PLANKTON-TYPES. TRICHOPLAXKTON 13 



of the numerous samples collected in 1898 by the Nathorst Expe- 

 dition to Spitzbergen. This species is »arctic» in the Atlantic only. 



The temperature of the water containing Tricho-plankton varied 

 from 0° to 10° and the salinity varied as a rule between 34 and 35 p. m. 



The route taken by the Tricho-plankton at the surface is from 

 the entrance to the Polar Basin, between Spitzbergen and Greenland, 

 along the east coast of Greenland towards Iceland. There it forks, 

 one branch going to E. Greenland, where it follows the current 

 round Gape Farewell, thence continuing along the western coast to 

 Disco, The same kind of plankton prevails in the Labrador current 

 and follows the east coast of N. America to about New York. The 

 other branch goes from Iceland above the submarine ridge to the 

 Faroes, the Shetlands and Western Norway. The bulk of the arctic 

 plankton that appears regularly in February in the Skagerak probably 

 gets there through the Norwegian depression and by means of an 

 undercurrent A part of the arctic plankton goes towards Scotland 

 and enters the North Sea, whence it proceeds to the continental coast. 



The distribution of the Tricho-plankton in the Atlantic varies 

 according to the seasons. In the winter (from November) this kind 

 of plankton extends, more or less intermingled with other kinds, 

 from the Newfoundland Banks to the Azores, to the coasts of the 

 Spanish Peninsula and to the western mouth of the English Channel. 

 In the early spring the arctic plankton dissappears more or less 

 completely. The Atlantic waters that contain arctic plankton become 

 mixed with water of higher salinity and drift in the -spring north- 

 wards, towards Iceland and the Fiiroe Channel. Such w^ater has a 

 salinity of about 35 p. m. and contains Chceto-plankton in abundance. 

 This kind of plankton is thus nothing but Tricho-plankton carried 

 into water of 35 p. m. salinity wherein Chcetoccros decipiens multi- 

 plies, but most of the other species of the Tricho-plankton die sooner 

 or later. It drifts during the summer towards Spitzbergen. 



The sudden appearance in the winter of arctic plankton over 

 the whole northern Atlantic cannot be accounted for by surface 

 currents, so it may be admitted that the cold southgoing current 

 along the American coast continues during the summer as an under- 

 current towards Europe, ascending to the surface from November 

 onwards. 



The Arctic coast-regions are characterized by a number of more 

 or less brackish forms, which together constitute the Arctic nci'itic 



