_ V — HOEK-INTRODUCTORY 



As we have now learnt to know the plankton forms characteristic for each 

 region and for each period of the year, we are to a certain extent prepared to 

 state where a species met with has come from; we can therefore use the plankton 

 as an indicator of the oceanic currents. As stated above, we can follow the At- 

 lantic forms along the west coast of Norway, also into the North Sea and the 

 Skager Rak (e. g. Arachnactis) ; we know the forms which reach to the Skager 

 Rak, Kattegat and Baltic through the Jutland current from the English Channel 

 and the southern part of the North Sea, as also the other forms again which are 

 carried with the Baltic Stream through the Belts into the Skager Rak. To the 

 south-east of Iceland, we find a sharp boundary between the Atlantic plankton to 

 the west and the Arctic plankton to the east, which accords with the hydrographical 

 boundary. We can follow the Atlantic plankton in the north along the west coast 

 of Iceland, and the Arctic plankton to the south as far as north of the Fseroe 

 Isles (East Iceland Polar Stream). The plankton is thus an important aid to 

 hydrograph}' and the greatTmass of observations, which the international investiga- 

 tions of the sea have brought together in the plankton tables, has in great measure 

 contributed to form a basis for this method of plankton-investigations. 



Further, the plankton investigations of recent years have shown, that the 

 distribution of the plankton is by no means uniform everywhere. The Arctic 

 waters are specially rich quantitatively in the summer months, and especially so in 

 the neighbourhood of the coast. This is also the case with the Lim Fjord 

 and the Kattegat, further with certain boundary regions between Atlantic and 

 Arctic waters (e. g. in the basin of the Denmark Straits, where the Atlantic 

 water of the Irminger Stream and the Arctic water of the East Greenland Polar 

 Stream cause a rotation of the so-called basin water). On the other hand, other 

 boundary regions (e. g. over the Fseroe-Iceland Ridge, where there is no rotation) 

 are imdoubtedly particularly poor in plankton. The why and wherefore of these 

 differences are but imperfectly known; we may hope, however, that future in- 

 vestigations may throw fuller light over these phenomena, though we can only 

 state them as facts at present. Their importance is great from the practical 

 standpoint also; we have to deal here with conditions which have this great 

 importance just because certain fish, e. g. the herring, seem to regulate their 

 wanderings according to the richness of the plankton. 



