CHYTRIDIALES 107 



lation cycle. Many chytrids of various genera occur in significant num- 

 bers on these plankton algae. In a series of three papers Canter and Lund 

 (1948, 1951, 1953) give a survey, from the limnological point of view, 

 of (1) the fluctuation in numbers of the important diatom Asterionella 

 in relation to fungal epidemics; (2) the incidence of parasitism of dia- 

 toms by chytridiaceous fungi; and (3) the interaction between para- 

 sitism and other factors determining the growth of the diatoms. 



A good example of their observations (Canter and Lund, 1948) 

 is the account of the parasitism of Asterionella formosa by Rhizophyd- 

 ium planktonieum (Fig. 10 H, I). Asterionella is the dominant diatom 

 over a considerable part of the year in Lake Windermere and has a 

 large spring and a small autumn maximum, 12,000 and 400 cells per 

 ml., respectively. 



The effect of Rhizophydium planktonieum on the periodicity of 

 Asterionella was observed in a section of the lake, Esthwaite Water, 

 a somewhat silted eutrophic area, the plankton population of which 

 is dominated by Myxophyceae and diatoms. Canter and Lund report 

 on two epidemics which took place in the late summer to early spring 

 period; that is, between the renewal in Asterionella of fall growth, 

 after the summer minimum, to the beginning of its spring growth pe- 

 riod. The data concerning the 1946 epidemic are given below: 



In 1946, the autumn growth period of Asterionella began between 

 September 24 and October 1. Numbers of Rhizophydium remained 

 low until October 28, when 10 per cent of the 322 live Asterionella 

 cells per ml. were infected. On November 2, infection was 26 per cent; 

 Asterionella cells, 390 per ml. Two days later (November 4) infection 

 was 2 per cent lower and the cell number was virtually unchanged 

 (388). On November 8, 26 per cent of the cells were parasitized whose 

 number had now declined to 266 per ml. From then on to November 

 26, both host cells and infection decreased {Asterionella to 45 per ml. ; 

 Rhizophydium to less than 1 per cent infection). The final fall in Aster- 

 ionella numbers may be attributed in part to mechanical depletion 

 due to the rapid rise in lake level (between November 18 to 26) and in 

 part, although no data are available, to dilution from vertical mixing 

 as a result of final and complete loss of temperature stratification. 



Variation in number of diatom cells per colony was correlated mark- 



