PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES, 401 
June 1st, 1909. Temp. of water 56° Е. (=133° C). Considerable 
increase in the number of Desmids. St. lunatum var. planctonicum abundant, 
and many of the rarer species becoming frequent. Spharocystis Sehroetert in 
great quantity, and Peridinium Wille: quite common, No Rhizosolenia was 
observed either in this month or in May. Very few Rotifers, but many 
Copepods and Nauplii, and much detritus. 
July 2nd, 1909. Temp. of water 62° F. (=17° C.). Largely а Desmid- 
plankton with Peridinieæ. Also much detritus. The small spiny Desmids 
and Cosmarium subaretoum very numerous. Rotifers fairly common. 
Rhizosolenia morsa reappeared in moderate quantity. 
In Ennerdale Water the dominant constituents of the phytoplankton are 
Chlorophyceæ, more especially Desmids. Neither the Diatoms nor the 
Myxophycez are at any time conspicuous, although quantities of the delicate 
Rhizosolenia morsa occur at certain times. The total number of species 
observed from August 1908 to July 1909 was 91, of which 49 (or 55 per 
cent.) were Chlorophyces, 24 (or 26 per cent.) Bacillarieze, 9 (or 39 per 
cent. Myxophycer, 5 (ог 5'5 per cent.) Flagellata, and 3 (or 33 per 
cent.) Peridiniew. 
If the foregoing table be compared with the list of phytoplankton found in 
this lake in May 1903 *, it will be seen that some 16 species which occurred 
at that time were entirely absent in the period from August 1908 to Julv 
1909. Ali these constituents were, however, of the more casual type. 
The plankton of Ennerdale Water has two very indistinct phases :— 
Т. December-May (cold period, with vernal rise of temperature). 
Mixed plankton ; Diatoms more or less frequent but none 
dominant ; Desmids in comparatively small numbers. 
И. June- November (warm period, with autumnal decline of temperature). 
Desmid-plankton, with a few of the Protococecales, few Diatoms, 
and Lyngbya limnetica. 
The dominant Desmids attain their maximum abundance in August, and 
maintaining their numbers through the autumnal fall in temperature, begin 
to visibly thin out in December. Of the few Protococeales which occur, 
Dietyospherium pulchellum, Askenasyella conferta, and Tetraspora limnetica 
also reaeh their greatest abundance in August and September, whereas 
Spherocystis Schroetert is most numerous in June. 
There is apparently no maximum of Diatoms in this lake, and with one 
exception almost all the recorded species are scarce. Particularly noticeable 
is the great scarcity of Asterionella and the star-dispositions of Tabellaria, 
* Vide W. & G. S. West, in ‘The Naturalist, April 1909. 
