13 



of the surrounding seas, it is most reasonable to assume that the 

 Plankton is really stationary, and that it is renewed by spores 

 from the bottom. 



The abundance of the Plankton in the Lim Fjord may, in our 

 opinion, be best explained from there always being in the mud of 

 the bottom spores sufficient, and ready to sprout, as soon as the 

 conditions necessary to their existence are favourable. Similar 

 conditions to those of the Lim Fjord are also to be met with in 

 the Scotch firths which are shallow, and where the water is calm, 

 so that the alluvial mud containing the organic remains can fall 

 to the bottom and settle there at a depth of but a few fathoms. 

 At certain seasons of the year they contain, according to G. MUB- 

 BAY [97] enormous quantities of Neritic Diatoms, especially Skele- 

 tonema costatum. 



Nor can it there be determined, without investigation, whether 

 this abundance of Plankton Flora is chiefly due to the amplitude 

 of the annual variations of temperature at the bottom, or the great 

 abundance of nitrogenous compounds. In every case the diatoms 

 make the conditions of nourishmeut still more favourable for the 

 small animals which live in the mud on the bottom. 



Also, according to VANHOFFEN'S investigations in the Karajak 

 Fjord, West Greenland [97], an abundant Diatomic Plankton exists, 

 which entirely disappears on the appearance of frost in October. 

 It is the power they have of forming spores that enables these sen- 

 sitive organisms to remain in such high northern latitudes, and 

 employ the short summer in producing, comparatively, enormous 

 quantities of organic matter. 



The Cattegat's Plankton has been very well investigated 

 by CLEVE [85, 96 and especially 97], and PETEBSEN [98]. The 

 depth in the Cattegat is not too great to allow of the annual 

 changes of temperature reaching the bottom, more especially as 

 the currents occasion great variations in the hydrographical con- 

 ditions (see PETTEBSSON and EKMAN [97]). It will thus be seen 

 from, for instance, the report of the Danish Commission (ROHDAM 

 [96]), that the bottom temperature in the Cattegat on the 26th 

 February 1892, averaged less than 5; in May about 6, while on 

 the other hand it was about 11 to 13 degrees in November. 

 The amplitude of the annual variation of temperature at the bot- 

 tom is thus at least 1.1 -f- 5 = 6; at most parts really much 

 more than this. 



Especially in the Northern portion of the Cattegat there exists 

 an abundant Neritic Plankton-Flora. During the spring, in March 

 and April, when the lowest temperatures have reached the bottom, 

 there crop up great quantities of Northern, in part Arctic species 

 (Thalassiosira Nordenskioldii, Th. grarida, Chcetoc. diadema, sociale), 

 cfr. CLEVE [96], Tab. II. 



During, the autumn, Southern forms may be met with in quan- 

 tities (for instance Chaetoceros didymum, Ch. Schiittii). According 

 to PETEBSEN'S investigations [98], there is, as a rule, a great dif- 

 ference between the surface Plankton and that of the bottom. 

 Whilst, in the salter layers at the bottom, there may be a rich 

 flora of various species of QuBtoettOt, and others, the surface layers 

 only possess quantities of Rhizosolenia alata, mixed with many dead 

 specimens of the flora from the waters of the bottom. 



The species which vegetate in the deep layers thus do not 

 appear capable of living in the less salt water, which on the sur- 

 face pours out from the Baltic. This is probably the reason why 

 the Plankton Flora of the Cattegat does not consist of so many 

 diverse species as that of the Skagerak. 



The Skagerak's Plankton has been still better investigated 

 than that of the Kattegat, and better than that of any other water 

 area of similar extent. 



The vegetable Plankton is best known from CLEVE'S works 

 [94, 96, 97]. This author, especially in his last great work [97], 

 has published a vast amount of information which has been partly 

 gathered at one time from a great area of operations, partly during 

 a lengthy period at established stations. 



To this may be added the very important contributions of 

 PETEESEN [98], besides a great number of samples which we have 

 collected and examined, and whose contents we shall treat of in a 

 subsequent work. 



The Neritic Plankton of the Skagerak is very similar to that 

 of the Cattegat, consisting, mainly, of the same species, with a few 

 more, and all these species appear at about the same season of 

 the year. Thus from CLEVE'S work [97], Table XI, will be seen 

 the Diatomic Planton at Vsedero for the entire year 1896. In the 

 latter half of February we meet with a Plankton, which, besides a 

 number of oceanic forms, contains chiefly Thalassiosira Nordenskioldii, 

 Th. gravidu, Coscinodiscus excentricus v. gelatinosa, Chcetoceros teres, 

 and a little later on Chcetoceros debile, diadema and sociale. These 

 are surplanted, little by little, by Chostoceros constrictum and Lepto- 

 cylimlrus danicus, which reaches its maximum in April. 



In May and June the Neritic forms retire, and are relieved 

 by the Plankton characteristic of the Baltic current RMzosolenia, 

 (alata var.) yracillima, and, besides, a majority of Peridines. At 

 Vsedero the Neritic species, Chcetoceros curvisetum, didymum, Schiittii, 

 Ditylum Brightwellii, only reappear in September. In the Gull- 

 mars Fjord they occur somewhat earlier, in July and August 

 (see 1. c. Tab. XII). In the Skagerak the circulation and 

 change of waters is so constant*) that no areas can be fixed as 

 having local characteristics either in hydrographical or biological 

 respects. 



It is true that there are some fjords, in which the bottom- 

 water is prevented from circulating throughout a great part of the 

 year, owing to reefs at the entrance ; and, of these, Gullmars Fjord 

 in Bohuslan, and Christiania Fjord have been thoroughly investig- 

 ated. In their upper water-layers both these fjords contain a Plank- 

 ton which in no important characteristic deviates from that of the 

 Skagerak, being, may be, especially at certain times, somewhat less 

 plentiful. From many observations in the Christiania Fjord, we 

 have derived the impression that the Diatomic Plankton, to a great 

 extent, enters the fjord from without. 



The upper water-layers too, circulate constantly with those of 

 the Skagerak, whilst the hydrographical conditions at the bottom 

 are so uniform, that while it certainly can be a home for a charac- 

 teristic Oceanic bottom-fauna, it cannot sustain any local Neritic 



*) See, amongst others, PETTERBSON and EKMAN [>7]. 



