the size distribution of sediment may serve as an 

 indication of the mode of its deposition. A 

 schematic representation of sorting of the material 

 suspended in water and its deposition in the four 

 zones of an estuarine system according to the 

 distance from the mouth of the river is shown in 

 figure 370, redrawn from Rochford (1951). At 

 the head of an estuary silt is transported by tidal 

 streams further than sand and because of floccula- 

 tion, caused by the influx of saltier water, smothers 

 the beds near the mouth (middle and left parts of 

 fig. 370). The complexity of the horizontal 

 pattern of currents is shown on the lower part of 

 the diagram. 



Particles immediately above the bottom move 

 by rolling, sliding, and jxunping. Outside of this 

 narrow "bed layer" (Einstein, 1950) the particles 

 in water constitute the suspended load; their 

 weight is continuously supported by the fluid 

 until they reach the lower part of the estuary 

 where they are deposited over the bottom and on 

 tidal flats. 



The problem of transport and settling of sedi- 

 ments in the sea is not well understood (Trask, 

 1950, p. 9) and is being intensively studied by 

 many oceanographic institutions of the world. 

 In tidal waters where oyster beds are located the 

 problem becomes more complex because the rate 

 of transport and settling is greatly influenced by 

 periodic changes in cmrent velocities, tm'bulence, 

 salinity, temperatiu'e, density and viscosity of 

 water, and size, shape, roughness, and specific 

 gravity of transportable particles. In the tidal 

 regime where salty oceanic water mixes with fresh 

 water another important factor called floccula- 

 tion enters into this already complex picture. 

 Small dispersed particles of clay and of organic 

 detritus have a tendency to aggregate in small 

 lumps. In the pm'ification of drinking water 

 flocculation is produced artificially by adding a 

 coagulant such as aluminum sulphate (Al2(S04)3- 18 

 HoO). In estuaries where sea and fresh water 

 mix the flocculation is caused by the change in the 

 electric charges of the particles which occur with 

 the change in the hydi'ogen ion concentrations. 



Laboratory experiments with kaolin suspension 

 show that flocculation of particles about 4^ in 

 diameter may occm- at constant pH (8.5) by in- 

 creased concentrations of Na+ at constant S04-_ 



concentrations (Whitehouse, 951, 11952).' The 

 results indicate the tendency of positive ions to 

 cause the flocculations of the negatively charged 

 particles. 



Flocculation in coastal waters may be observed 

 from the deck of a ship at the time of freshets 

 along the coastline of the Carolinas and Georgia. 

 Large patches of aggregated silt particles show 

 clearl}' in the sahy offshore water while the brown- 

 ish color of less saline water at the mouth of a 

 river remains uniform. 



Rochford (1951) states that flocculation of silt 

 takes place in the salinity zone of less than 19°/oo 

 and higher than 10°/oo (fig. 370). Since floccula- 

 tion depends on several factors including tempera- 

 ture, pH, and the type of sediments in suspension, 

 one may expect a great variation in the rates of 

 settling in the various estuaries of the Atlantic 

 and Gulf coasts. 



The gross effect of sedimentation may be meas- 

 ured by determining the depth of deposition of 

 silt over an oyster bottom per unit of time and 

 area. Less noticeable, but highly significant, is 

 the deposition of a thin layer of sediment over 

 the hard surfaces to which many organisms, in- 

 cluding oyster larvae, attach. A deposit of loose 

 sediment only 1 or 2 mm. thick is enough to make 

 the sm'face of shells and rocks unsuitable for the 

 attachment of larvae and to cause failure of 

 setting. I have observed such conditions many 

 times in certain sections of Oyster River, Chatham, 

 Mass.; in the Wiweantic River of the Cape Cod 

 area; in the small rivers and creeks emptying into 

 Delaware Bay; and in the Rappahannock and 

 York rivers, Va. U^ndoubtedly similar conditions 

 may be found in many other places where silt is 

 transported by estuarine currents. Light sedi- 

 mentation is not harmful to populations of adult 

 oysters, but may be hea\^ enough to interfere 

 with their reproduction. 



' Whitehouse, U. Grant. 



1951. A study of chemical sedimentation and of physical oceanography 

 sponsored at Texas A. and M. College by the American Petroleum 

 Institute through the Scripps Institution of Oceanography of the 

 University of California (A.P.I. Project 51). Progress report tor the 

 quarter ending December 31, 1951. The Te.Nas A. and M. Research 

 Foimdation, Project 34, The Agricultural and Mechanical College of 

 Texas, Department of Oceanography, College Station, Texas, pp. 

 (Preliminary report). 



1952. A study of the chemistry of oceanic sedimentation sponsored at 

 Texas A. and M. College through the Scripps Institution of Oceanog- 

 raphy of the University of California (A.P.I. Project 51). Progress 

 report for quarter ending December 31, 1952. The Texas A. and M. 

 Research Foundation, Project 34. The Agricultural and Mechanical 

 College of Texas, Department of Oceanography, College Station, 

 Texas, 11 pp. (Preliminary report). 



FACTORS AFFECTING OYSTER POPULATIONS 



411 



