476 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



water may also fluctuate seasonally to a marked extent, which again 

 may be the basis of alterations in the inhabiting population. It is 

 often contended to be as a result of the prevailingly low temperatures 

 that cold-loving Diatoms predominate in the lakes of central Europe 

 in winter and spring, followed by Peridinians in summer and by 

 Cyanophyceae when the temperature has reached 20° C. However, 

 as pointed out by Mr. Robert Ross {in litt.), many other Diatoms are 

 by no means cold-loving, and it may well be that the control of this 

 cycle is more chemical than physical, silica depletion for example 

 being very important. Hot springs, having a constantly high tempera- 

 ture, support a largely peculiar and often very limited flora' — as do 

 snow and ice at the lower end of the scale of temperatures allowing 

 plant activity {see pp. 489-91). 



Although the food requirements of Algae and many other water- 

 plants are still but poorly understood, it is clear that the presence 

 or absence of certain mineral salts, derived from the substratum or 

 inflowing currents, is of outstanding importance in helping to deter- 

 mine both the composition and the luxuriance of the vegetation 

 developed. Quite apart from questions of salinity, small freshwater 

 lakes are particularly dependent on the chemical and physical nature 

 of their beds. As already explained in Chapter XI, chemical 

 ' reaction ' (aciditv, neutrality, or basicity) and especially nutritive 

 salt-content are of fundamental significance in determining details 

 of development of aquatic vegetation — especially plankton — and, of 

 course, also to the dependent animal populations, in bodies of fresh 

 water. Hence the classification of the latter largely on the basis of 

 productivity into ' oligotrophic ' (poor in nutrients, with a hard rocky 

 bottom and rapidly deepening water), ' dystrophic ' (also poor in 

 nutrients but rich in humus and acidic in reaction), and ' eutrophic ' 

 (poor in humus though commonly silted and shallow, rich in nutrients 

 including combined nitrogen, phosphorus, and often calcium). Par- 

 ticularly are the nitrate and phosphate contents of decisive importance 

 in the matter of biological productivity in fresh waters as well as 

 salt ones. Also of great significance are the contents of dissolved 

 oxygen and carbon dioxide, which vary at dift'erent depths and in 

 different seasons — cf. Fig. 165 for oxygen fluctuations. 



In oligotrophic lakes, cold-water Fishes such as Trout are often 

 plentiful ; these lakes commonly show succession towards the eutro- 

 phic type. Dystrophic lakes, on the other hand, usually lack these 



^ For example in hot springs above 45*" C. only Schizophytes appear able to 

 persist, as indicated on p. 499. 



