HOEK— INTRODUCTORY — IV — 



the occurrence, distribution and passive importance of the plankton forms, are 

 quantitative hauls and quantitative determinations of the composition of the 

 hauls. The signs used at present, cc (very common), c (common), -(-(neither 

 common nor rare), r (rare) and rr (very rare) may suffice for the hydrographical 

 aims of the plankton investigations, but other quantitative determinations are 

 necessary for the study of the plankton as microscopic substratum of the organic 

 life in the sea. 



It should not be forgotten for a moment, however, that the regular collection 

 of plankton on the seasonal cruises formed a section of the hydrographical and 

 not of the biological program, both at the Stockholm (1899) and Christiania (1901) 

 Conferences. That the plankton-studies of the first two years suit the demands of 

 the hydrographical investigations and determinations to a greater degree than 

 those of the biological, is therefore sufficiently clear. 



The view which is at present held regarding the distribution of the plankton 

 in the northern seas, so far as the hydrographical aspect is concerned, rests mostly 

 therefore on qualitative investigations and determinations. This view may be 

 again presented here brieflj^ 



As already mentioned, the investigations have succeeded in determining the 

 distribution of the various plankton-organisms very exactly, so that we are now 

 in a position, not only to state the distribution of each single species, but also to 

 group together the species which have a common distribution: that is, we can 

 now describe the communities of the plankton-forms as they occur in the different 

 regions. First of all\ we must distinguish between the plankton-communities 

 which are restricted to the coastal waters (the neritic groups) and those which are 

 at home in the open sea (the oceanic groups). Each of the larger regions, however, 

 produces several plankton communities, according to the period of the year. Thus, 

 for example, the Arctic waters in the neighbourhood of the coasts during the spring 

 months contain a very rich vegetable plankton (neritic Tasnio- and Siraplankton), 

 whilst the open Polar Sea contains predominantly an oceanic plankton at this 

 time (Chaetoceras criophilum), which later in the year is replaced by another plankton- 

 community, distinguished specially by numerous specimens of Ceratium arcticum. 



The same holds for the organisms met with in the Atlantic waters which 

 stream through the Fseroe Channel into the Norwegian Sea, along the west coast 

 of Norway or further south, right into the North Sea. The forms which specially 

 characterise this water are, amongst others, Ceratium intermedium, Arachnactis, 

 Salpa and Doliolum. 



Along the coasts, that is, wherever there is coastal water, we also meet with 

 plankton-communities, as already mentioned, which are characteristic for these 

 water-masses. The success has also been attained here of demonstrating a regirlar 

 series of plankton-communities changing with the periods of the year. 



It is everywhere the vegetable plankton especially which plays the chief role 

 in this connection; the various larger regions have all their special plankton- 

 communities, as e. g. Iceland, Norwegian west coast, the southern North Sea, the 

 Danish waters, the Baltic. 



I For the summary which follows the Bureau is indebted to Museumsinspektor C. H. Ostenfeld. 



