PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES, 413 
Tabellaria fenestrata, most of the colonies being transitional to the var. 
asterionelloides. Cwlospherium increasingly abundant, and  Peridinium 
Wille’ in slightly increased amount. Rotifers numerous. Entomostraca 
few. 
Aug. 2nd, 1909. Temp. of water 59° F. (=15° C.). Mixed plankton in 
which Cælospherium Kützingianum is the dominant constituent. Desmids 
more conspicuous, and Sphærocystis Schroeteri fairly common. Diatoms 
numerous, mostly Tabellaria and Asterionella. Rotifers fairly common. 
Entomostraca rare. 
Loch Lomond has a mixed plankton of Chlorophyceæ and Diatoms, in 
which quantities of Cælospherium Kützingianum occur nearly all the year 
round. The total number of species observed from September 1908 to 
August 1909 was 50, of which 25 (or 50 per cent.) were Chlorophycem, 
14 (ог 28 per cent.) Daeillariez, 8 (ог 16 per cent.) Мухорһусеге, 1 (or 
2 per cent.) Flagellata, and 2 (or 4 per cent.) Peridinies. 
There are two more or less distinet phases in the phytoplankton of Loch 
Lomond :— 
І. December-June (cold period, with vernal rise of temperature). 
Chlorophyceæ at their minimum ; Diatoms rapidly increasing in 
numbers towards the latter end of the period, and culminating in 
a great maximum of Melosira granulata and Asterionella gracillima. 
II. July- November (warm period with autumnal decline of temperature). 
Chlorophyce: attain their maximum, along with a great maximum 
of Cvlospherium and a considerable abundance of certain Diatoms, 
such as Tabellaria. 
Of the fourteen species of ВасШагіее observed in the plankton, seven 
become more or less common at some period of the year. Two of these, 
Asterionella qracillima and Melosira granulata, become dominant : the former 
from June to August, when it is associated with quantities of Tabellaria 
fenestrata ; and the latter in June, when it occurs in such prodigious 
quantity that the lake may be described as possessing a Melosira-plankton. 
In both April and May, among the thousands of straight filaments of Melosira 
were a few coiled ones such as those recorded by Ostenfeld * from a lake in 
Iceland. The two species of Tabellaria are most abundant in the summer 
months, but Surirella robusta var. splendida attained its maximum in March 
and April. 
Of the Chlorophyceæ, several Desmids were common in July, August, and 
September, and a few fairly abundant up to November or December. 
Ankistrodesmus faleatus was also common in the autumn. Both Sphærocystis 
Schroeteri and Tetraspora lacustris were in considerable abundance from 
* С. IH. Ostenfeld, in Botanisk Tidsskrift, xxvi. (1904) p. 233, f. 5 (p. 232). 
