F 



or the hydrographical researches of Pettersson and Ekman the North Sea was ex- 

 plored four times during the year 1899, viz: in February, April — May, July — August and 

 November. At the same time samples of water were taken for chemical analysis and of 

 plankton for microscopical examination. Some other samples have been taken in March 

 at 59° — 60° N. 4° — 5° W. by a steamer Thyra, in May by the Swedish expedition to Green- 

 land under Prof. A. G. Natiiorst, in June by the steamer Rurik on the way to Spitz- 

 bergen, in July by the Swedish corvette Freya and in September by the returning 

 Greenland expedition. At the biological stations at Plymouth, S:t Vaast la Hogue and 

 Helder samples were colleeted almost every week, during the whole year at Plymouth, 

 from January to March at S:t Vaast and from Janna ry to September at Helder. For this 

 valuable assistance I beg to tender the directors of the said stations, Mr. E. J. Allén, 

 Dr. P. P. C. Hoek and Mr. Eugene Malard my best thanks. Again, at the expenses 

 of the Fishery Association of Gothenburg, samples of plankton were colleeted almost every 

 week at Måseskär and Väderöboda, off the west coast of Sweden, and, though with less 

 regularity, in the open Skagerak. 



I. The North Sea in February 1899. 



The hydrographical map of the North Sea, February 1899, has already been 

 published by Pettersson and Ekman (Bih. till K. Sv. Vetensk. Akad. Ffandl. Vol. XXV. 

 Part. II. N:o 1.) It is seen from that map, that water with the salinity 35 p. m. extends 

 north of a line from Newcastle towards the Skagerak, S. W. of Norway, where it en- 

 counters water of lower salinity. South of the said line water of 34 p. ni. salinity 

 prevails to the Continental coast region. 



The microscopical examination proved that the water of 34 p. m. salinity was 

 practically sterile. The water of 35 p. m. salinity was poor in plankton, which contained 

 as most characteristic forms Halosphmra and Coscinodiscus concinnus. On the space, where 

 the two kinds of water meet, the plankton was not so scarce and increased steadily in 

 quantity towards the Skagerak. This plankton contained, besides the above specic-, 

 chiefly tripos-plankton, that was the prevailing kind N. W. of the Danish Peninsula. There 

 entered also in the composition of the plankton a certain amount of Ceratvum longipes. 

 The intermixture of tripos- and concinnus-plankton ruled in the bank water, west of the 



