444 Ecology 



uncontaminated lakes, and except in extreme northern latitudes some of 

 them are perennial constituents of the plankton. A few species frequently 

 have enormous maxima; in north temperate regions Asterionella gracillima, 

 Tabellaria fenestrata var. asterionelloides and several species of Melosira are 

 the most notable ; in the large African lakes Nitzschia nyassensis and several 

 species of Melosira and Surirella are at one season among the dominant 

 constituents of the plankton. The desmid-flora of these lakes, except those 

 containing an abundance of Surirella, is usually poor, and few species occur 

 in quantity. 



Uncontaminated lakes (or lakes with very pure water) contain fewer 

 diatoms, and such as do occur in the plankton rarely attain even a small 

 maximum. The desmids may be very numerous, depending to a great 

 extent upon the nature of the geological formation, and there is sometimes 

 a rich desmid-plankton. The lakes of the Carnarvonshire mountains are 

 excellent examples, being amongst the least contaminated of all the British 

 lakes. In these lakes there are relatively few diatoms (only ll'l per cent, 

 out of a total of 162 species) and many desmids (62'4 per cent.), and some 

 of them possess a very rich desmid-plankton. 



Lakes which possess a mixed plankton of diatoms and desmids are 

 probably of an intermediate character with regard to the nature and amount 

 of the dissolved salts in the water. The Myxophycere are to a great extent 

 absent from the lakes with very pure water, but an examination of the 

 occurrence and distribution of the plankton-species of Blue green Alga^ 

 indicates that the factors which control their relative abundance are some- 

 what different from those which govern the prolific occurrence of diatoms 

 (W. & G. S. W., 12). 



There is also a considerable reduction in the amount of Asterionella and 

 of the star-dispositions of Tabellaria, or even an entire absence of them from 

 lakes with pure water. These two genera of diatoms are almost entirely absent 

 from the great African lakes, most probably owing to too high a temperature 

 of the water (G. S. W., '07), but their absence from certain British lakes 

 appears to be directly concerned with purity of the water. Wastwater in 

 the English Lake District furnishes a good example of a lake from which 

 these star-dispositions of the frustules of diatoms are absent ( W. & G. S. W., '12). 



The plankton-community as a whole is a very ancient one and this fact 

 is particularly emphasized in the case of those lakes which possess a distinct 

 community of plankton-desmids. The general periodicity of the plankton- 

 constituents (i.e. the seasonal changes in the composition of the plankton) is 

 much the same in different parts of the world, diatoms dominant in the early 

 spring, Green Algse, and Blue-green AlgcV attaining their maximum later in 

 the year. 



Neither plankton-desmids nor those which occur in other habitats undergo 



