14 



Plankton As already mentioned the bottom temperature through- 

 out the year remains at 5 to 7 degrees. 



On account of the circulation in the upper layers of water 

 of the Skagerak and Cattegat, the chief thing to he d.-t/i mined 

 is the extent to which the individual parts t the roast in tin- 

 two seas contribute to the joint stock of Plankton. Probably 

 all the surrounding shores are centres of development of Neri- 

 tic forms. 



The circulation makes the conditions of life -" uniform that, 

 in all probability, those species which are almost alike sink to tin- 

 bottom with their spores, and from these spring to life again along 

 the entire coast, just as, for instance the fixed nlga-flora is some- 

 what uniform on all the shores of the Skagerak. The circulation 

 favours also the Neritic forms, as it conveys the warmth of summer 

 and the cold of winter along the coasts down to a considerable 

 depth, so that the range of temperature may he extraordinarily 

 great. Thus, for instance, in October 1897. there were warm layers 

 of water crowded up in the north eastern portion of the Skage- 

 rak. of such a depth that the temperature (at Langesund) was 12 

 down to a depth of 250 metres. 



The Neritic Plankton of the North Sea does not appear to 

 be nearly so plentiful in individual forms as the Skagerak. 



This is shewn up chieHy by PETKK-KN'- investigations [98. 

 Table I]. The North Sea proves to be not only less full of Plank- 

 ton than the Lira Fjord and Cattegat. but the species met with in 

 it are, paramountly, oceanic. Of Neritic forms, it is really only 

 BuhluJphia mobiliensis, which occurs in any nameworthy quantity. 



CI.KVK'S Tables for the North Sea [97. Tables VI. VIII. IX] 

 also indicate a majority of oceanic forms. The German Expedition 

 to the North Sea, found lesser quantities of peridins and diatoms 

 than in the Baltic and Gulf Stream [see SCHUTT. 93], whilst G. 

 HVRBAY [97] found great quantities of Neritic diatoms during the 

 spring along the North Sea seaboard of England, especially Skele- 

 tonema costatum. On the North Sea seaboard of Norway, from 

 Lindesnses to Stadt. no previous studies had been made. The 

 results of our investigations there will he found in Table 1. station 

 16; Table 3, St. 7376, and Table 5, St. 179185. The Sta- 

 tions 13 and 179 183 are off Stavanger: 46 and 183 185 

 off Marsten Lighthouse near Bergen, and 7376 off Bremanger 

 near Stadt. 



The Summer Neritic Plankton on the Norwegian littoral of the 

 North Sea is typically represented in the samples obtained by 

 HJORT off Marsten (st. 4) on the 26th June 1895. This Plankton 

 is rich in species, amongst which most of the autumnal forms of 

 the Skagerak are met with. But the relative conditions in respect 

 to numbers are somewhat different, and some species, for instance 

 Chectoceroe Schiittii are replaced in the North Sea by others, and 

 thus Chasioeerm WUlei and Ch. cinrtum are characteristic forms of 

 the North Sea littoral. 



It will be seen that, in the Tables, we have divided the 

 Neritic Diatoms into Summer, Winter and Arctic forms. The 

 latter are not met with in the Skagerak or on the North Sea 

 littoral, and the Summer and Winter forms are divided chiefly 

 according to the seasons in which they occur in greatest numbers 

 in the Skagerak. 



Even in the Skagerak, in which the Summer and Winter Plank- 

 ton is very different, the boundary between the Summer and the 

 Winter forms is not always easily drawn, and some individuals of 

 trne Summer forms (for instance Chtetocero* d'uli/mum) may be met 

 with in the middle of winter, whilst Winter forms may be encoun- 

 tered in small numbers at the warmest period of the autumn. 

 CLEVE'S Tables shew, however, that each species attains its maxi- 

 mum at a fixed period of the year. On the West Coast this is 

 not so marked. We thus see that so late as the 26th June, no 

 -mall numbers of true Winter forms (Thalassiosira Nordenskvihlii. 

 graritla. Ch&toceros diadema) are to be met with. 



In the Bergen Fjord*), almost similar species are to be met 

 with throughout the entire summer, as those contained in the above 

 mentioned samples from Marsten. From Table 3, St. 73 75, it 

 will also be seen that a like plankton, if even somewhat poorer, is 

 to be found all the way up to Bremanger, where, it keeps but 

 close in to the coast, which however agrees with the fact that it 

 consists of Neritic species. 



An impression of the Winter Neritic Plankton of the West 

 Const will he gathered from Table 5, Stations 179 182. The 

 samples of Plankton were collected by NOKDGAAKD whilst making 

 the hydrographies] section off Stavanger ,7th 9th February 96), 

 and off Marstenen near Bergen (14th 15th February). On the 

 surface the Plankton was still somewhat sparce, but a little deeper 

 down, especially at the outer stations (181, 184, 185), where the 

 Atlantic water with its greater temperature comes nearer to the 

 surface (40 metres at Station 184), there was found, amongst the 

 oceanic forms a considerably large number of Neritic species. 

 GBAN found that the same kind of Neritic species were abundantly 

 developed between Lindesnses and Stadt in March 1898. The 

 species there were the same as those in the Skagerak, the 

 quantative relations only being somewhat different. The Winter 

 forms were not so paramount as in the Skagerak, and there 

 was not so small a number of true Summer species amongst them. 



This agrees with the hypothesis that on the West Coast the 

 variation of the temperature between summer and winter is not 

 so great as in the Skagerak, the conditions on the littoral of the 

 North Sea, both in respect to hydrography and biology being more 

 oceanic, whilst in the Skagerak they are more "continental", if we 

 can employ this term in relation to an area of water. 



It is possible too, that the bottom of the North Sea does not 

 present so favourable conditions to life as might be imagined. The 

 North Sea, as we may perceive from our charts, is also to a great 

 extent filled with oceanic bodies of water. It is a restless sea in 

 which the boundary of the mud (J. MURRAY'S "mud- line") is first 

 met Jwith at a depth of 80 fathoms. The bottom consists for the 

 most part of sand and gravel, bare of all higher alga-vegetation, 

 notwithstanding that the depth is not so great as to preclude its 

 growth if it found a suitable soil. It is possible that the spores 

 fall more easily to the bottom, and are retained in more tranquil 

 parts where the bottom is covered with mud at a depth of a few 

 fathoms. The only Neritic Diatom which appears in plentitude in 



*) Samplei of Plankton from this fjord have been kindly placed at mir 

 by NORDOAARD. 



