40 ASTRID CLEVE-EULER, QUANTITAT1VE PLANKTON RESEARCHES IN THE SKAGER RAK. 



mate cells and being no doubt identical with Th. fallax, described by Meunier from the 

 Arctic Expedition of the Duke of Orleans. Dominating are further Biddulphia avrita, 

 Thalassiothrix nitzschioides and especially the small Sceletonema costatum, that is accu- 

 maluting downwards, towards a layer of about 31 %o salinity and 3°,3. Such a pronoun- 

 ced sinking can not yet be stated for the dominating Thalassiosirae, though their occurring 

 with resting spores in the under layers show the vegetation to approach its end for the 

 season. The highest number of diatoms registered in this series was 253,500 ind. per 1, 

 found at S. Skag. in a depth of 30 m, but the numbers at Extra II stop only a little 

 behind and keep high up to the surface. When comparing with the interesting results 

 of Mr Gran from the same route in February 15th — 22th 1912, we find that his material 

 was incomparably richer in plankton-algae; such enormously high numbers of frequency 

 as 915,350 ind. per 1 in maximum for S. Skag. and corresponding inundations of dia- 

 toms in the upper parts of the vicinal stations, into which this crowded Baltic water was 

 spreading, may perhaps be considered exceptional. At any råte, it is evident only from 

 the data known hitherto, between what wide limits the absolute production during the 

 diatomaceous maxima does vary. Of course, the yearly variations may to some extent be 

 due to the more or less advanced season at a given date, announcing itself for instance by 

 differences in temperature for analogous water-layers. It thus seems probable that a 

 low temperature, as that of less than + 1°, ruling in the Baltic water, restricted to a thin 

 layer in 1912, is more fit to give rise to a rich development of the Sira-plankton, than the 

 somewhat higher temperature of about + 2°,s, found in 1914. Differences of this and 

 other kinds being ultimately the ef fect of the varying course and intensity of the currents, 

 for the greatest part at least, we are aware of the most important röle_of the lat t er for the 

 biology of the sea. 



When reaching the salter stations, we find conditions at 8. Skag. 5 similar in 1912 

 and 1914 so far, as to the thorough absence of Baltic water with its rich vegetation, but 

 the salt and rather uniform water is considerably warmer in 1914 than in 1912, 6° — 6°,ö 

 against 0°,7 — l°,o, and is holding more of Paralia. 



At S. Skag. 6, the superficial addition of Balt ic-Current -forms, found in 1912 is 

 missing in 1914, which doubtless stånds in connection with the utterly scanty develop- 

 ment of the Sira-plankton at the Norwegian coast. So, S. Skag. 8 is exceedingly poor 

 throughout, and S. Skag. 9 exhibits in a properly diluted surface-water as little as 8,860 

 diatoms per 1, where Mr Gran found up to 329,200, truly by a somewhat deeper lying 

 30 % -limit. 



On the whole, the waters of the Baltic Current prove to be sharply separable also 

 this time from the deeper water-masses, both hydrographically, as regards temperature 

 and salinity, and biologically, as regards the plankton-vegetation. Only the first kind 

 of water is rich in plankton. In the latter, only Paralia is of any importance. 



Concerning the Peridiniales, they are present, chiefly in lower forms, such as Qym- 

 nodinia and Hemidinia, to a not very high number in the Baltic Sira-plankton, reaching 

 in maximum 3,000 ind. per 1. The Qyrnnodinia show signs of a beginning multiplying in 

 the surface. Ceratia are very sparse everywhere. 



Our most coinmon Protozoa, the infusorians Mesodinium and several Laboete, are 



