434 Ecology 



according to quality, i.e., according to the nature of the constituents of the 

 plankton, but he had not realized the basic principle underlying this difference 

 in quality. Desmid-plankton was unknown at the time and the great 

 importance of the geological formations of lake-basins had not been recognized. 



Wesenberg-Lund also states that pond-plankton (heleoplankton) is more 

 variable from pond to pond than lake-plankton. This may be true in 

 Denmark and in the Baltic lakes, but it is not often true of other regions. 

 Take three of the English lakes situated within a few miles of one another 

 Ennerdale Water, Wastwater and Winderrnere ; 91 species have been recorded 

 in the phytoplankton of Ennerdale Water, 50 species in that of Wastwater 

 and 65 in Windermere; yet only 15 species are common to all three lakes, 

 and the dominant species are for the most part different (W. & G. S. W., '12). 

 The Central African lakes afford another example of widely differing 

 plankton ; 85 species have been recorded in the phytoplankton of Tanganyika, 

 of which 61 do not occur in the other great lakes (G. S. W., '07). Wesenberg- 

 Lund differentiated between the plankton of deep lakes and that of shallow 

 lakes, stating that deep lakes were characterized first and foremost by their 

 enormous diatom maxima and shallow lakes by * water-bloom.' These state- 

 ments require careful revision, since they were based upon a limited and very 

 inadequate experience. The deep lakes of North Wales and the deepest lake 

 (Wastwater) in the English Lake District have no great diatom maxima in 

 either spring or autumn. The same is also true of many of the deep 

 Scottish lochs. On the other hand, the great African lake Tanganyika, 

 which on the whole is a deep lake, has a very great diatom-flora. It seems 

 highly probable that the great diatom-maxima of certain lakes have little 

 connection with the depth. ' Water-bloom,' which is caused by a great 

 development of certain species of Blue-green Algae, is, however, a feature of 

 shallow lakes in lowland areas. It is a conspicuous feature of Lough Neagh 

 (Dakin & Latarche, '13) just as it is of the Danish Lakes. Wesenberg-Lund's 

 statement that Chlorophyceae are practically absent from deep lakes is very 

 erroneous, since Green Algae often form upwards of 70 per cent, of the phyto- 

 plankton of deep lakes. 



The principal groups of Algae represented in the freshwater phytoplankton 

 are the Myxophyceae, Peridiniese, Bacillariese and Chlorophyceee, and a few 

 comments upon the occurrence of each of these groups may prove useful to 

 the student. 



MYXOPHYCE^E. In those lakes in which Blue-green Alga3 are conspicuous, 

 the various species are usually abundant in the warmer part of the year, more 

 particularly in the early part of the autumnal decline in temperature. The 

 lakes of the British Islands and Scandinavia are less dominated by Blue-green 

 Algse than most other European lakes, only Ccelosph&rium Kutzingianum, 

 Gomphosph&ria Ndgeliana and Oscillatoria Agardhii occurring in such 





