14 



Wlieii we see that sucli masses of diatoniaceæ, in this case Chætoceros 

 boreale and Thalassiothrix Frauenfeldii, are carried into the Cattegat by tlie 

 salt (c. 3,4 p. et.) under-current, it seems quite probable that these masses, 

 when they are killed by the lower saUuity (c. 2,5 p. et.) (and perhaps by the 

 higher temperature), which they neeessarily must meet farther southward, either 

 sink to the bottom or drift out again, as their specific gravity is higlier or 

 lower thau that of the water; sueh dead diatomaceæ are a very common phe- 

 uomenon, I dåre say, in o^ur seas. Column 6 in the table, May 6., 97, eouse- 

 quently the day after the first observation, showed nearly the same conditious 

 at Nordre Rønner, while column 8 shows tliat the living plankton at Hirts- 

 holmene goes up to the very surface; but here the water is certainly also 

 more than 3 p. et. up to the very surfaee. The density of the plankton was 

 here very eonsiderable; more than 9 gr. were gathered from 5 — O fathoms. 



Table II shows, however, that dead plankton was found also at other 

 piaces than at Læsø. Far down the Sound and in the eastern Cattegat it 

 occurred in May 1897, but always in water of a saliuity lower than 3 p. et. 

 Having poiuted out, as above mentioned, the rich diatom-plankton in the salt 

 water and in the under-eurrent, which comes from the north in the northern 

 Cattegat, I wanted very much also to find this plankton farther northward in 

 the Skager Rack, wlience I siipposed it must come. Unfortunately a fortnight 

 passed before this could be done; I do not suppose, however, that the lapse 

 of this time has had any eonsiderable intiuence on the result. Columns 1 — 4 

 show the result of our plankton fishery in the Skager Rack from the very 

 middle of this sea (Tromlingerne in Norway, in NW ^4 N, at a distance of 38 

 miles) towards the Skaw, at the surface as well as down on various depths; 

 but nowhere, except near the Skaw, on 58 fathoms of water, the density of 

 the plankton was found to be anything like that in the shallow northern 

 Cattegat. Cli. boreale and Thai. Frauenfeldii were found here, certainly, but 

 they were nowhere common; the peridiniece were predominant. It was parti- 

 cularly the oceanic species of the diatomaceæ that occurred; of the ncritic oues 

 only Leptoajlindrus danicus was of any greater importance. 



The rich plankton icith Chætoceros boreale, conseqiienfli/ , did not come 

 from the north; we must therefore compare the rich grotvth of the diatomececc in 



ery of dead diatomaceæ, particularly SIcelefonema, in the upper water of some Scottish 

 Lochs, and he says with respect to this: »I helieve this phenomenon, which at first puzzled 

 me greatly, to be due to the decline or loss of salinity of the water«. — He does not 

 seem, however, to have undertalien any specical ineasureinents of the sahnity; but, 

 spealiing from my own experience, I have no doubt that he is right. 



