'^ which are usually encountered in estu- 



volume of the basin in acre feet, and t time in 

 days since the pollution started. 



In this formula it is assumed that the influx 

 and efflux occur discontinuously once a day. By 

 assuming that the influx of contaminated water 

 and the efflux are contmuous, Tuckerman (see: 

 Galtsoff, Chipman, Engle, and Calderwood, 1947, 

 p. 100) arrives at the foHowing formula: 



p^po.[l-{l-K)'] 



where poo is the hmit which the proportion of 

 contaminated water approaches after a long time 



and X=^^^- Computations made by using the 



two formulas indicate the same value for the 



nm=—^! but differ in the rate at which this 

 ' a-\-b 



limit is approached. For the small values of 



aries, the two rates are practically identical and 

 the simpler Hotteling formula may be applicable. 



In many estuaries the water is stratified. With 

 relation to stratification and circulation patterns 

 Pritchard (1955) distinguishes four types of 

 estuaries — highly stratified (type A), moderately 

 stratified (type B), and vdrtually homogeneous 

 (types C or D). The reader interested in the 

 dynamics and flushing in different types of es- 

 tuaries should consult the original contributions 

 of Pritchard (1952a, 1952b, 1955) and Pritchard 

 and Kent (1953) in which the complex hydro- 

 dynamical problem is analyzed. It is sufficient 

 for a student of oyster ecology to realize that 

 vertical and horizontal distributions of oyster 

 larvae will be different in each of the four principal 

 types of estuaries. 



A free-swimming organism such as bivalve larva 

 cannot be considered in the same manner as a ma- 

 terial dissolved in water or as an inanimate body 

 passively transported by water movements. Lar- 

 vae of bivalves, barnacles, and other invertebrates 

 may have a tendency to swarm and, therefore, 

 their distribution may not be uniform even in a 

 homogeneous environment. Oyster larvae react 

 to changes in the environment by periodically 

 closing their valves and dropping to the bottom or 

 by swimming actively upward or in a horizontal 

 plane. Consequently, they may be carried up- 

 stream or downstream according to their position 

 in the water column. Field observations in the 



estuaries of New Jersey and Chesapeake Bay 

 (Carriker, 1951; Manning and Whaley, 1955; 

 Nelson, 1952) in which the salinity of water in- 

 creases from surface to bottom indicate that ver- 

 tical distribution of larvae is not uniform. The 

 late umbo larvae of C. virginica have a tendency 

 to remain within the lower and more saline strata, 

 and are probably stimulated to swim by the change 

 in salmity at flood tide. A brood of larvae swim- 

 mmg withm a given salinity layer will be passively 

 moved in the direction of the current. Nelson 

 observed that in Barnegat Bay, N.J., the brood of 

 larvae of setting size was carried about 3 miles up 

 the bay in a single evening spring tide. In 

 Yaquina River, a small tidal stream along the 

 ocean shore of Oregon, swimming larvae were 

 transferred by tidal currents and set several miles 

 above the natural beds (Fasten, 1931; Dimick, 

 Egland, and Long, 1941). There are many other 

 places where setting grounds are far above the 

 spawning grounds. Since the seaward discharge 

 of water in an estuary usuaUy exceeds the land- 

 ward movement, it was difficult to visualize a 

 mechanism by which the larvae can be transported 

 up an estuary and left there. The existence of 

 such a mechanism became apparent, however, 

 from the hydrographic studies by Pritchard (1951). 

 He found that estuaries may be considered as being 

 composed of two distinct layers: an upper layer 

 in which the net movement is toward the mouth 

 (positive movement), and the lower layer in which 

 the net movement is toward the head of the estuary 

 (negative movement). There is a boundary be- 

 tween the two layers which may be called "the 

 level of no net motion" (fig. 366). Since the net 

 movement seaward does not result in a net dis- 

 placement of the fines of constant safinity in the 

 upper layer, there must be a progressive transfer 

 of the deeper, higher safinity water of the lower 

 layer upward, across the boundary level, to be 

 included in the seaward transfer. The role of the 

 strongest tidal currents is primarily that of pro- 

 viding energy for the mixing processes. Compu- 

 tations made by Pritchard show that superimposed 

 on tidal oscillation there is a residual or nontidal 

 seaward drift on the surface and a net landward 

 drift along the bottom. He appfied his theory to 

 a study of the seed oyster area of the James River, 

 Va., and found that below a depth of about 10 feet 

 there is a net (or residual) upstream movement of 

 water at an average speed of slightly more than 

 one-tenth of a knot. This is the type of estuary 



402 



FISH AND WILDLIFE SERVICE 



