NATURE ADRIFT 



the example of the midges. Often wc can find no satisfactory explanation; it 

 may be caused by patchy distribution of micro-chemicals, it may be just a 

 random chance. Whatever the cause, the patchiness is very real and makes it 

 diflicult to assess plankton with any quantitative accuracy. Two nets towed 

 side by side may easily differ in their catch by as much as three times; samples 

 taken one after the other, in as near as possible the same water, are just as differ- 

 ent. To smooth out these differences a long tow helps, or one can take an aver- 

 age of several replicate samples. One can get fairly detailed pictures of the 

 plankton content of a small body of water, the smaller the more detailed, but 

 only rough estimations of a large area. It is impracticable to sample in detail 

 the plankton of the North Atlantic Ocean, or even of the North Sea, but as 

 long as the investigator is aware of the probable limits of error in his samples 

 he can make reasonable assumptions. 



This chapter has tried to show how distributions vary and some of the 

 causes of these variations. Some are physical, chemical or other external 

 factors which affect the well-being of the organisms themselves, or perhaps 

 their predators or their food. Food is the basic necessity, for without it the 

 populations cannot exist, no matter how favourable their physical environ- 

 ment. We thus have to be careful to distinguish distribution due to the organ- 

 isms' own constitution, and those due to the limited tolerances of their 

 favourite food supply. This difference is important in the plankton itself, but 

 it is much more obvious in fish and swimming creatures that can deliberately 

 change their location in the search for food. 



120 



