Dredging results in the alteration or displacement of natural habitats. The 

 exposed areas may be nearly devoid of life and are often different from the 

 originals in physical and chemical properties (Copeland and Dickens 1974). 

 Since most dredging produces an unstable sediment configuration, further 

 deposition may occur and further dredging may be required. Repeated dredging 

 prevents a community of long-lived species from becoming established in the 

 dredged area. 



Dredging also may alter the water circulation pattern. Changes in water 

 temperature, salinity, dissolved oxygen, and sediment distribution may result. 

 These changes are the most long-term impacts of dredging on benthic 

 communities (Kaplan et al. 1975). Marked changes in the composition of 

 species that have been associated with changes in sediment types resulting 

 from dredge and fill operations have been documented (Kaplan et al. 1975). 

 Dispersal of pelagic larvae that are carried by currents are also likely to be 

 affected. 



Excessive turbidity is another common result of dredge operations. During the 

 operations, dredging disperses large quantities of silt into the water column. 

 This may have detrimental effects up to a half-mile from the site (Copeland 

 and Dickens 1974). The extent of turbidity depends largely on the type of 

 sediment being dredged, the method of dredging employed, and the local water 

 currents. As deposits disturbed by dredging resettle, a veneer of silt may 

 form a false bottom that is easily resuspended by tidal and wind-driven 

 currents. Suspended or resuspended silt may smother the sessile animals that 

 inhabit the bottom or it may foul the respiratory apparatus of filter-feeding 

 species (Copeland and Dickens 1974) . 



Excessive turbidity also decreases the penetration of light into the water 

 column, thereby limiting plant growth and reducing primary production 

 essential to the support of the entire food web. Reduced light penetration is 

 of special concern in deep water, where light penetration may already be low. 



Lowered primary production (photosynthesis) usually reduces the oxygen supply. 

 The dissolved oxygen levels may be depressed further by microbial activity on 

 suspended sediments and particulate organic materials in the water column 

 (Clark 1977). Low levels of dissolved oxygen have been responsible for 

 massive kills of benthic invertebrates and can place considerable stress on 

 benthic systems around dredge sites (Radash 1976). 



Zooplankton are seriously influenced by the increased turbidity resulting from 

 dredging. Loosanoff (1961) and Loosanoff and Tommers (1948) reported that 

 clam and oyster larvae placed in a turbid environment were affected 

 significantly. At 0.75 g/1 of silt, the growth of oyster larvae was retarded. 

 Hoss and coworkers (1973) reported low mortality but high incidence of erratic 

 behavior in zooplankton exposed to low concentrations of effluents from spoil 

 disposal areas. Such altered behavioral patterns reduce reproductive success 

 among the affected organisms. 



Dredging in a polluted area can cause the resuspension of heavy metals which 

 can be absorbed by plants and animals. When disturbed by dredging oxygen- 

 demanding substances held within the sediments can lower levels of dissolved 

 oxygen, a condition which may be lethal to many species (see "Water Pollution 

 and Heavy Metals," below). 



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