KAPLAN, WELKER, and KRAUS: EFFECTS OF DREDGING 



and productivity of an estuarine area. Raising 

 the level of the marsh above the inundation zone 

 will replace the highly productive Spartina 

 community with the less biologically useful 

 Phragmites communis. Much of the food of detritus 

 feeders comes from the disintegrating plant 

 material of the Spartina marsh and, in the ab- 

 sence or depletion of this food source, the species 

 mix and/or proportion of detritus to deposit on 

 filter feeders may be permanently changed. 



Even the removal of shell from a mud bottom 

 has been suggested as a reason for the exclusion 

 of certain species from a dredged bay. Barnard 

 and Reish (1959) suggest that the amphipod, 

 Metaceradocus occidentalis and the polychaete 

 Scyphoproctus oculatus were in danger of losing 

 their habitat as the upper shell and rock laden 

 layers of the mud substratum were removed by 

 a dredging operation. 



The distribution or removal of materials during 

 dredging in a body of water with even minimal 

 flushing action results in immediate, temporary, 

 and long-term changes in its ecology. The inter- 

 action of organisms with this rapidly changing 

 environment is poorly understood. Estuarine 

 organisms are noted for their ability to withstand 

 environmental vicissitudes, yet this adaptability 

 may be overstressed by one or another aspect 

 of the dredging process. For example, Postma 

 (1967:226) refers to the difference in the distribu- 

 tion patterns of dissolved and suspended ma- 

 terials. He points out that dissolved materials 

 have a net transport from regions of high con- 

 centration to regions of low concentration, causing 

 a rapid dispersal of the dissolved matter and its 

 consequent removal from the source area: "In 

 the case of suspended matter the reverse often 

 occurs. This material may be trapped and accumu- 

 lated in the nearshore environment." Thus, a 

 benthic organism in the vicinity of a dredging 

 operation can be subjected to a short-term rapid 

 surge of dissolved nutrients in its environment, 

 with all of the concomitant interactions this 

 represents. Superimposed on this relatively fleet- 

 ing enrichment of the water would be the longer- 

 term deposition of suspended sediments. The 

 interaction between the two, such as the adsorp- 

 tion of organic compounds on suspended clay 

 particles (e.g. amino acid complexes binding 

 strongly to clays) (Siegel, 1966), the effects of 

 flocculation, etc., is poorly understood. The pre- 

 sence of the dissolved organic compounds lib- 

 erated by the dredging process also can have 



beneficial effects on the benthic organisms. 

 Siegel quotes Stephens and Schinske (1961) who 

 found that glucose, glycine, and aspartic acid 

 can serve as energy sources for marine inverte- 

 brates. Organic matter may also supply a growth 

 factor such as vitamin B12 or may inhibit the 

 growth of bacteria by its antibiotic effect (Saz 

 et al., 1963). It may promote growth by solu- 

 bilizing trace metals, thus making them available 

 (Johnston, 1964). Udell et al. (1969) analyzed 

 marsh grasses and found a number of vitamins, 

 including vitamin B12. The destruction of peri- 

 pheral marsh by spoil deposition may eliminate 

 a constant source of vitamins and other nu- 

 trients made available by the disintegration of 

 the Spartina. 



The effects of the dispersion of light rays in 

 the turbid water of a dredged bay is also in- 

 completely understood. It is unlikely that in- 

 creased turbidity can destroy benthic flora 

 through light deprivation in shallow waters. 

 Clendenning (1958) studied the relationship be- 

 tween photosynthesis and light intensities for 

 Macrocystis pyrifera laminae. Compensation 

 (light intensity where photosynthesis balanced 

 respiration) occurred at 15 foot candles using 

 white light. First evidences of saturation occurred 

 at about 400 foot candles and maximum photo- 

 synthetic rates occurred at 1,600 foot candles. 

 Since the intensity of daylight delivered to the 

 water surface is about 10,000 foot candles, it is 

 unlikely that the light values would so depreciate 

 in shallow water as to seriously impair photo- 

 synthesis. On the other hand, the authors ob- 

 served a colony of Ruppia after dredging and the 

 leaves were covered by a light brown flocculent 

 material which had been deposited from the water. 

 Large areas of Enteromorpha and Aghardiella 

 showed a similar canopy of fine sediment. It is 

 possible that the deposition of opaque material 

 from the water onto leaves and stipes in areas 

 of negligible current velocity might pose a threat 

 to the plants by inhibiting photosynthetic activity 

 even though the turbidity of the overlying water 

 is not high enough to reduce adequate light 

 penetration. 



The estuarine environment is particularly sus- 

 ceptible to particle deposition. Although it shares 

 the factor of close proximity to the source of the 

 particulate matter with open beaches, the beaches 

 have a longshore drift factor which tends to 

 distribute particulate matter. It is well known 

 that beach sands are well sorted. Estuarine areas, 



473 



