was placed along the northern end of South Bay. As a result, Boca Chica Pass 

 filled in, circulation in South Bay became nonexistent, the average depth 

 decreased from 1.2 to 0.4 m (3.9 to 1.3 ft), and the oyster population was 

 destroyed. There was also a decrease in fish and invertebrate populations 

 (Breuer 1962). 



Although dredging-induced changes are often detrimental, as in South Bay, 

 they could conceivably be beneficial by eliminating "pollution traps" through 

 improved water exchange between a polluted estuary and the open sea (Odum 

 1970). 



Continental Shelf Disposal 



Continental shelves, like estuaries, are highly productive areas for 

 marine fisheries. However, adverse impacts of dredged material disposal are 

 not as severe to continental shelves as to estuaries because water over con- 

 tinental shelves has greater dilution, mixing, and assimilative capacities. 

 Compared to estuaries, continental shelves are not the scene of as many crit- 

 ical physical-chemical-biological processes. An exception to the above state- 

 ments is the impact of disposal on coral reefs. (See assessment of impacts at 

 the dredging site--bottom impacts). Another disruption of an ecosystem was 

 noted with disposal in the surf zone of a rocky kelp bed area in Oregon (U.S. 

 Army Engineer District, Portland 1978). 



Although adverse environmental impacts are of less concern on the conti- 

 nental shelf, other constraints such as transportation costs and available 

 equipment become more critical. Much of the discussion of the previous section 

 about estuarine disposal applies to continental shelf disposal. However, many 

 impacts occurring in estuaries will be less severe or will not occur on the 

 continental shelf. This discussion (and the section about deep ocean disposal) 

 is brief because of a lack of studies and our desire not to repeat the same 

 information contained in the section on estuarine disposal. 



Water column impacts . Potential impacts to the continental shelf water 

 column are similar to impacts to the estuarine water column. Except for some 

 special cases, such as in the New York bight (Gunnerson and Wanson 1975), im- 

 pacts to the continental shelf water column should be minimal to non-existent. 

 In the New York bight, apparently the circulation is not adequate to dilute 

 and disperse the large volume of waste material (of several types) that is 

 disposed there. 



Lee et al (1975), Blom et al . (1976), Brannon et al . (1976), Chen et al . 

 (1976), and Burks and Engler (1978) discussed the release of nutrients and 

 potential toxicants to the water column during disposal operations. Signifi- 

 cant releases of manganese and ammonia can be expected. Lesser releases of 

 iron, cadmium, zinc, and orthophosphate may occur. Normal dilution should 

 reduce these materials to harmless levels, but there are possibilities of ad- 

 verse effects over portions of the continental shelf with poor circulation. 

 The most likely impact would be the stimulation of algae blooms by ammonia. 

 Lee et al . (1977) found no significant water column impacts from offshore 

 dumping at Galveston, Texas. 



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