FISHERY BULLETIN: VOL. 72. NO. 2 



on the other hand, have a circulation cycle which 

 favors deposition. Postma (1967:229) states: "The 

 estuarine circulation therefore acts as a 'sediment 

 trap' in which water flows freely seaward, but 

 particles heavier than the water are retained." 



Flemer et al. (1968) list a numher of factors 

 associated with the effects of dredging on animal 

 survival and suggest that suspended sediments 

 probably affect many sites in the energy flow 

 sequence of the benthic community. 



Several studies have been made on the effects 

 of siltation on the survival of pelecypods. 



Loosanoff and Tommers (1948), Davis (1960), 

 Davis and Hidu (1969), and Loosanoff (1962) 

 described harmful effects of heavy sediment loads 

 on eggs, larval development, and adult pelecypods, 

 while Lunz (1938), Wilson (1950), Mackin (1956), 

 and Dunnington (1968) showed that adult oysters 

 do not suffer appreciable physiological damage 

 unless subjected to very heavy siltation or buried. 



Pfltzenmeyer (1970) described the effects of 

 dredging and spoil deposition in Upper Chesa- 

 peake Bay. The dredging process did not cause 

 major topographical or stratigraphical changes 

 since the spoil was fundamentally identical with 

 the substratum upon which it was deposited and 

 it spread out to form a thin layer over the bottom, 

 undisturbed by strong currents. Species mix and 

 biomass were markedly reduced immediately 

 after dredging, but recovered to original levels 

 after 18 mo. 



Of interest in Pfitzenmeyer's study is the super- 

 imposition of the natural cycles of certain mol- 

 luscs on the data related to dredging. The pele- 

 cypods Macoma phenax and M. balthica were in a 

 period of natural decline during the period of the 

 study, while i?an^ia cuneata experienced a short- 

 lived population explosion, reaching a density of 

 10,000 clams per m^. One year after the study, 

 the Rangia population had disappeared. These 

 rapid and extreme fluctuations in the population 

 densities of organisms profoundly affected bio- 

 mass calculations because of the relatively large 

 size of the pelecypods, compared with, for example, 

 the three permanent dominants, two of which 

 were an isopod and an amphipod. If the Rangia 

 population increase had not compensated for de- 

 creases in the Macoma populations during the 

 study, it is possible that there might have been 

 significant differences in the results. If the dredg- 

 ing had substantially altered the substratum, e.g., 

 by removing the silt to a depth sufficient to 



expose the sand underneath, the recovery of the 

 populations might have required a period of 

 substratum stabilization before achievement of 

 normal populations. 



Pearce (1970) studied a spoil deposition area 

 of the New York Bight known as the "dead sea." 

 He describes the benthic environment as severely 

 affected by the deposition of large quantities of 

 spoil. He found contamination by heavy metals, 

 pesticides, and petroleum derivatives. The central 

 portion of the spoil area contained no living 

 macrofauna; peripheral areas were frequently 

 barren or impoverished; interstitial waters of spoil 

 sediments had extremely high coliform counts. 



In laboratory experiments where the crusta- 

 ceans Homarus americanus and Cancer irroratus 

 and the xiphosuran Limulus polyphemus were 

 exposed to sludge and spoil sediments, high 

 mortalities, and pathological conditions were 

 described. 



Pearce concluded, ". . . sewage sludge and dredge 

 spoil deposits are incompatible with most normal 

 biological phenomena," (p. 66). He blames this 

 condition on: 



1) adults being killed by toxins, anoxia, or 

 inundation by solid wastes; 



2) interference with or destruction of eggs and 

 larvae; and, 



3) active avoidance by adult and larval 

 organisms. 



A number of reasons suggest themselves to 

 explain why the results of Pfitzenmeyer's and 

 Pearce's studies are so diametrically opposed. 

 For one, Pearce's study area was one of constant 

 spoil deposition; Pfitzenmeyer's had only one in- 

 undation. Secondly, Pfitzenmeyer records rela- 

 tively normal concentrations of oxygen while 

 Pearce indicates that oxygen concentrations were 

 frequently 2-3 ppm lower in the water above the 

 spoil. 



Finally, there seems to be a very high degree of 

 contamination of the dredged sediments with 

 heavy metals, insecticides, and petroleum frac- 

 tions in Pearce's study, which is absent in Pfitzen- 

 meyer's. 



A number of studies was performed on the 

 effects of dredging on oyster production. Breuer 

 (1962) reported major changes produced by dredg- 

 ing spoil deposition in South Bay, Tex. Water 

 circulation was impaired by reducing the size of 

 the entrance. Water depth decreased, much of the 

 oyster population was silted over and destroyed, 

 and high local turbidity was evident. 



474 



