33< 



DEPTHS OF THE OCEAN 



special study of diatoms before he commenced co-operating about 

 1890 with the well-known hydrographers, Otto Pettersson 

 and Gustaf Ekman. They commenced their labours in the 

 Skagerrack, that remarkable little sea where so many different 

 water-masses meet and pass each other ; and it very soon became 

 clear that different currents might each possess synchronously its 

 own particular flora, and therefore there was the possibility of 

 ascertaining where the water-masses came from, by determining 

 their flora.^ All that was requisite was to know the distribution 

 of the different species in contiguous parts of the sea. The 

 investigations were accordingly extended, and samples were 

 collected by ordinary steamers in the North Sea, the Norwegian 

 Sea, and the Northern Atlantic, in addition to the collections 

 that were gradually formed chiefly through the efforts of 

 Swedish, Norwegian, and Scottish scientific expeditions. 

 Cleve also studied the annual changes in the plankton, and had 

 weekly collections made at selected stations on the Swedish 

 coast. The scope of his investigations was further enlarged, 

 for his unique knowledge of forms enabled him to determine, 

 not merely all pelagic plants, but also little by little, a whole 

 series of animal-families which proved no less useful than the 

 algse as " guiding forms " to determine the character and origin 

 of the plankton. 



Cleve believed that he could distinguish a series of plankton- 

 types characteristic of defined marine areas. Particular species 

 were therefore assigned by him to one or other of these main 

 types. But whereas outside the Skagerrack each of the plankton- 

 types had its own characteristic distribution, within this sea the 

 same types were found to predominate, each in its own character- 

 istic season. From February to April there were the same 

 species that we have learnt to connect with the coasts of Green- 

 land and Spitzbergen in the Polar Sea, and from May to June 

 there was a plankton resembling that of the Western Baltic. 

 During the course of summer and autumn there were, first of all, 

 species like those belonging to the southern part of the North 

 Sea, and afterwards Atlantic and more northerly forms. Cleve 

 was led to conclude that these changes in the Skagerrack were 

 due to the fact that it is supplied during the course of the year 



1 " While passing through the Japan Stream the tow-net observations indicated water from 

 two different sources. When in the colder streams there were very many more small diatoms, 

 Noctilucce, and Hydromedusse than in the warmer streams, where the same pelagic animals that 

 were obtained all the way from the Admiralty Islands prevailed. Many similar instances 

 occurred during the cruise, where the approach to land or the presence of shore water was 

 indicated by the contents of. the tow-nets" (Narrative of the Cruise, Chall. Exp., vol. i. 

 p. 750, 1885 ; see also Summary of Results Chall. Exp., pp. 893 and 895, 1895). 



