176 



Mr. W. West and Dr. G. S. West, 



[Oct. 30, 



the plankton of the Eivers Ouse, Avon, and Cam in England, and the Biver 

 Lochay in Scotland. 



It would appear that some phytoplankton occurs all the year round in the 

 British rivers, due mostly to the absence of severe winters, and that in the 

 winter months the living constituents are mostly Diatoms. In fact, Diatoms 

 dominate throughout the entire year, the most important genera being 

 Asterionella, Synedra, Melosira, Surirella,, and Fragilaria. Fritsch* summarises 

 the phytoplankton of the Thames in the course of a year as follows : — 



Mixed Plankton (with Asterionella-TpXi&so) — * Melosira — * Synedra — Mixed 

 Plankton. 



We find Melosira varians conspicuous in the spring-plankton of British 

 rivers and persisting in quantity through the summer until the late autumn. 

 Synedra Acus is one of the dominant summer species in the British rivers, 

 although it is a spring or an autumn form in the Oder, the Danube, and the 

 Illinois Biver. Surirella biseriata and S. robusta, var. splendida, are often 

 conspicuous in the summer-plankton, the former being the more abundant, 

 which is never the case in the lake-plankton. 



Certain Protococcoideae occur in the late summer, but never in great 

 quantity. Species of Pediastrum, Scenedesmus, and a few other genera are 

 the most frequent, and occasionally odd specimens of a Cosmarium or 

 a Closterium may occur. 



Flagellates and some of the Volvocaceae occur mostly in the spring months 

 and generally attain their greatest abundance before the maximum summer 

 temperatures. Endorina elcgans is most irregular in its occurrence*, and is 

 often found in quantity in midsummer or even in early autumn. We have 

 never found the Volvocaceae so numerous in river-plankton as they often 

 become in the plankton of lakes and pools. Pandorina morum is the most 

 frequent both in the rivers and small pools. 



The backwaters of the rivers are largely the breeding-places of the 

 plankton-organisms, and as pointed out by Kofoid, their contributory 

 function to the plankton of the river is at its maximum during the decline of 

 the floods. It is during such times that vast accumulations of plankton-units 

 are carried into the main stream. 



Melosira varians, Fragilaria capvcina, and Gyclotella Kiitzingiana are 

 perennial plankton-organisms in the rivers we have examined. 



X. General Comparison of British Lake-areas. 



In summarising our present knowledge of the phytoplankton of the 

 British lakes, it has been one of our first duties to obtain a record of the 



* Fritsch, loc. ext., 1903, p. 637. 



