causes of movement and mixing of water, which 

 may be due either to tide, wind, or river flow. 

 Rochford (1951) points out the significant differ- 

 ences between brackish water and estuarine en- 

 vironment. According to his ideas, brackish water 

 refers to a dynamically stable environment of 

 lakes, lagoons, etc., in which sea water, diluted 

 by fresh water, is not necessarily influenced by 

 tidal movements. On the other hand, the estua- 

 rine environment, influenced by tidal rise and 

 fall, is dynamically unstable. 



The persisting factors of a typical estuarine 

 envh'onment are seasonal salinity variations and 

 circidation exchange between the river and sea 

 water. The intruding salt water forms a wedge 

 or prism with its base at the mouth and the tip 

 at the upper part of the estuary. The position of 

 the wedge along the bottom and its dimensions 

 depend to a gi-eat extent on the flow of the river 

 water. 



Circulation and mixing of water is a highly 

 complex problem adecjuately discussed in the 

 papers of Rochford (1951), Ketchum (1951a, 

 1951b), Pritchard (1951, 1952a, 1952b), and in 

 the textbook. The Sea, vol. II, edited by Hill 

 (1963). 



It is important for a biologist to understand 

 that the type of circulation that prevails in a 

 specific estuary depends on physical structure, 

 i.e., size, depth, bottom configuration, etc., river 

 flow, and vertical salinity gxadients along the 

 enth'e length from head to mouth. Circulation 

 pattern and mixing have important biological 

 implications in the study of the distribution and 

 transport of sediments, pollutants, and plank- 

 ton, including free-swimming larvae of sedentary 

 invertebrates. 



The volume of fresh water entering at the head 

 of an estuary occupies the upper layer and 

 exceeds the volume of the up-estuary flow in the 

 lower and more saline layer by an amount sufficient 

 to move the fresh water toward the sea (Pritchard, 

 1951). As one moves seaward, the volume of 

 saline water contributed by the ocean increases 

 while the river water, entered at the head of the 

 estuary is being removed. The process of removal 

 of river water, called flushing, continues ttu-oughout 

 the estuary from its mouth to the so-called "inner 

 end" which is defined by Ketchum (1951b) as 

 "the section (of an estuary) above which the 

 volume required to raise the level of the water 

 from low to high water mark is equal to the 



volume contributed by the river dm-ing a tidal 

 cycle." Consequently, there will be no net 

 exchange of water through this section dm-ing a 

 flood tide and the water above the section should 

 be completely fresh. 



The sahnity at any location below the inner 

 end varies with tide but retm-ns to substantially 

 the same level on successive tidal stages. Assum- 

 ing that a net seaward transport of fresh water 

 during any tidal cycle is equal to the volume 

 introduced by the river in the same period of time, 

 and that there is no net exchange of salt water 

 through the cross section during the same tidal 

 cycle, Ketchum (1951b, 1954) advanced a simpli- 

 fied theory which permits an easy calculation of 

 the proportion of water removed by the ebb tide. 

 This theory is based on the assumption that in 

 each of the volume segments, limited by the 

 average excm'sion of water on the flood tide, the 

 water is completely mixed at high tide. Accepting 

 this assumption, which is obviously a simplification 

 of the actual conditions, the rate of exchange in a 



given segment (r^) has the value rp= p ^ in 



which Pn is the intertidal volume and Vn the 

 low tide volume of the segment n. The average 

 length of time required for the river water with 

 a particle suspended in it to move thi-ough a 

 segment of an estuary is called flushing time, 

 which is defined as a ratio obtained by dividing 

 the volume of river water, calculated from the 

 salinity data, by river flow. The ratio is expressed 

 in number of tides. In Raritan Bay, N.J., a 

 sm'vey made by the Woods Hole Oceanographic 

 Institution indicates the flushing time for the 

 entire estuary was 60 tides. 



If a stable pollutant is discharged at a constant 

 rate at the head of an estuary and is uniformly 

 mixed as it is transported downstream, its propor- 

 tion in the water can be calculated by using the 

 formula of Hotelling which was applied in deter- 

 mining the concentration of poUutant over 

 Olympia oyster beds (Hopkins, Galtsoff, and 

 McMiUin, 1931). According to this formula the 

 proportion p of a contaminant in a basin is: 



a+bl 



l-{l-a+b) 



V 



] 



where a is the rate of discharge of contaminant 

 in acre-feet per day, b the rate of influx of water 

 into the basin in acre-feet per day, V the total 



FACTORS AFFECTING OYSTER POPULATIONS 



401 



