KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 7. 51 



Summary. Several of the principal features of this series having been treated more 

 at lengtli above, we shall here chiefly have to summarise the total changes in the quality 

 and quantity of the plankton in relation to the last February-series. If we first look at 

 what is left of the then dominating winter-diatoms such as the Thalassiosirae and other 

 discs, Biddulphia aurita and Thalassiothrix nitzschioides, we shall find this Sira-plankton 

 to be best conserved in the deeper layers of the coast stations, as well in the Swedish, as 

 in the Norwegian, waters where it is indigenous, but missing in the other parts of the 

 basin. The remnants of that vegetation are not wholly inconsiderable, as will be seen 

 from the tables 63, 65, 71, 73; they amomit from a few hundreds up to one or two 

 thousand ind. per 1 of the more ordinary discs, but Biddulphia aurita is now almost 

 totally lacking. The dominating coast-diatom now reaching the depths is, however, 

 Scelelonema costatum, with a maximum of more than 120,000 cells per 1 near the bottom 

 of the Swedish station Skag. 0. In February, this diatom was also quite frequent, but 

 had its maximum of 144,000 ind. per 1 in the middle layers. We are thus in April in the 

 beginning of its resting period. Another frequent winter-diatom, now scantily represented 

 and heaping mostlv in the under layers, is Thalassiothrix nitzschioides. 



Going back to the April-series of 1913, we also find the winter-forms still spread 

 especially at somewhat deeper levels, and it may thus be considered a regular feature of 

 the Skager Rak-waters to keep the survivals of a Sira-plankton, »flowering» in high- 

 winter, among the låter developing spring-vegetation of a different diatomaceous 

 formation. 



As to the exact amount of the diatom-production in spring, it may be sufficient 

 to ref er to the foregoing tables, any average data being inpossible to state, in view of the 

 exceedingly great yearly variations, cfr the records for Leptocylindrus in 1913 and 1914. 

 Concerning the hitherto known winter-maxima in the Skager Rak, they present still 

 greater differences. While Mr Gran could count until 580,850 diatoms per 1 near the Nor- 

 wegian coast and 915,350 diatoms per 1 near the Swedish one in February 1912, my num- 

 bers for the same stations in February 1914 do not exceed 241 ,900 and 247,600 respectively. 



Some main results of the quantitative plankton-researches in the 



Skager Rak. 



Finally, I will make an attempt to get at some general results and relations, that 

 may now be said to exhibit themselves more clearly than before, thanks to the quanti- 

 tative work hitherto carried out in the Skager Rak. I should wish to draw attention to 

 the following points. 



I. Plankton-associations in the Skager Rak. 



Several natural associations of plankton-organisms, appearing in given seasons and 

 regulary succeeding each other have been distinguished about 20 years ago by P. T. Cleve. 

 A yearly cycle of development may thus be said to characterise even such a troubled 

 corner of the sea as the Skager Rak, though really the individual aspects of different 



