KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 7. 53 



are frequent too, and låter in summer, Prorocentrum micans reaches a relatively high 

 råte of frequcncv. But diatoms are represented only b}^ small quantities of Rhizosolenia 

 alata f. gracillima, and t lie total amount of cells does never exceed, for the Ceratia, 1,700 

 pro 1 and generally not half of it. Vegetable cells in total reach up to 3,000 — 5,000 pro 1 

 and animals — principally Laboeee — to about asmuch. This means one or a f ew per cent 

 only of the winter- and spring-vegetations, when referring to the number of individuals; 

 but if we consider the mäss of organic substance produced, the proportion is improved 

 in favour of the summer- vegetation, since a Geratium-ceU. will hold in volume several 

 of the common diatoms. 



4. While the TW;;o-y-plankton is developing in a surface-layer of some 10 — ISmdepth, 

 the underlying, salter water houses another association of peridinians, among which 

 Ceratium longipes is the main species. and diatoms, among which Guinardia flaccida, 

 Paralia sulcata and Cerataulina Bergonii rank in the first place. We might call it the 

 Guinardia-association, and ha ve to distinguish it sharply from all associations hitherto 

 spöken of, because it is not endogenetic, but allogenetic according to the terminology 

 of Airivilliijs, ?'. e. imported by the now predominating southern currents. The Baltic 

 affluents being only feeble during the warm season, a considerably higher salinity is 

 found up to the surface than is the case during the winter-season. It is of interest to have 

 provided cvidences of a quite brisk multiplying of Guinardia and Cerataulina at the highest 

 levcls — 30 to 20 m below the surface — which they attained in high summer. Cfr table 

 7p. 10. It maybe remembered, however, that the produetion stops at some fewthousand 

 cells pro 1, not at hundreds of thousand, as in winter. No doubt, the lack of sufficient 

 light stånds for much in this result. 



The slowly developing Tripos- and Guinardia-a.ssocia.tioiis 1 finally intermingle and 

 become especially rich in Iitoral forms, since they rule in our waters for a comparatively 

 long time, giving way, for all we knowfrom now investigated series, to no other types of 

 plankton until the ^Va-plankton, with Tholassiothrix nitzschioides as a pioneer, reappears 

 with the cold, Baltic affluents at about the turn of the year. 



Having thus been lead to distinguish four well defined plankton-associations in the 

 Skager Rak, of which three are endogenetic and one allogenetic, but all of a typical and 

 regular occurrence, we have further to deal shortly with some other formations, more 

 sporadically met with in the boundary. They are partly of a northern, partly of a southern 

 tyj)e, and will, as foreign to the Eastern Skager Rak at any råte, make their appearance 

 in the Central and in the deep Norwegian parts of this arm of the Sea. Among the former, 

 I have to note a Chaetoceras-sociale-a,ssocia,tion, with Ch. dioderna, that enters with deep- 

 water-currents along the Norwegian coast right into the bottom of the Gullmårefjord 

 in early spring of certain years, ljut is absent in other years, when the same great depths 

 keep absolutely empty of plankton, or almost so. 



In other years, on the contrary, as for instance in 1914, an association of purely 

 southern character, revealing its origin to be the English Channel, will hivade Central 

 Skager Rak. Rhizosolenia Stolterfothii and Rh. fragilissima being its leading species, 



1 Guinardia, suffering more from the cooling off of the waters, disappears earlier in the autuinn, 

 whereas Paralia coutinues to multiply and gets the predomiuance in November. 



