ITis/ : PhytoplanktoH of English Lake District. 261 



the water. In the autumn, there would also be an increase in 

 the available food-material due to the death and decomposition 

 of short-lived summer forms, and also to the slight concentra- 

 tion of dissolved material in the water poured into the lakes. 

 The aeration would obviously be greatest in the times of 

 greatest disturbance of the surface-water. 



It is also probable that the intense light of the summer 

 is detrimental to any great increase of Asterionella. 



Cydotella compta has two maxima, one in June, and one in 

 September, but these are not nearly so well marked as in 

 AsterioneUa. 



On the whole, most of the Diatoms attain their greatest 

 abundance in the autumn. A few species never completely 

 disappear from the plankton, and can be found in the living 

 state throughout the entire year. Such are Surirella robusia, 

 AsterioneUa gvacillima, and Tahellaria fenestrata var. asterionel- 

 loides. 



The characteristic var. asterionelloides of T. fenestrata was 

 most abundant in September, with the highest water-tempera- 

 ture, and scarce during the cold winter months. The typical 

 chain-form of this species with a zig-zag disposition of the 

 frustules, which is also the normal littoral and pond form, was 

 only observed in the plankton in the month of June. There is 

 no evidence in this lake of any seasonal change from spring 

 forms with a zig-zag disposition of the frustules to pelagic 

 summer and autumn forms with star-dispositions, such as is 

 mentioned by Wesenberg-Lund to occur in Denmark. The 

 chain-form was not observed in the plankton until the star- 

 dispositions were quite common, and it was only seen in that 

 one month. It would thus appear that the var. asterionelloides 

 is well established in Windermere, and that the small maximum 

 is due solely to the multiplication of perennial colonies. Prac- 

 tically no variation in the frustules of these colonies was ob- 

 served, the somewhat elongated proportions being very con- 

 sistently maintained through the entire year. 



Myxophyce.e. Of the few members of this group found 

 in the plankton of Windermere, Ccrlosphcsrium Kiitzingianum 

 is. the most conspicuous, attaining its greatest abundance in 

 September (temp. I4.4°C.), in which month four out of the 

 seven recorded species of blue-green Algae occur. 



Oscillatoria Agardhii occurs in gradually diminishing quan- 

 tity from September to December (temp. 14.4^ — 3-2°C.). 



1909 July I. 



