20 



At Station IX, the Tiicho-Plankton predominates, the three 

 other communities only in a subordinate degree. At Station X 

 ■very small (quantities of both Tricho-, Disco- and Chseto-Plankton 

 are met with, but none of them predominating. 



It was the same at the following Stations which were established 

 at different spots in the sealing area. Such solitary members of 

 the various Oceanic Diatoms are to be met with almost everywhere 

 throughout the North Atlantic Ocean at all seasons of the year. 



Thus Vanhoffen [97], during midwinter, up the Karajak Fjord 

 in West Greenland, found solitary individuals, at one time of the 

 Chcetoceros deeipiens, boreale or atlantkum, at another of the Rhizo- 

 solenia styliformis or semispina, and also of the Coscifiodiscus oculiis 

 iridis and radiatus. 



It was all a chance whether the net brought up one or other 

 of these species, as they were very scarce (see Vanhoffen's Table 

 1 c, p. 288, and Gran [97, 2]). There is thus nothing to prevent 

 our assuming that the various Oceanic Diatomic communities can 

 become developed independently in quite different localities. 



From this it is conclusive that the distribution of these com- 

 munities cannot be employed in forming conclusions concerning the 

 connection of currents through widely separated tracts of Ocean. 

 The Typical forms in Cleve's Types are so widely spread that 

 their occurence in masses cannot, as a rule, indicate those parts of 

 the ocean in which the stratum concerned originated. 



At short distances, where the hydrographical conditions are 

 not allowed time to change, the Plankton may, however, yield 

 very important information. Thus it is known that great quan- 

 tities of cuttle fish and medusaj are conveyed, during the 

 Autumn, to the Skagerak from the North Sea and English 

 Channel. 



These large animals have greater powers of resistance than 

 the one-celled Diatoms, but the same current carried with it too. 

 an abundance of Styli-Plankton, which contained marked southern 

 forms. Thus, the Oceanic Bhizosolenia robusta, Norm., was found 

 off Langesund on the oth November 1897. It is common in the 

 Mediterranean but not met with previously in a higher northern 

 latitude than off Plymouth. 



But such migrations of the Plankton communities can only be 

 deemed possible, provided the conditions of life remain somewhat 

 unaltered. On lengthy journeys, the conditions become changed — 

 we need only to look at the charts to see how the temperature of 

 the Gulf Stream drops as it approaches the North — and the 

 Plankton organisms be surplanted by other species. 



It is quite another matter that, owing to tlie Diatoms being 

 very sensitive to hydrographical variations, the investigation of the 

 Plankton, combined with readings of temperature and chemical 

 re earch, becomes a means of studying the hydrography of the 

 various ocean currents. 



Thus, inter alia, our PI. 4, shews that two adjoining ocean 

 currents can possess a very different Plankton. 



But as the Plankton organisms are dependent on the liydro- 

 graphical relations (temperature, salinity) they cannot enlighten us 

 on any point other than what we also could discover by accurate 

 hydrographical investigations. 



Given equal conditions, the same Plankton organisms will be- 

 come developed provided the necessary germs are present, and this 

 fact as a rule holds good in respect to the Pelagic Diatoms. 



In respect to the N critic Diatoms the conditions are much 

 more complicated, as they possess many more species. Cleve re- 

 cognises 5 types. 



1. Didymus-Plankton or Southern Neritic Plankton (Nm) Typical 

 Forms 



Chcetoceros didymum 



— curvisetum 



— Schuttii 

 Biddulphia mohiliensis, and others 



2. Northern Neritic Plankton (Ns) with its Typical Forms 



Leptocylindrus danicus 

 Sheletonema costatum 

 Chcetoceros laciniosum 

 Lauderia annulata 



3. Arctic Neritic Plankton (Ng) 



Achnanthes tceniata 

 Fragilaria oceanica 

 Chcetoceros furcellatum 

 Thalassiosira hycdina 



4. Concinnus-Plankton (Nc) 



Coscdnodiscus concinnus 

 Biddulphia mohiliensis 



5. Halosphsera-Plankton (Nh) 



Halosphcera iridis 

 To which must be added 



6. Sira-Plankton 



Thalassiosira NordensMoldii 



— gravida 

 Chcetoceros diadema 



— sociale 



Cleve has included this in tlie Oceanic Disco-Plankton. 



All these Neritic types of Plankton, which at different times 

 are met with in Skagerak, are supposed to be in constant movement 

 with the currents, but none of them are said to proceed from the 

 Skagerak itself. Cleve says ([97] ]>. 8) "These Neritic forms move 

 with the currents along the coast;, and I believe there are no 

 stationary species among them." 



Thus Type No. 1 (Nm) is presumed to enter the Skagerak 

 from the southern parts of the North Sea; No. 2 (Ns) from the 

 "West Coast of Norway; No. 3 (Ng) belongs to the Arctic shores 

 of Greenland and Spitzbergen ; No. 4 and 5, the North Sea, and 

 the Sira-Plankton. it is })resumed, comes from the Coast of Greenland. 



None of tlie Types therefore are supposed to belong to the 

 Skagerak itself, and yet there is found there a greater mass of 

 Neritic Plankton than is to be met with at any other spot on the 

 seaboard of the North Sea or Nortliern Ocean. It is only the 

 Cattegat and the Lim Fjord that possesses Neritic Plankton in 

 greater abundance. 



i 



