18 



attention, oi- which, as regards some individuals, have entered the 

 waters from adjacent currents. 



In this way one can, in any case, most easily explain the 

 gradual changes in the composition of the Plankton, as we know 

 it. from the great oceanic currents, especially the Gulf Stream 

 (see J. Murray 97). 



It is, therefore, necessary to make fuller investigations respecting 

 the extent to which the Plankton organisms can travel in that stra- 

 tum of water to which they helong, without their being destroyed 

 by the change of conditions. 



According to Petersen's investigations, the vaiious organisms 

 do not possess equally great powers of standing such changes. 

 A^ hilst large animals, such as the medusa Pilema octopus are 

 carried along alive, during Autumn, with the bodies of water which 

 flow from the North Sea into the Lim Fjord, and idso whilst Noctiluca 

 is carried in, in great numbers, the Diatoms of the North Sea (during 

 the Autumn Rhizosolenia stylifonhis) only come as far as the nearest 

 station at the entrance of the Fjord. 



As the water invariably flows from the North Sea through the 

 Lim Fjord to the ('attegat, the Diatoms arriving from the North 

 Sea must perish in the western portion of the Lim Fjord. 



In the central ))art of the Lim E'jord they are replaced by a 

 rich Neritic Flora, which, however, in its turn, cannot proceed further 

 with the water to the Cattegat, but sinks to the bottom before it 

 reaches the eastern mouth of the fjord. 



Thus in the Lim Fjord the movement of the Diatoms is limited 

 to a most considerable extent. In that fjord the conditions of life 

 become changed with great rapidity, as, for instance, the salinity*) 

 can decrease from .30.4 7oo (Station 2) to 25.8 "/oo (at the eastern 

 Station). 



In the open sea, the Diatoms might well traverse very great 

 areas. 



Our greatest interest is centred in our ac(iuaintance with tlie 

 changes that take place in the ocean currents along the coasts where 

 our great fisheries exist. And when the oceanic bodies of watei- 

 ajiproach the land they must undergo hydrographic changes which 

 are sufficiently great to change their Diatomic flora. 



Contact with the bottom, and with other currents must, there, 

 supply the waters with the germs of a new Plankton, which, in 

 such a case, develop rapidly. Several observations have been made 

 which indicate that the Plankton increases in bulk where various 

 oceanic currents meet (see, amongst others, Petersen [98 | pp. \/^, 

 Lt ; Vanhoeeen [96 j, and our own observations from the West 

 Coast, Table 5). 



In' individual cases it may, therefore, be very difficult to deter- 

 mine which forms have been carried in with the oceanic currents, 

 and which have been previously latent, as s])ores on the bottom or 

 single individuals floating in the coastal current. 



Of late, Swedish scientists have come to tolerably advanced 

 conclusions respecting the distribution of the Plankton, and, mainly 

 on these, built their hypotheses of the system of curi'cnts of the 

 North Atlantic. 



Their views have been given in several works ; first in those 

 of AuRivjLLius [96], based on the animal Plankton, subsequently 

 in those of Cleve [97] which are based exclusively on the distri- 

 bution of the Diatoms. Various phases of the hypothesis have, 

 moreover, been discussed in the wiitings of Cleve, Ekman, Hjokt 

 and Petteesson [97], concerning the condition of the Skagerak 

 during the period of the Herring fishing, and also in Petteesson 

 and Ekman's two last treatises on the subject [97 and 98]. 



The hypothesis may, shortly, be indicated by saying that, the 

 layers of watei' in the North Athmtic move between each other without 

 becoming mixed to any great extent, except in the shallows on the 

 coast. They ca.n be recognised by their Plankton oi'ganisms, just 

 as geological strata are distinguished by thei:' leading fossils. 



When those various types of Plankton, which in Summer are 

 to be met with in different geographical areas in the Noitb Atlantic 

 and on its seaboard, occui-. periodically, in consecutive order in 

 the Skagerak, this is contingent upon the various layers of watei' 

 flowing in from each its geographical pale. 



The southern Xeritic species which frequent the Skagerak in 

 September, October and November, should thus enter it from the 

 southern portions of the North Sea along the coast of Juthind 

 together with oceanic forms from the eastern parts of the Atlantic. 



In November, some northern Neritic forms ought to arrive, 

 probably from the Norwegian coast, as well as Central Atlantic 

 forms (the so-called Cha3to-Plankton) ; in December, West Atlantic 

 oceanic species (Ti'icho-Phmkton), and, in February, Oceanic and 

 Neritic sjjecies from the coast of Greenland (Sira-Plankton), which 

 must, through the increased bulk of the Polar current, be conveyed 

 right across the Gulf Stream. 



During the coui'se of the spring, these are surplanted by the 

 Central Atlantic l-'huikton, and Neritic foru)s from northern shores, 

 and, during the summer, the East Atlantic species reappear in the 

 deeper strata, whilst the organisms of the Baltic current (Tri|)0S 

 Plankton) fill the less saline waters of the surface. 



In his last great and conq)reliensive work, Aueivillius gives 

 a description of the Animal Plankton of the Skagerak. from the 

 study of which he has arrived at similar i-esults, which we will refer 

 to in our concluding remarks. 



We will now enter more fully into Cleve's hypotheses, in 

 order to see whether they, in our opinion, can be maintained. 



Cleve recognises the following types of Plankton. 



Oceanic Types 



1. Sira-Plankton (symbol Si). Typical forms 



*) Peteksen [98J Table S. 



2. Tricho-Plankton 



3. Cha;to-P]anktc 



T 



1 lialassiosira Kor- 



densJmldii 

 Cusclnodiscus oculus 



iridis 

 Thalassiothrix lon- 



gissima 

 Bhizosolenia semi- 



sp'nia 

 i Cfuvfoccros decipirns 

 " [ — horeaJe 



