22 



ASTRID CLEVE-EULER, QUANTITATIVE PLANKTON RESEARCHES IN THE SKAGER RAK. 



Sccing thus one vegetation just ended, another eveu beginning, one feels inclined 

 to name this a season of yearly revision or making up before starting the new cycles, 

 for which a general intermingling of the waters must be a useful preparation. Then such 

 a meeting of different plankton-types and spreading of cells and germs to all levels gives 

 apparently the best chances for every species finding låter the conditions, most propi- 

 tious to its development. 



Though the total amount of organisms is less variable from the surface to deeper 

 regions at this season than at all others, yet there are occasional irregularities through 

 the columns, caused by the mode of occurrence in long chains of the dominating plant, 

 Paralia. 



Series III. April 25th~26th 1913. 



This series was taken at a season, when it could be expected tliat the great spring- 

 niaximum of diatoms was come to an end, and it was intendcd to elucidate the quantity 

 of plankton, produced by this second great »flowering» in the year. A first mass-pro- 

 duction falls, as known, in the high winter and will be treated in a following series, but 

 for 1913 no such February-series was collected. 



Table 27. S. Extra II. 57° 43' N., 11° 7' E. 26. IV. 1913. 





o 









n 



£ o 



o 





CD o 





*a 



C S 



e 



en 



a 



CD 



CD 3 





H * 



CO 



t» 



Q 



c 



i cd • - 



o -o 



« ^ K C8 



2 « 



CO L. 



-o 



Ji 



C2 

 CD en 



is 



•2 I 





CC •- 



© CD 

 Cj 73 



Oj2 



CD 



q a, 



<D 





CD ^ 



05J3 



a o c 

 Q C 



10 

 20 

 40 



0,69 



4,18 

 4,59 

 4,70 



21,37 | 10.77 



28, ni 22,27 



32.63 i 25,87 



33.64 ! 20,66 



(200) 



304,200 280 400 



8,050 300 j 2,650 



450 i 2,750 I 200 j 1,650 



(100), 



50 8,050 ' 

 50 i 4,950 



350 ! 050 ' 



1 ,200 



200 

 4,000 



650 

 + (300) 



375,910 



17,550 



17,050 



As this season, when masses of diluted water are brought into the Skager Rak by 

 the Baltic Current, we find here, at the mouth of the Kattegat, a thin and outfreshed 

 surface-layer, that is, practically spöken, nearly free from diatoms, holding only more or 

 less damaged or empty cells. But immediately beneath, about the 10 m-level, diatoms 

 have developed in enormous masses, among which Leptocylindrus danicus is the all- 

 dominating form. This species reaches here an absolute maximum of 364,200 ind. per 1, 

 that is depassed nowhere in the boundary, as far as it was investigated. Next to that 

 leading form of the spring-formation, we find Chaetoceras decipiens and Rhizosolenia 

 hebetata f. semispina as its characteristic associates. Also Chaetoceras debile and sociale 

 occur in this Chaeto-plankton of 1913, but no othér species of that large genus in note- 

 worthy quantities. Especially the almost total absence of Ch. contortum and Ch. con- 

 strictum is remarkable, both the named species being common in the Chaeto-plankton 

 of the Skager Rak in other years, as f. i. in April 1914, as will be shown låter on. Of the 



