PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES. 407 
[erum now quite common. Other members of the Chlorophyeez and also 
Diatoms increasing in numbers. Rotifers fairly common. 
June 4th, 1909. Temp. of water 54° В. (12:22 C.). The Chlorophycew 
have now attained their maximum abundance, both as regards number of 
species and number of individuals. — Peridinzum Wille; common. Rotifers 
numerous, 
July 5th, 1909. Temp. of water 51° F. (=10°5 C.). Staurastrum jaeuli- 
Jerum very abundant. Elakatothria gelatinosa very common and at its 
maximum. — Peridinium Willei common. Rotifers abundant. 
In Wastwater the chief constituents of phytoplankton are Chlorophyce:e 
and Diatoms, although Peridinium Willei is а conspicuous summer consti- 
tuent. The total number of species observed from August 1908 to July 1909 
was 50, of which 25 (or 50 per cent.) were Chlorophycez, 20 (or 40 per cent.) 
Bacillarieze, І (or 2 per cent.) Phæophyceæ, 2 (or 4 per cent.) Myxophycee, 
І (ог 2 per cent.) Flagellata, and 1 (or 2 per cent.) Peridinieze. 
Of these constituents only seven became really common, and only four of 
these were at any time abundant. Thus the actual bulk of phytoplankton is 
rather small. АЦ the species recorded from the plankton of this lake in 
September 1906 * were observed in the twelve months under consideration. 
The plankton of Wastwater has two fairly distinct phases :— 
I. January-May (cold period, with vernal rise of temperature) Chloro- 
phyceæ at their minimum ; Diatoms frequent but none dominant. 
П. June- December (warm period, with autumnal decline of temperature). 
Chlorophyceze at their maximum, dominated by great abundance of 
Staurastrum jaculiferum. General maximum of Diatoms in June, 
although marked by’ abundance of Chlorophyces. — AAizosolenia 
morsa attaining a great maximum in December. 
Although 20 species of the ВасШате» occur in the plankton of Wastwater, 
only one of them, /éhizosolenia. morsa, ever becomes very abundant. This 
delicate Diatom appears rather suddenly in September and at once gets very 
numerous, attaining а great maximum in December. 1t then rapidly declines 
and disappears in March, appearing again in small quantity in May. Ц thus 
reaches its great maximum during the autumn fall in temperature. Melosira 
granulata was one of the rarest constituents of the plankton, and Asterionella 
and the star-dispositions of Zabellaria were entirely absent. 
Of the Chlorophycez, the one outstanding constituent was Staurastrum 
jaculiferum, which occurred in enormous abundance, without any diminution, 
from June to November. The individuals were very finely developed, with 
long spines, and even throughout the winter months were not at all infre- 
quent. —Oocystis lacustris and Gloocystis gigas were fairly common in June, 
T 
* Vide W. & G. 5. West, in ‘The Naturalist,’ Apri] 1909. 
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