1904-5.] The Plankton of Thingvallavatn and Myvatn. 1097 
While studying the seasonal variations in our own lakes, I 
was struck by the thought that if the variations are occasioned 
by outer conditions varying in accordance with the tp. varia- 
tions of the water, we must expect the variations to be most con- 
spicuous in those lakes which have the most pronounced annual 
variations in tp. It has now become evident that the seasonal 
variations are great and very conspicuous in a great many lakes of 
Denmark, South Sweden, and North Germany. If the theories 
held by Ostwald and myself prove to be correct, we might 
expect the seasonal variations to be inconspicuous or wholly 
absent in arctic or alpine lakes with their much slighter amplitude 
of the tp. scale. 
It will be easily understood, that the various results and sup- 
positions arrived at through my explorations would be strongly 
corroborated if it were possible to compare them with those from 
more northern latitudes. 
If the results mentioned above were correct, w r e should expect to 
find the following conditions with regard to the arctic and sub-arctic 
lakes. 
The Myxophycese would most probably be almost wholly absent, 
perhaps with the exception of Anabcena flos aqua, and Oscillatoria 
rubescens. The main part of the phytoplankton will be the Diatoms, 
of which especially Melosira , Asterionella , and Tabellaria w r ill be 
of importance ; on the other hand, Fragilaria crotonensis , with 
its higher max. tp., probably will be absent. The Chlorophycese 
will play only a small part in the phytoplankton : Sphcerocystis 
and some others form an exception to the rule. The Yolvocinese 
will probably be very scarce; according to A. Cleve (1899), 
Lagerheim (1900), and Levander (1901), no Yolvocinese have 
been noted in lakes in the Lappmark, the Bear Isle, and the 
Murman coast, while Yanhoffen (1897), Borgesen (1898), and 
Ostenfeld (1904), have found Volvox as well as Eudorina in 
small ponds in Greenland (c. 71° lat. north) and Iceland. 
Of the Peridinese, Geratium hirundinella , the only species 
which commonly is of importance in the plankton, probably will 
not attain a conspicuous frequency in more northern lakes. Here 
it is very rare, only a few specimens having been found by 
Levander (1901) in the lake Enare, and by Ostenfeld (1904) in 
