INTRODUCTION 19 



Just how significant these parasites are in the fresh-water habitat 

 has been made abundantly clear by the researches of Canter (1946-53) 

 and Canter and Lund (1948, 1951, 1953). Their investigations of certain 

 English lakes indicate not only the widespread presence of chytridia- 

 ceous parasites but their occurrence in sufficient numbers to exert a 

 destructive influence on populations of phytoplankters (see "The Chy- 

 trid Epidemic," p. 105). 



Although the aquatic Phycomycetes of flowing waters have been 

 little investigated, there is no reason to believe that they will differ 

 from those in other fresh-watei habitats. In one of the few such studies 

 undertaken, Waterhouse (1942) detected a seasonal rhythm of sapro- 

 phytic forms in an English river. The molds appeared in September 

 or October, rose to a maximum in December, January, and February, 

 and disappeared in summer. A species of Pythium was the only phy- 

 comycete that was recovered throughout the year. The disappearance 

 of the fungi in summer is considered most likely to result from the rise 

 in temperature, falling water level, and decrease in hydrogen-ion con- 

 centration. Other factors or combination of factors, such as percentage 

 of sewage effluent present, and the weather may also be influences. 

 Moreover, the sporadic appearance of some species may be due to 

 their transport from higher reaches. Waterhouse concluded that, al- 

 though the flora of a river was less rich than that of a pond, it was 

 more varied and not so static. Since her study, Harvey (1952) has exam- 

 ined, in a preliminary fashion, the role of aquatic fungi in stream sani- 

 tation and their relation to the processes of natural purification. 



Although it is not so well known, epidemics may be produced by 

 certain aquatic Phycomycetes in marine hosts. 1 Of marine organisms 

 affected, the stalked littoral diatoms, such as Licmophora, seem most 

 susceptible to invasion by Phycomycetes. In species of that genus losses 

 of 50 to 88 per cent of the population have been reported (Petersen, 

 1905; Sparrow, 1936b; Hohnk, 1939; Aleem, 1950d, 1953). Attacks 

 by Olpidium laguncula on the alga Dumontia incrassata have had a mark- 

 ed effect upon its periodicity (S. M. Lodge, in Aleem, 1953) and on the 

 Isle of Man, for example, two particular populations of it were wiped 



1 See Vishniac (1955a, 1956) and Johnson (1957b) for a discussion and descrip- 

 tions of marine Phycomycetes. 



