ZoBeU 



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Marine Microbiology 



Table XV. — Bacterial population of inflowing and outflowing water collected at the mouth 

 of Mission Bay (from ZoBell and Feltham, 1942): — 



and La Jolla, California. At high tide the water covers an area of four 

 square miles and at low tide it is reduced to channels and lagoons aggre- 

 gating less than half this area. The average depth of Mission Bay is only- 

 one meter, so a considerable portion of water is exchanged at each tidal 

 cycle. Therefore an analysis of the outflowing water gives information 

 on the bacterial population of the Bay water while the inflowing water is 

 more representative of marine conditions. The rapid movement of the 

 water tends to suspend much material. 



Diurnal fluctuations in the bacterial population: — Fischer (1894a) 

 reported finding somewhat more bacteria in surface waters of the North 

 Atlantic during the hours of darkness than during periods of intense sun- 

 light. Similar diurnal fluctuations in the bacterial population of surface 

 water were noted by Bertel (191 2) in the Mediterranean Sea and by 

 Lloyd (1930) in the Clyde Sea. However, only a limited number of ob- 

 servations were made by these workers. Bertel's conclusions were based 

 upon only three series of samples. 



Bacteriologists at the Scripps Institution of Oceanography have plated 

 water samples periodically throughout the day and night at the end of the 

 pier as well as at several stations in the Pacific Ocean off the coast of 

 southern California and in the Gulf of California. The results from nearly 

 a hundred series of samples failed to show any regular diurnal fluctuations 

 in the bacterial population of water collected from depths of from one to 

 five meters. In 26 series, more bacteria were found at night than during 

 the day-time, but in 21 series the reverse condition was found. In 47 

 series the night and day counts were the same within the limits of accu- 

 racy of the plate count procedures; namely, ± 10 per cent. In those 

 series where there was a significant difference in the abundance of bacteria 

 found during different times of the day, the difference was no greater than 

 what might be expected from the sporadic distribution of microorganisms 

 in sea water. 



As will be discussed below, it is doubtful whether the penetration of 

 the ultraviolet radiations of sunlight is sufficient to cause any perceptible 

 diurnal fluctuation in the bacterial content of sea water, particularly in 

 view of the fact that the topmost few meters of water are fairly well mixed 

 by wave action. The diurnal vertical migrations of zooplankton or nekton 

 may influence the diurnal distribution of bacteria in the euphotic zone 



