1904-5.] The Plankton of Thingvallavatn and Myvatn. 1109 
know anything about the inner causes, but we know a little with 
regard to the others. Based upon the numerous plankton in- 
vestigations of the last twenty years, it may be affirmed that 
the tp., including the physical conditions depending upon tp. 
(viscosity, gravity), of the water, and its chemical composition, 
together with the light, are the effective factors in relation 
to the plankton organisms. The chemical composition of the 
water (contents of air and of salts) plays an important part with 
regard to the marine plankton, but in fresh water the tp. and the 
light are the most effective factors. It is not easy to decide 
which of these may he said to predominate — or, more correctly, 
in some cases the tp. is the critical factor, in other cases 
(probably more exceptionally so) the light predominates. Our 
table will give some examples of both cases. Preceding this, it 
must he mentioned that in July-September 1902 the main part 
of the plankton consisted of Crustaceans, which occur in such 
large quantities as to hide the phytoplankton completely. Conse- 
quently, the signs of quantity with regard to the plant organisms 
in the samples obtained during these months stand very low in the 
table. Apart from this, we learn that Splicer ocystis and Oocyst is 
are dependent of the variations of the tp. as regards their 
frequency. They have their max. in October 1902 at a tp. of 
5°-7 - 5°, Sphcer ocystis has a secondary max. in June 1903 at 
7°-8’5° ; it follows that we are right in stating that the two 
species here and in the year in question reach their max. at about 
7 ° ; the lower tp. as well as the higher thus being unfavourable 
for their propagation. Nor does their growth depend upon the 
light, as the light in June is of much greater power than in 
October, and Splicer ocystis is of about the same great frequency 
at these two periods. It is also the tp. which regulates the 
growth of the two Rliizosolenice and the Cyclotella ; they reach 
their max. in June 1903 at 7°-8 , 5°. 
The case is not so clear with regard to the three Diatoms, , 
which are the most characteristic forms in the plankton, viz., 
Asterionella, Melosira italica , and M. islandica. We learn that 
Asterionella is rare in July- August, and from that we conclude 
that 9°-ll° is too high a tp. ; hut it grows very well in tps. 
varying from l°-8'5°, perhaps with a faint decrease in quantity 
