compromise their effectiveness. By 

 changing inundation frequencies, average water depth, and duration of 

 submergence, stipulations could be used to restrict such occurrences. 



Soil toxicants may enter levee soils as a result of line leakage or rup- 

 ture. Such toxicants may be composed of either concentrated brine or 

 petrochemical derivatives. Site plant complexity may be reduced as 

 intolerant species are replaced by hardy pioneer species capable of 

 tolerating extreme soil conditions (i.e., high soil salinity, toxic ion 

 concentration, or anaerobic conditions). Such effects are usually loca- 

 lized (radius of only a few meters) due to the restricted mobility of 

 such substances in soil substrates. Large-scale spills or discharges are 

 treated in the section of this discussion on spills and cleanup. 



Disturbances associated with line installation typically cause short-term 

 displacement of sensitive wildlife species from otherwise favorable 

 habitats nearby. Following activity completion, these species reenter 

 nearby sites if they remain essentially unaltered. Shortterm displace- 

 ment may reoccur during infrequent maintenance or repair sessions. 



Key attribute alterations 



Alterations associated with line installation are a function of line size, 

 site location, and placement method. Temporary surface lines, permanent 

 overhead lines within road easements, and buried utility lines and flow- 

 lines typically generate short-term impacts of limited areal extent that 

 generate little, if any, noticeable effects on consumer components. Gas 

 and oil transportation pipelines, on the other hand, modify important 

 ecosystem attributes. Key alterations involve: (1) the immediate 

 removal of existing plant assemblages and habitat components necessary to 

 local consumer groups; (2) the longterm substitution of new plant cover 



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