ZoBeU 



Marine Microbiology 



Table XVIII. — Average number of bacteria per ml. of water found at diferent depths in the 

 Clyde Sea by Lloyd (ipjo); — 



its in the Clyde Sea. Lloyd (1930) found that the number of bacteria 

 in surface water fluctuated widely, "apparently in relation to factors 

 which are not seasonal but irregular." 



Due to differences in the depth, size, water movements, inflowing 

 streams, seasonal changes, stratification, and other conditions in different 

 lakes, it is difficult to generalize on the vertical distribution of bacteria in 

 lakes. Moreover, the annual turnover tends to minimize differences in 

 the vertical distribution of bacteria. According to Henrici (1939), there 

 is no sharp differentiation between the numbers of bacteria in the epilim- 

 nion and the hypolimnion even during periods of stratification, although 

 he and most other workers have found somewhat more bacteria in the 

 epilimnion than in the hypolimnion. The epilimnion is the upper stratum 

 of sea or lake water in which the temperature is essentially uniform. In 

 the stratum next below, known as the thermocline, there is a sharp drop in 

 temperature per unit of depth. The hypolimnion is the lowermost region 

 or stratum in which the temperature is nearly uniform. 



Kleiber (1894) failed to find any differences in the abundance of bac- 

 teria at various depths in Lake Zurich, but he examined water from only 

 a few levels. At first. Minder (1920) made similar observations on the 

 vertical distribution of bacteria in Lake Zurich, but later, when he exam- 

 ined samples from depth intervals of 5 meters. Minder (1927) found the 

 largest bacterial populations at a depth of 1 5 meters : 



Surface 



5 meters 



15 meters 



30 meters 



50 meters 



623 bacteria per ml. 

 700 bacteria per ml. 

 826 bacteria per ml. 

 503 bacteria per ml. 

 571 bacteria per ml. 



In Lake Ritom in Switzerland whose high salt content in the hypolim- 

 nion prevents an overturn of water, Duggeli (1924) found marked dif- 

 ferences in the types and numbers of bacteria at different depths. There 

 were numerous heterotrophic aerobes in the surface layers and virtually 

 none in the deeper water. Sulfate-reducing bacteria were abundant in 

 the more saline bottom layers which had a high content of H2S. Purple 

 sulfur bacteria thrived in an intermediate zone where the H2S-containing 

 water approached the overlying oxygenated water. A similar bacterial 

 stratification occurs in the Caspian Sea in which there is little or no dis- 

 solved oxygen below 300 meters. The Caspian Sea is a salt lake having a 

 salinity ranging from 10 to 2o°/oo, the average being around i2.8°/oo. 



Bacterial stratification is even more marked in the Black Sea. Heter- 

 otrophic aerobes are abundant in the upper layers where the salinity is 15 

 to iSVoo and virtually absent in the H2S-rich water at depths exceeding 

 150 meters where the salinity is 22 to 25Voo- Only sulfate reducers and a 

 few other anaerobic types of bacteria occur in the Black Sea at depths 

 exceeding 150 meters. 



