384 MICKOSCOPIC MARINE ORGANISMS IN HYDROGRAPHY. 



it only once, in February of this year, gathered at the bottom of the 

 Christiania fjord (100 m.), where the temperature amounted to 7'5° C, 

 and the salinity to o4-76 per thousand, the highest figures obtained 

 by the hydrographical examinations of all the samples gathered in 

 February, 1896. 



On the other hand, this plankton was frequently found mixed with 

 the next type in samples collected at the time named. 



(4) The fourth type, the Sira-planldon, consists also mainlj'- of 

 diatoms, but of different species, the most characteristic being Thal- 

 assiosira AordensJcidldii and 27i. gravida, Chaeioceros graenlandicus, 

 Ch. socialis, Gh. scolopendra, Ch. teres, Nitschia seriata, many of which 

 belong to the Arctic seas, and some of which are new to science. 

 Among the cilioflagellates the most abundant is a variety ardica of 

 Ccraiium tripos, distinguished by Dr. Aurivillius as a constituent of the 

 plankton of Baffin's Bay. 



There can be no doubt about the Arctic origin of this type. It 

 occurred in the Skagerack and Kattegat this year in February and 

 March, always more or less mixed with (3) and (1). In the Skagerack 

 the water witli Types (3) and (4) was covered by a shallow layer of 

 water with Type (1) ; but in the Kattegat it reached the surface. The 

 admixture of Type (3) shows that the water on or before its arrival 

 at the coast of Sweden was mixed with Atlantic water. The tem- 

 perature and the salinity were found to vary greatly, owing to the 

 admixture of the slightly saline Baltic water, at this time of the year 

 very cold. 



I have observed the same type of water in some slides collected 

 on the west coast of Scotland by Mr. George Murray, and sent to me by 

 Mr. Grove. These samples had been gathered in the spring of 1888 — a 

 year remarkable in England as an unusually cold one. 



As far as the plankton researches are advanced at present we may 

 conclude that the surface-water around the Swedish coast consists in the 

 summer of water from the North Sea mixed with Baltic water; that in 

 the autumn its place is taken by water from the southern part of the 

 North Sea ; and in the winter by water from the Northern Atlantic and 

 the Arctic Ocean. Whether these changes occur regularly every year, 

 or in certain years only, cannot be answered for the moment. Probably 

 the last change is in correspondence, as Professor Petersen has recently 

 suggested, with variations in the amount of water which the Gulf 

 Stream carries past Iceland, westwards to Davis Strait, and eastwards 

 to the Arctic Ocean. 



I think I have proved by the above that the examination of plankton 

 is a matter of the greatest interest, not only in relation to hydrography, 

 but also to meteorology and to fishery questions. There can be no doubt 



