bo 
c 
PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES. 45 
which is found only in Wastwater, and Tetraspora. limnetica, which occurs 
only in Ennerdale Water, are both summer forms. Botryococcus Brauni 
is not often abundant in the British lakes and is a late summer form. 
BACILLARIE®.—Ûnly in the more contaminated lakes do the Diatoms 
attain great maxima comparable to the vast quantities which periodically 
appear in many of the central European lakes. The Pennate Diatoms are 
much more numerous, and for the most part much more conspicuous than the 
Centric Diatoms. 
The evidence at present available shows that although many of the 
plankton-species of Diatoms occur in greatest quantity in the spring, some 
of them attain their maxima in the summer and autumn, and several of 
them have a double maximum, one in the spring and the other in the 
autumn. Ап instance of the latter is afforded by Asterionella gracillima in 
Windermere *, in which the spring maximum is of greater bulk but not so 
prolonged as the autumn maximum. 
The double maximum (spring and autumn) of Diatoms in general does 
not appear to be so marked in the freshwater plankton as in the marine 
plankton f, and the double maximum of the same species, as instanced by 
Asterionella gracillima, Cyclotella compta, Rhizosolenia morsa, and others, is of 
unusual interest. 
A number of the freshwater Diatoms are perennial constituents of the 
plankton of certain lakes and сап be obtained in a living condition all the 
year round. Some of them attain their maxima in the spring and others in 
the autumn, and sometimes in both. 
The occurrence of Rhizosolenia morsa is very interesting. It apparently 
had a double maximum in four of the lakes examined. In Ennerdale Water 
it had a small maximum in April (water-temperature 5°5° C.), and a much 
larger one from October to November (10°-8°3° C.) ; in Windermere it was 
rare in Мау (44? C.) and very rare in September (14:47 C.), not occurring 
in any other months ; in Wastwater there was a small spring maximum in 
May (1:8? C.) and a large maximum in December (8:9? C.), in Loch Katrine 
there was a moderate January-February maximum (6:17—E4? C.) and a 
larger July maximum (155° C.). The fact that the autumn maximum 
is the larger is of interest in comparison with the marine plankton of 
Western Europe, in which the autumn maximum of Diatoms consists largely 
of species of the genus Rhizosolenia. Of interest, also, is the occurrence 
of this same species in the Yan Yean Reservoir, Victoria, with only one 
ж We have already discussed this double maximum (cf. W. & С. S. West, l. с. 1909, 
рр. 193, 260, апа 261), which has likewise been observed in Fragilaria crotonensis by 
Wesenberg-Lund (‘Plankton Investigations of Danish Lakes,’ Copenhagen, 1908, p. 50). 
t Consult Herdman, Presidential Address to the Linnean Society, Proc. Linn. Soc. 1908, 
рр. 23, 24; also Herdman & Riddell, in Trans, Biol, Soc, Liverpool, xxv. 1911, pp. 68-65, 
