Chapter V — 63 — Distribution in the Sea 



which Taylor (1940) collected from the lake at a distance 200 meters 

 offshore from the point of entry of the river. In his studies on the fate of 

 bacteria washed into Lake Zurich by inflowing rivers, Kleiber (1894) 

 noted that the bacterial population dropped off sharply with increasing 

 distance from land, the zone of rich flora extending only about 20 meters 

 from the mouth of the river. 



Similar observations on the horizontal distribution of bacteria were 

 made in Lake Mendota by Fred el al. (1924). They found 400 to 600 

 times as many bacteria in samples taken at the mouth of a storm-water 

 sewer as in samples of water collected 100 meters offshore. The bacterial 

 population of Lake Mendota, Wisconsin, was not influenced by the in- 

 flowing water after heavy rainfall or at times of melting snow for a dis- 

 tance of more than 1000 to 2000 meters from shore. These investigators 

 concluded that, although land drainage is a continual source of contam- 

 ination in the lake, the adventitious organisms introduced from the land 

 do not remain long in large numbers, and except under unusual condi- 

 tions, they do not occur very far from shore. 



Effect of the tide : — The number of bacteria found in the entrance of 

 San Diego Bay is influenced by the tide, which moves in and out twice a 

 day. After the tide has been flowing in for live or six hours, the water 

 may contain no more than a few thousand bacteria per ml. as compared 

 with hundreds of thousands per ml. after the tide has been ebbing out of 

 the Bay for similar periods. The bacterial content of the water a mile or 

 two from the entrance to San D.ego Bay is also influenced by the ebb and 

 flow of the tide. The larger bacterial population of the Bay water is at- 

 tributed to an increased content of organic matter and suspended solids, 

 both of which promote the growth of marine bacteria. 



Movements of the water caused by tides have no effect on bacterial 

 populations except in very shallow water where bottom deposits are sus- 

 pended or in regions where large quantities of organic matter may be car- 

 ried seaward by tidal currents. ZoBell and Feltham (1934) collected 

 samples from the end of the Scripps Institution pier at high and low tide 

 over a period of 14 weeks, finding an average of 348 bacteria per ml. at low 

 tide and 294 per ml. at high tide. On 15 days the low tide samples had a 

 larger bacterial population, on 1 2 days the high tide samples showed the 

 larger counts and on 26 days the results of high and low tide samples were 

 virtually the same. No other diurnal variations were noted during this 

 time. 



In those parts of the world where the tidal range is much greater than 

 it is in the San Diego region, one might expect the influence of tidal cur- 

 rents on the composition of water to extend farther from land than it 

 does here. 



According to Lloyd (1930), the influence of tidal movements is more 

 marked when there is a slightly shelving coast-line with a comparatively 

 broad intertidal zone which supports much plant and animal life. In the 

 lochs which she investigated in the Clyde Sea area off Scotland, "the 

 steeply sloping sides afforded only a narrow intertidal zone, and the tidal 

 variations do not appear to have much effect on the bacterial content of 

 the water." 



The bacterial population of sea water collected from the mouth of Mis- 

 sion Bay fluctuates with the tide as shown by the data in Table XV. Mis- 

 sion Bay is a shallow inlet of the ocean located midway between San Diego 



