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



At both the last stations, there is a sharp biological limit, when the råte of salinity 

 attains 34,8 %o> by a temperature of 6° — 8°. Above this limit, that curiously enough lies 

 higher up near the coast (S. Skag. 9), we have the ordinary scanty Tripos-plankton, the 

 Ceratia gaining upon the Gymnodinia and Glenodinia again nearer land. Also the Protozoa 

 are numerous in the surface-water, attaining 1,980 ind. per 1 at 20 m. The relatively verv 

 high maximum of 5,200 ind. per 1 at the same level of S. Skag. 8 was due mostly to a frequent 

 occurrence of Lohmaniella ovijormis Leeg., Lohmann's »kleiner kugeliger Ciliat». When 

 passing into the lower, colder and salter water, the Peridinia and Protozoa get abruptly 

 sparse again, while the diatoms augment in frequeney, thanks to Paralia, that attains 

 its maximum first in the great depths (710 ind. per 1 at 220 m). 



Of a special interest are the samples from great depths at S. Skag. 8, south of the 

 Norwegian coast. They prove to be exceedingly poor in plankton. The perpetual sinking 

 of especially less vital, or dead, organisms from the upper parts does not, thus, lead to 

 any lasting accumulations in the depths. In shallow basins, they may do so, at least for 

 a time, as can be found for instance from the tables 50 and 63 (p. 35 a. 41), until some 

 under-current or other movement of the waters comes and sweaps the bottom clean again. 

 Even the heavy Paralia shows only a most insignificant increase in the undermost sample 

 from 675 m below the surface. We will låter come back once more to this important fea- 

 ture of the vertical distribution. 



Series V. February 25th— 26th 1914. 



Once before, in 1912, a February-series was taken and investigated by Mr Gran. 

 Together with the present one, it offers an opportunity of comparing the development 

 of the winter-plankton in different years. Collected at a period, when the winter-maxi- 

 mum may be considered to be at an end, to judge from the frequent occurrence of spores 

 especially at deeper levels, and the spring-flowering just going to begin, the Series V will 

 give us a valuable starting point for getting some idea of the subsequent plankton-pro- 

 duction in spring. It must, however, be observed, that such an estimation must for the 

 present be confined to a direct comparison between the numbers of frequeney observed, a 

 mathematical dealing with the data in order to calculate the total produetion being quite 

 impossible in view of the few observations on one side and all unknown factors of de- 

 struetion, råte of loss by drifting away, etc, on the other. Anyhow, a look at the follow- 

 ing April-series will allow some evaluation of the changes, brought by the spring- »f low- 

 ering ». 



At this winter-season, the southern part of the Skager Rak is filled with a cold and 

 rather dilute water to a deep level, the 30 % -limit falling nearly at 30 m. In this, the well- 

 known winterly Sira-plankton has dcveloped to a moderate extent. The råtes of frequeney 

 of the leading discs may be seen from the table 50 beneath, and prove far behind the perhaps 

 exceptionally high numbers, stated by Mr Gran two years earlier. He gives such startling 

 high dates for the south -eastern waters as upto 636,600 ind. per 1 of the most common spe- 

 cies, Thälassiosira Nordenskiöldii. No doubt the present series represents a more advanced 



