CHAPTER 11 



'Indicator' species and water nioi'enients 



Living things arc all adapted to certain conditions of life, but some are 

 much more tolerant than others. For example, polar bears live in the cold 

 cHmate of the Arctic and penguins m the Antarctic; a giraffe prefers it warm; 

 a hippopotamus likes it wet; a parrot prefers a forest but a bison the open 

 plain. All of them will survive in the very similar conditions in a temperate 

 zoo when protected and given the right food, because they arc fairly tolerant. 

 Many tropical species just will not live in zoos except in specially heated 

 animal-houses because they are less tolerant of the colder temperate climate. 

 Some will happily take a variety of foods and arc not difticult to rear, but a 

 panda is most particular about his bamboo shoots. 



So it is, too, amongst the plants and animals of the plankton, and we hnd 

 those that are associated only with a high salinity, others with a lower salinity, 

 and yet others where salinity does not seem to matter. Other limiting factors 

 are temperature, depth, food, and also those more delicate chemical factors 

 such as vitamin and hormone content. Thus the different species in the 

 plankton are distributed according to environmental conditions, the tolerant 

 ones rather loosely, and the less tolerant ones between quite closely defined 

 limits. This was mentioned in Chapter 8 (p. no) as it affects the geographic 

 or seasonal distribution of the plankton. 



We can, however, take it a step farther than this. By choosing suitable 

 species and studying their distribution we can follow their movements and so 

 learn more about the currents that carry them. The principle is quite simple. 

 As water moves from one place to another it gradually mixes with the 

 surrounding water, and it will become cooler, or warmer as the case may be. 

 Its physical and chemical characteristics also mix until its identity is lost and 

 the differences can no longer be measured. But although the plankton also 

 becomes mixed the actual individuals carried by the currents cannot be 

 changed and they remain recognizable until they die and disintegrate. If 

 the chosen organisms are easily identified the water mass can be labelled at 

 sight simply by towing a plankton net and looking at the catch. This has been 

 realized for centuries, but only in a vague way until 1935 when Dr. F. S. 

 Russell of the Plymouth Laboratory pointed out how useful such labels were 

 and how easily they could be obtained. Since then most of the water masses 

 in the north-east Adantic, and in several other places in the world, have been 

 labelled. Because certain species in the plankton could be used to indicate the 



K 139 



