KAAKK MÜNSTKK STKøM M.-N. Kl 



II. ACCOUNT OF THE PLANKTON OF THF LAKFS 



INVESTIGATED. 



r. V'ansjø i.s a .shallow lake of consick rabh; si/c, situated in the 

 district Østfold in South-eastern Norway. (Northern latitude 59 26'.) - 

 The surrounding area is for a greater part cultivated, but there are also forests 

 and peaty moors of considerable extent. 



The plankton is quite unique in Norway as it is distinctly Baltic in 

 its appearence. (Concerning the terms Baltic and Caledonian vide the last 

 part of this paper.) — That is what could be expected as the lake is .shal- 

 low, and the water doubtlcssl}' contaminated from the surrounding culti- 

 vated fields. — It must, however, be emphasized that lakes in the western 

 parts of our Country, e. g. the Jæder lakes, whose physical conditions are 

 very similar to those of Vansjo, have a quite different plankton with con- 

 siderable affinities to that of the h-ish lakes. - Several facts may be the 

 reason of this. The rainfall in Western Norway is very heavy, whereas 

 the eastern parts of the Coimtry are relatively dry. The winter is mild and 

 the summer as a rule cool in the western districts, whereas the contrary 

 is the case in the eastern districts. These different climates, the Atlantic 

 and the Continental may to a certain degree be responsible for the difference. 

 No doubt also the western lakes are less contaminated, and the surrounding 

 area more peaty. (Really contaminated lakes as for instance the Danish 

 are a great rarit}' in Norway, where the most of the surface is almost virgin.) 



Two plankton samples were examined, the first from Jimc 21. i8cj6, 

 the second from June j. i8gcj. (The first a surface-sample, the second 

 a vertical one from a depth of 6 m's to the surface.) 



The first sample contained Goiuphosphceria lacitstris vnr. compacta and 

 Melosira graiiiilata in such prodigious quantities as to cause water-bloom. 

 — Of the other Algæ Pcdiastnim Boryaiiuni was abundant, and Astcriourlla 

 gracillima, TabcUaria fcncstrata var. astcrioncUoidcs, Auabania Lciaiiicniiatiiiii, 

 and Coclosphcerium Kuetzingianitui were more or less common. The phyto- 

 plankton may be characterized as mainly a mixed Myxophycé and Diatom- 

 plankton. 



The second collection resembled the first, but was not so rich. (Even 

 with regard to the fact that it was a xertical one.) Melosira grainilata was 

 very abundant, Goniphospliceria lacitstris var. compacta, TabcUaria fcncstrata 

 var. astcrioncUoidcs, and SiirircUa robusta var. splcndida were common. — The 

 zooplankton was comparatively poor. 



2. Mjøsen (Near its outlet at Minne.) — MJ osen is the greatest lake 

 of Norway. It covers an area of 362,4 square kilometres, and its maximum 

 length is about 100 kilometres. It is a very deep lake, the maximum depth 

 being about 443 m's, that is, as the surface is situated at an altitude 

 of 121 m-s, the deepest part is 322 m's below the level of the sea. The 



