(1?67) found in Boco Ciega Bay, Florida, that the recolonization of new chan- 

 nels by invertebrates v;as negligible after 10 yr and that none of the 49 

 species of fish caught in the channels (as compared to 80 species caught in 

 undredged areas) were derrersal. The decrease in numbers of invertebrates in 

 the channels was attributed to the soft silt-clay dredged sediments compared 

 to the sand-shell undredged sediments. Decreased oxygen supply in and above 

 the dredged channel substrate was also considered a prime factor. 



More serious impacts of new channel construction may be chances in circu- 

 lation patterns, salinities, sediment input and deposition, sediment supply to 

 the coast, and nearshore wave refraction and diffraction patterns (Rees 1977). 

 These impacts are usually detrimental but on occasion could be beneficial. An 

 increase in water exchange rate between a bay and the ocean could be benefi- 

 cial in instances when there is insufficient dilution to disperse contaminants 

 or nutrients. 



Changes in the bottom topography due to construction of the Mobile (Ala- 

 bama) ship channel have contributed to the problem of annual oxygen depletion 

 in Mobile Bay. Water in sinks in the bay bottom becomes depleted of oxygen; 

 occasionally the oyxgen depleted water is moved shoreward by wind and v/ave 

 action resulting in stress to the biota (May 1973a). 



Kaplan et al. (1974) noted a drastic decrease in productivity following 

 the dredging of a navigational channel through a small, shallow bay in Long 

 Island, New York. Biota were reduced in the bay and in the dredged channel. 

 The authors blamed changes in current velocity and concomitant modifications 

 in substrate type as well as land use changes brought on by the new channel. 



Dredging can be extremely destructive to coral reefs. Bak (1978) noted 

 decreases in light penetration from turbidity caused by dredging, a loss of 

 the zooxanthellae, and eventual death of certain coral species. Calcification 

 rates were suppressed in two species of corals during and following the dis- 

 turbance from dredging. A thorough discussion of impacts of dredging and sus- 

 pended material on coral reefs is found in Stern and Stickle (1978). 



After a ship channel is constructed, it becomes a sink for sediments that 

 often contain large amounts of potential contaminants. Resuspension or reacti- 

 vation of these potential contaminants is great when maintenance dredging is 

 conducted or when ships move through the area (Lee 1976, Smith 1976). 



Other Impacts 



The greatest impacts from new channel construction often are related to 

 increased industrial development which may alter drainage patterns and reduce 

 water quality. 



A potential adverse impact of dredging is the entrainment of slow-moving 

 nekton. Large scale mortality of dungeness crabs has been blamed on hydraulic 

 dredges in Grey's Harbor, Washington (memorandum dated 11 February 1977 from 

 R.H. Latta, Corps of Engineers, Seattle, Washington). 



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