Limnoplankton 435 



abundance as sometimes to dominate the plankton, and then only in con- 

 taminated lakes (i.e. those with a relatively high percentage of dissolved 

 salts). Myxophycese occur very abundantly in many of the shallower lakes 

 of the European continent situated on the more recent geological formations 

 (Schroder, '00 ; Lemmermann, '03 ; etc.) and the same is true in N. America 

 (Marsh, '03). Gomphosphteria lacustris may sometimes occur in great quantity 



Fig. 269. Chart of the periodicity of five of the dominant constituents of the phytoplankton of 

 Loch Katrine, Scotland. The temperatures are in degrees Centigrade. 



and in Lough Corrib in the west of Ireland it completely dominates the 

 summer-plankton (W. & G. S. W., '06). Chroococcus limneticus is also of 

 frequent occurrence and in the larger pools may often be quite abundant. 

 Various species of Anabsena (notably A. Flos-aqu&, A. drew a Us and 

 A. Hassallii) and Aphanizomenon Flos-aquas often occur in quantity, and they 

 may attain great maxima, along with Oscillatoria Agardhii and Glmotrichia 

 echinulata, in pools and smaller lakes on the occasions signalized by great 

 masses of 'water-bloom' (vide p. 32). This phenomenon, which does not 



282 



