40 



The Medium 



the vertical distribution of plankton. Differences in the density of 

 fresh water and sea water at three temperatures within the biological 

 range are shown in Table 2. As a result of these density differences, 



TABLE 2 

 Sample Values for the Density of Water at Different Temperatures 



relatively sharp stratification of water masses may come into being in 

 lakes and in oceanic areas. If other influences do not interfere, 

 warmer water tends to remain over colder water, and fresher water 

 tends to float on top of the more saline. 



Murray and Hjort, 1911, copyright Macmillan & Co. 



Fig. 2.8. Calocalanus pavo, a tropical planktonic copepod. The length of the 

 crustacean's body is about 2.5 mm. 



Although the differences in density shown in Table 2 may seem 

 very small, they are sufficient to influence profoundly the circulation 

 of water in lakes and in the ocean, and to make the difference be- 

 tween floating and sinking for planktonic organisms. Plankton some- 

 times tends to sink through the upper stratum of less dense water, 

 but to stop sinking when it reaches a stratum of higher density below. 



