Ch. 6— Impacts and Mitigation • 125 



wood, cardboard and paper products, as well as fer- 

 rous metals. Leaching of organic matter such as 

 garbage and wood waste can lead to an increased 

 biological oxygen demand (BOD) and reduced lev- 

 els or large fluctuations in dissolved oxygen (DO). 

 Such changes in water chemistry can cause stress 

 to aquatic populations and changes in species di- 

 versity. 



Biological and Ecological Variables 



Population Abundance, Diversity, 

 and Productivity 



Productivity, abundance, and diversity are im- 

 portant factors in evaluating the potential impacts 

 of a certain activity on a wetland. Highly diverse 

 wetland ecosystems with high overall productivity 

 but low abundance of many species may be affected 

 heavily by activities that change the limiting fac- 

 tors for selected species, thereby unbalancing the 

 whole structure (species composition) of that eco- 

 system. A less diverse ecosystem may be impacted 

 less by the same activities. Spartina marshes, which 

 admost can be considered a monoculture, are known 

 to be highly resistant to changes in salinity and 

 might not be affected significantly by, for exam- 

 ple, the reduction of freshwater inflows to the 

 estuary from upstream use of water for cooling a 

 powerplant. 



Presence of Key Species 

 Important to an Ecosystem 



The severity of impact from a particular activi- 

 ty will be greater if the adverse effects focus on a 

 key species in the wetland ecosystem. For exam- 

 ple, detritus-based food chains can easily be dis- 

 rupted by activities that would lower the abundance 

 of snails and small crustaceans that help produce 

 detritus by shredding the marsh grasses. 



Habitat Diversity and Carrying Capacity 



Fish and wildlife may require different habitats 

 during their lifecycles, in each season, and even dai- 

 ly, in order to meet their needs for food, water, cov- 

 er, and reproduction. Wetlands offer a variety of 

 habitats for a variety of species and life stages. 

 Habitat diversity often has been assessed as an in- 

 dication of the importance or health of a wetland. 



The degree of impact on a wetland often will de- 

 pend on which habitats are adversely affected; for 

 example, fish that use coastal marshes may be di- 

 verted from their normal routes by large changes 

 in salinity and flow (24). 



Operations Variables 



Frequency, Duration, and Season of Activity 



The frequency, duration, and season of a devel- 

 opment activity in or affecting a wedand will modify 

 the severity of impact. Frequent channel-mainte- 

 nance dredging, for example, might limit the recov- 

 ery of an adjacent wetland from the temporary ef- 

 fects of sediment resuspension, especially where 

 there is high exposure to wind and waves. Oil ex- 

 ploration may have rather minor and temporary 

 adverse effects on waterfowl if access to wetlands 

 is limited during the breeding, nesting, and rear- 

 ing season. Similarly, construction of a highway 

 through a wetland will have less impact on water 

 quality and wUdlife if the construction is rapid and 

 efficient, avoids the period of high spring runoff, 

 and is carried out before or after the waterfowl 

 breeding season. 



Location of Activity Within an Ecosystem 



The location or orientation of development proj- 

 ects within a wetland can alter the magnitude of 

 their impact considerably. One example would be 

 the placement of highway fill in a wetland. If the 

 causeway fill is placed parallel to the direction of 

 surface sheet flow and subsurface flow, the prob- 

 lems of blocking wetland drainage or channeling 

 the flow through culverts wiU be minimized (44). 

 In another example, if pipelaying in wetlands is 

 confined to the "push-ditch" method and the 

 equipment can operate on dry soil at the edge of 

 the wetland, the impacts will be less than if the 

 equipment is operated from mats in the wetland. 



Distribution, Scale, and Type of Activity 



The type, scale, and spatial distribution of con- 

 struction or development in a wedand must be con- 

 sidered in order to estimate reliably the project's 

 impact. Wedand filling, if confined to a single area 

 of marsh while leaving other areas undisturbed, 

 may be preferable to a patchwork of fills distributed 



