440 Ecology 



in the English Lake District, If. granulata has a large maximum in April 

 with a water-temperature of 1'7 C. ; in Loch Lomond it attains its maximum 

 from May to June (temp. 5 --13'3 C.); in Lough Neagh the Melosira 

 maximum corresponds closely with that of Windermere, occurring from 

 February to March. In the Yan Yean Reservoir the maximum is attained 

 in the middle of the warm period with a water-temperature of 21 C. ; more- 

 over, auxospore-formation took place at approximately the period of highest 

 temperature. This is in striking contrast to the behaviour of Melosira 

 iskmdica (a closely allied species to M. granulata) recorded by Ostenfeld 

 & Wesenberg-Lund ('06) in the plankton of Thingvallavatn, Iceland, in 

 which auxospore-formation occurred from December to January with a 

 water-temperature of 1--2C. In the European area some lakes are 

 ' Melosira-lakes ' whereas others are not, and this difference occurs in lakes 

 in close proximity. , Similarly, of the large African lakes, Nyasa and Victoria 

 Nyanza are both ' Melosira-lakes,' whereas Tanganyika is not. In the two 

 first-named lakes there are many species (M. nyassensis, M. ikapoensis, 

 M. ambigua, M. italica, M. argus, M. granulata, M. Agassizii, M. Schroederi, 

 etc.), almost -all of which are represented by several varieties. In fact, the 

 I/e/ostra-flora is both phenomenal and dominant in the early spring (consult 

 O. Mliller, '04 ; Ostenfeld, '09 ; Woloszynska, 14). In most cases the Melosira- 

 filaments are approximately straight, but in several instances Lemmermann, 

 Volk, Ostenfeld, and others, have recorded the occurrence of spirally coiled 

 filaments. 



The various species of Cyclotella are important as plankton-forms, 

 especially in many European lakes (vide Bachmann, '11), and Stephanodiscus 

 A straw is sometimes very abundant both in European and African lakes. 

 St. Niagara is dominant at one period of the year in certain Canadian lakes. 

 Coscinodiscus lacustris is a very noteworthy feature of the plankton of Lough 

 Neagh in Ireland (W. & G. S. W., '02 ; '06 ; Dakin & Latarche, '13). 



CHLOROPHYCEYE. The only groups of Green Algae of importance in the 

 plankton are the Protococcales and the Desmidiacese, and almost all the 

 different forms attain their maximum vegetative abundance during the 

 autumnal fall in temperature. 



Of the Protococcales several genera are more or less important. Several 

 species of Pediastrum, and especially the greatly perforated varieties of 

 P. simplex and P. duplex, may occur in abundance, although the usual 

 pond-forms are not common except in the shallower and more lowland lakes. 

 Sph&rocystis Schroeteri is frequent in some lakes, but quite absent from 

 others. Species of Scenedesmus and Crucigenia, although not occurring in 

 any great quantity, are often characteristic of the plankton of certain lakes. 

 The same is true of several species of Oocystis. On the other hand, Dictyo- 

 pulcliellum may occasionally occur in great abundance. A few 



