KAPLAN, WELKER. and KRAUS: EFFECTS OF DREDGING 



Mackin (1961) reviewed the literature on the 

 biological effects of dredging, with special refer- 

 ence to oyster survival. Most of the authors he 

 cited found that oyster mortality was caused by 

 direct inundation with spoil resulting in suffoca- 

 tion. Beyond the area of deposition, oysters and 

 fishes were unaffected. 



Mackin found that at low current velocities 

 turbidity is not an important factor in oyster 

 mortality at levels up to 700 ppm. Such levels 

 were higher than those found beyond 250 ft from 

 the outlets of the three types of dredges studied. 

 He also argues that oxygen levels are not appreci- 

 ably decreased under conditions normally found 

 on oyster beds. 



SUMMARY AND CONCLUSIONS 



The results of the present study differ from 

 those reported in most other investigations of the 

 effects of dredging in that profound changes are 

 reported in macrobenthic animal populations 

 throughout the bay. Abundant evidence is avail- 

 able concerning long-term depreciation of stand- 

 ing crop in dredged channels (cf. Taylor and 

 Saloman, 1968; Odum, 1963; Murawski, 1969; 

 and O'Connor, 1972), but these reports show 

 limited residual effects beyond the immediate 

 region of the channel and/or spoil areas. This 

 difference in results is attributed to the fact that 

 most previous studies reported on the creation of 

 channels through relatively large bodies of water 

 such as Chesapeake or Boca Ciega Bays. Spoil 

 distribution effects and changes wrought in cur- 

 rent velocity and sediment deposition are mini- 

 mized when the ratio of the dredged area to total 

 bottom area and contained water volume is large. 

 Long flushing time and reduced inlet size of small 

 estuarine bays exaggerates the hydrodynamic 

 effects of channel construction. Wind-induced 

 sediment transport and- the effects of spoil deposi- 

 tion on the surrounding peripheral marshes are 

 factors which complicate the evaluation of the 

 effects of dredging, especially in small bays. 



In areas of high human population density, 

 combined dredging-landfill operations have be- 

 come common and their effects have been felt 

 primarily in the small shallow bays which could 

 provide (if dredged) good anchorages for pleasure 

 boats and picturesque settings for homes. Yet 

 these small bays, edged with Spartina marshes, 

 are primary trophic energy sources in the economy 



of the sea. It appears that further long-term 

 investigations of the effects of dredging on 

 these bays is warranted. 



A summary of the areas of investigation and 

 conclusions follow: 



1. The dredging process caused turbidity 

 throughout the bay. Light penetration was re- 

 duced to 0.4 m during dredging, but the particu- 

 late matter released was rapidly dissipated. It is 

 unlikely that turbidity affected light penetration 

 enough to interfere with photosynthesis. How- 

 ever, a canopy of flocculent material deposited 

 on the plants as the result of the deposition of 

 suspended bottom material may have interfered 

 with primary productivity in the low current 

 velocity areas of the bay. 



2. Water transport patterns were greatly modi- 

 fied as the result of dredging. Current velocity 

 in the eastern half of the bay was reduced 

 approximately 50%, while small increases were 

 noted for the middle portion of the bay, which 

 previously had negligible velocities. 



The main mass movement of water shifted 

 from channel A to channel B as the result of 

 deepening the latter channel. 



Dye studies revealed that flushing time of the 

 bay as a whole was not appreciably changed. 



3. Correlations between sediment particle size 

 and changes in current velocity suggested that 

 the distribution of sediment types in Goose Creek 

 would be permanently changed as the result of 

 modified current velocities. 



4. Values of particulate phosphorus, silicates, 

 and chlorophyll a increased substantially. Dis- 

 solved organic phosphorus and nitrates increased 

 slightly during the post-dredging year. 



A number of authors have reported increases 

 in phytoplankton and/or benthic productivity as a 

 result of increased nutrient levels, but no defini- 

 tive correlation could be observed in the course 

 of this study. 



5. It was found that wind-driven currents af- 

 fected the distribution of nutrients and bacteria 

 in the bay. In view of the predominance of strong 

 northwesterly winds over the year and the 

 shallow, slowly moving water of the western 

 half of Goose Creek, it was suggested that sedi- 

 ment deposition in this region was primarily a 

 function of wind-driven currents. The assertion 

 by Flemer (1968) that late fall is the best season 

 for dredging is disputed on the basis of a high 



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