ZoBell — 74 — Marine Microbiology 



others which multiply at temperatures several degrees higher than those 

 encountered in any marine environment. 



In environments less stable than the ocean or in regions where there 

 are no other limiting factors, temperature is one of the most important 

 ecological factors. In lakes and reservoirs, for example, there is often an 

 indirect correlation between the vertical and seasonal distribution of 

 bacteria with temperature. Moreover, seasonal changes in temperature 

 are responsible for the annual overturn of the water which is character- 

 istic of lakes in the temperate zones (Welch, 1935). Taylor (1940) found 

 no direct effect of temperature on the number of bacteria in water in 

 English lakes, the annual temperature of which ranges from 2° to 20° C. 



The bacterial population of sea water stored in glass receptacles is 

 largely a function of the temperature of incubation until a state of equi- 

 librium is reached. Thereafter the bacterial population is independent of 

 temperature within the range of from 0° to 25° C. 



The temperature range of growth of marine bacteria and the influence 

 of temperature on their activities are discussed elsewhere in this volume. 



Seasonal distribution of marine bacteria: — Very few workers have 

 been in a position to make sufficiently extensive or continuous observa- 

 tions on the bacterial populations of oceans or lakes to show definite sea- 

 sonal cycles of abundance. In Lake Zurich, Pfenniger (1902) observed 

 a summer minimum, with spring and autumn maxima. He attributed the 

 autumn maximum to the food supplied by dead plankton organisms. 

 After making similar observations in Lake Ziirich, Minder (1920) as- 

 cribed the summer minimum to the lethal action of sunlight, but he failed 

 to consider several other concomitant factors which are known to influ- 

 ence the abundance of bacteria in bodies of natural water. Ruttner 

 (1932) correlated the spring and autumn maxima observed in Lake Lunz 

 with the semiannual turnover of the lake. The highest counts occurred 

 in the spring following the runoff of melted snow. 



Conversely Fred et al. (1924) failed to find any definite seasonal cycles. 

 These workers, who recognized the complexity of factors which influence 

 the seasonal distribution of bacteria, observed the abundance of bacteria 

 in Lake Mendota for three consecutive years. In 1920 the highest plate 

 counts were observed during the summer, in 192 1 the maximum occurred 

 in the autumn, and during 1922 in the spring. 



In Lake Alexander in Minnesota, Henrici (1938) found a gradual in- 

 crease in the bacterial population as indicated by plate counts as well as 

 by the submerged slide technic from the time that the ice went out in 

 April until August when the maximum count was obtained. The bac- 

 terial population decreased throughout September and October, at which 

 time the survey was discontinued. Only minor fluctuations were noted. 

 The fluctuations were probably less than could be accounted for by the 

 sporadic distribution of microorganisms in water and variations directly 

 attributable to sampling and counting errors. Significantly, the curve 

 for the abundance of bacteria in Lake Alexander followed that for the 

 abundance of plankton during the months when observations were made. 



Lloyd (1930) found the number of bacteria in the Clyde Sea to be 

 remarkably constant throughout the year for all layers except the surface. 

 The bacterial content of surface waters fluctuated widely, but there was 

 no evidence of rhythmic seasonal variation. In interpreting her data, 

 Lloyd says, "Any variations outside the limits of the accuracy of the 



