Chapter V — 73 — Distribution in the Sea 



hours of darkness and sink to deeper water as the intensity of sunlight 

 increases (Spooner, 1933; Clarke, 1933; Russell, 1936). The abun- 

 dance of diatoms, dinofiagellates, and other phytoplankton in water is 

 directly associated with light intensities (Stanbury, 1931; Pettersson 

 et al., 1934). 



Temperature as an ecological factor : — In a stable environment like 

 the sea where the lack of organic matter or solid surfaces are known to 

 limit the bacterial population, it is doubtful whether the temperature has 

 much influence on the number of bacteria found there, although the tem- 

 perature unquestionably influences the kinds of bacteria and their activ- 

 ities. The total bacterial population is merely the dynamic balance be- 

 tween the rate of reproduction and the death rate of bacteria in a given 

 environment. Increasing the temperature may increase the rate of multi- 

 plication of bacteria within certain limits, but it may also increase their 

 rate of death. 



In all except surface waters, the temperature of the marine environ- 

 ment is practically constant throughout the year. In surface waters the 

 annual temperature range is usually only a few degrees Centigrade. If 

 organic nutrients and other conditions essential for growth are present, 

 minor increases in temperature may materially accelerate the rate of bac- 

 terial multiplication and metabolic activity — but only temporarily. At 

 the increased rate of assimilation of organic nutrients, a lack of the latter 

 soon restricts further multiplication, in spite of the increased temperature, 

 until more organic matter becomes available from some source. In the 

 meantime, the increased temperature may have accelerated the death 

 rate of the bacteria so that the increased temperature merely causes a 

 temporary fluctuation in the bacterial population. 



Evidence for these views is forthcoming from data on the abundance 

 of bacteria in sea water having different temperatures. The largest bac- 

 terial populations in the sea occur on the sea floor which, in general, is the 

 coldest part of the ocean. However, little difference has been found in the 

 numbers of bacteria in shallow bottoms in tropical seas where temper- 

 atures of from 20° to 25° C. prevail and in deep sea bottoms where the 

 temperature is perpetually lower than 5° C. In warm shallow bottoms, 

 bacteria multiply more rapidly, partly because the temperature is higher 

 and partly because more organic matter is settling from the overlying 

 water, but they are also dying off more rapidly. In cold deep bottoms 

 which receive only a limited amount of organic matter, bacteria multiply 

 more slowly but they live longer. When a dynamic balance is struck, it 

 appears that the total bacterial population is independent of temperature 

 except in so far as the temperature influences the availability of organic 

 matter, the principal limiting factor. 



It has already been mentioned that an annual temperature change of 

 about 8° C. in the surface water temperature at La JoUa has not caused 

 any detectable fluctuations in the seasonal distribution of bacteria over a 

 period of ten years. Similarly there are no differences in the number of 

 bacteria in surface waters at different latitudes which can be correlated 

 directly with temperature over a range of near 0° C. in arctic waters to 

 25° C. in tropical waters. However, the temperature does influence the 

 metabolic rate of bacteria and the kinds which are present. As will be 

 elaborated elsewhere, there are marine bacteria which multiply at tem- 

 peratures considerably lower than any temperature found in the sea and 



