424 MR. W. WEST AND PROF. G. 5. WEST ON THE 
(XENERAL CONCLUSIONS CONCERNING THE PHYTOPLANKTON OF 
BRITISH LAKES AND ITS PERIODICITY. 
The following conclusions are not merely drawn from the data afforded by 
the collections from the seven lakes previously described, but also from our 
observations on the plankton of dozens of other lakes in all the lake-areas of 
the British Islands. 
The coldest water-temperatures occur from February to April and are 
frequently met with in the month of March. The highest temperatures of the 
water are as a rule in July and August. 
The greatest actual bulk of plankton occurs when the Entomostraca are 
most abundant, but the greatest amount of phytoplankton іх found in the 
late summer and autumn, during the autumnal decline in temperature. 
In all the lakes examined there are certain more or less well-marked phases 
in the phytoplankton, each phase dominated by one or more of the con- 
stituents. It is not an easy matter to compare the phytoplankton of one 
lake with that of another, as the annual phases of one probably do not 
correspond. with those of the other. Take three of the English lakes— 
Ennerdale Water, Wastwater, and Windermere ; Ennerdale Water has 91 
species in Ше phytoplankton, Wastwater 50 species, and Windermere 
63 species; yet only 15 species are common to all three lakes, and the 
dominant species are for the most part different. 
CHLOROPHYCE®.— Almost all the Green Algæ attain their maximum 
vegetative abundance during the autumnal fall in temperature. 
a. Desmidiacee. The Desmid-phase is during the decline in temperature 
from August to November, and they are most abundant in September (more 
rarely in August), although in some lakes certain species reach their 
maximum in June. The plankton Desmids appear to require very similar 
physiological conditions, and the maximum vegetative activity of the group 
as a whole is just after the highest temperature has been reached : that is, 
in September, or in some casex in early October. In some lakes certain 
species are perennial, but these also have their maximum at the same 
season аз the others. We have previously pointed out that this is approxi- 
mately the period of abundance of Desmids in the littoral region and in the 
bogs *. 
b. Protococcales. ОЕ this group there are few genera of importance in 
the plankton, and the frequent pond species of Scenedesmus, Pediastrum, 
and Crucigenia are very uncommon except in the shallower and more lowland 
lakes. Spheroecystis Schvoeteri and Dietyospherium pulchellum are mostly 
summer and autumn constituents in the larger lakes. Elakatothria gelatinosa, 
ж W.& G.S. West, in * The Naturalist,” May 1909, р. 192. 
