278 



Mr. W. H. PearsalL 



In the first place, the primitive lake waters are remarkable for their high 



Desmid plankton. If it be assumed that the Desmid plankton requires such 

 a high ratio of alkalies to other bases, it then becomes possible to explain not 

 only the distribution of this type of plankton in this lake area, but also its 

 presence in certain Scotch, Irish and Welsh lakes, for which data have been 

 published. Moreover, the scarcity of Desmid plankton in European waters 

 as a whole would then be due, on this assumption, to the scarcity of pure 

 waters in which alkalies predominate {i.e., excluding the sea and brackish 

 waters). While it would lie outside the limits of the present paper to provide 

 the data necessary to establish these statements, it may be assumed pro- 

 visionally that Desmid plankton characterises waters with a high ratio of 

 NaaO + KsO 

 CaO + MgO ■ 



The abundance of Diatoms in the plankton of evolved lakes can also be 

 correlated with the chemical characters of the lake waters. Since Diatoms 

 have walls composed chiefly of silica,* they are obviously dependent on the 

 presence of a certain amount of silica dissolved in the water. Thus a scarcity 

 of silica — as in the primitive lakes — is accompanied l)y a scarcity of Diatoms, 

 and these organisms are most abundant in those lakes containing the largest 

 amounts of dissolved silica, e.g., Windermere, Ulls water and Esthwaite. On 



with a Diatom plankton and such waters are normally rich in nitrates. These 

 factors may, therefore, be as necessary as the high proportions of silica. 



A third element of marked interest in the plankton is the presence at times 

 of abundant Myxophycese in Windermere, Ullswater and Esthwaite, well 

 shown in the figures for August, 1920. It is noteworthy that these lakes 

 have waters in which the organic content is normally higher than in the more 

 primitive lakes, and it may be pointed out that Crummock Water has also a 

 rather high organic content and a considerable element of Myxophyceae. It 

 is premature to say whether or no there is a direct connection between these 

 facts, but it seems probable that they are in some way connected, as a 

 dominance of Myxophycefe is not unusual in waters having a high organic 

 content. 



A comparison of the bulk of the phytoplankton in these different types of 

 lakes leads to the conclusion that the greatest hulk occurs in the evolved lakes. 



CaO + MgO 



ratio, and only the waters of this type possess a predominant 



the other hand a low 



]N^a20 + K20 

 CaO + MgO 



ratio is equally characteristic of fresh waters 



* Whipple and Jackson record the fact that 49'48 per cent, of the dry weight of 

 Asterionella is silica (' Journ. New England Waterworks Assoc.,' vol. 14, 1899). 



