KAPLAN. WELKER, and KRAUS: EFFECTS OF DREDGING 



cause a net transport of materials towards the 

 lee shore. Wind storms can so pile up water at 

 the mouth of the bay that flood tide current veloci- 

 ties would be considerably above the normal 

 range, causing erosion of the banks of tidal 

 channels and exaggerated depositional patterns, 

 or winds can depress the natural flushing action 

 of the ebb tide, increasing the deposition of light 

 particles. A number of the aforementioned factors 

 have not been considered in the literature in 

 detail, perhaps because most investigations are 

 concerned with relatively large and deep bodies 

 of water. However, Biggs (1968) concludes that 

 most of the suspended material in Upper 

 Chesapeake Bay came from the bottom and had 

 been stirred by wind-waves and currents. 



Inman ( 1949) refers to three basic factors in the 

 transportation and deposition of sediments: 

 degree of bottom roughness, settling velocity, 

 and threshold velocity. He shows that as current 

 velocity drops in a downstream direction, particle 

 size also decreases. The degree of sorting, 

 however, is at a maximum in sediments with a 

 median diameter near the grade of fine sand 

 (0.18 mm). Threshold velocity for grain diameters 

 less than 0. 18 mm increases with decreasing grain 

 size. Since the threshold velocity is much greater 

 than the setting velocity for smaller particles, 

 suspended particles entering a bay will, when 

 deposited, have a tendency to remain a part of the 

 substratum rather than move about by surface 

 creep or resuspension. On the basis of these 

 characteristics of fine sands, Sanders (1958) 

 deduces that they must represent a very stable 

 environment. He also emphasizes the role of clay 

 as an efficient binding agent for organic matter, 

 thus influencing the number of deposit feeders 

 present. The simple clay-silt proportion governing 

 the population size of Sanders' deposit feeders 

 is not apparent in the distribution of filter feeders, 

 where more complex factors are at work. 



McNulty et al. (1962) related low current veloc- 

 ity to the accumulation of a detritus layer on the 

 sediment surface capable of supporting large 

 populations of detritus feeders. 



Rhoads and Young (1970) suggest that biogenic 

 reworking lowers critical erosion velocity and 

 increases the instability of the substratum as 

 manifested by a high resuspension rate and in- 

 creased turbidity close to the silt-water interface, 

 placing selective pressure on suspension feeders. 



In the present investigation, maxima in bio- 

 mass production occurred in areas of coarse and 



fine sand in the channel (stations B, C, and H) 

 with current velocities of the order of 56 cm/sec 

 and 17 cm/sec, before dredging. 



In the bay as a whole 14 of 113 individual 

 dredge hauls yielded dry weights above 80 g/m^. 

 Since the distribution of organisms was so patchy, 

 these extraordinarily large standing crop mea- 

 sures are perhaps the best index of the productiv- 

 ity of the various substrata. The highest biomass 

 was recorded for station 2. However, this con- 

 sisted almost exclusively of Crepidula fornicata, 

 an epibenthic gastropod which requires the 

 scouring action of a rapid current to establish 

 a substratum of stones upon which it clings with 

 a broad foot. Stations 7 and 9 had high and 

 medium current velocities (41.5 and 12 cm/sec) 

 and supported extensive colonies of the poly- 

 chaetes Clymenella torquata and Notomastus la- 

 tericeus, as well as large pelecypods (Mya, Ensis, 

 Mercenaria) in the case of station 9. Both C 

 torquata and A'^. later iceus are deposit feeders 

 inhabiting sandy sediments. 



Stations 16, 17, and 22 were in regions of 

 almost negligible current velocity which were 

 characterized by a substratum of silt over fine 

 gray sand. The major weight contributors at sta- 

 tions 16 and 17 were Sclerodactyla (Thyone) and 

 large Mercenaria, with the polychaetes, Capitella 

 capitata, Polydora ligni, Scoloplos robustus, and 

 S. fragilis making important contributions. Poly- 

 dora is almost exclusively an inhabitant of mud, 

 while the other worms are found in sandy mud. 



All of the above-mentioned worms are deposit 

 feeders whereas Sanders groups Mercenaria and 

 Sclerodactyla together as suspension feeders. 



Deposit-detritus feeders were important con- 

 tributors to the biomass in Goose Creek, in both 

 the sandy and muddy habitats. These animals are 

 more or less substratum-specific, as can be seen 

 on their distribution graphs (Figure 12) and in 

 Sanders' data. Changes in current velocity have a 

 profound influence on the nature of the sub- 

 stratum and, consequently, on animal distribu- 

 tion. This is especially true in the regions of the 

 sandier sediments. Stations 2, 7, and 9 had 

 reductions from 50 to 75% of pre-dredging 

 velocities. In the western portion of the bay, wind- 

 driven currents are the predominant means of 

 sediment transport, and, although some changes 

 in the mid-bay region could be expected due to 

 increased current velocities, these would not have 

 a substantial influence on the soft sediment of 

 the western half of the bay. 



471 



