Microplarikton fr/mt the Fimwh Wahr* (lurimj Ihr Monlh of May 1912. 33 



It is difficult to cotne to any conclusion as to the vegetating period of the spore- 

 less forms from the material examined. Compared with station F 30, which will be 

 dealt with next, station P 23 has a richer plankton than F 30, which is situated to the 

 south of F 2.'5 and has nearly the same hydrographical conditions. On that account it 

 is very probable, that the plankton of station F 30 is decreasing, while that of station 

 F '23 is not as far developed as that of station F 30. Frobably the plankton of station 

 F '23 will also be reduced. This circumstance is indicated by the fact that among the 

 plankton many arctic forms are found, which will not thrive in warm water. The arctic 

 forms will be mentioned låter (p. 38). 



STATION F 30. 



The hydrographical data do not differ much from those of the previous station. The 

 température, salinity and the oxygen tension show somewhat higher nnmbers than at 

 station F 23. Tbe température • decreases from 1,83° (0 mètres) to 1,62° (40 mètres) 

 and then increases again to '2,26° (100 mètres). The salinity varies in the npper layers 

 above 50 mètres between 5,59 °/ o an( l 5 >57 °/ 00 and increases deeper to 6,20 °/ o (100 

 mètres). The water is supersaturated with oxygen at any rate to the depth of 60 mètres ; 

 observations from 90 mètres and 122 mètres show smaller numbers (89 % and 89,4 %). 



This station too is an example of great homogenity both as regards to the hydro- 

 graphical data and the distribution of the plankton. The organisms are found fairly eqnally 

 to the depth of 80 mètres, then they decrease to the depth of 100 mètres. While most 

 of the species occur in small numbers, the diatom Thalassiosira baltica takes a peculiar 

 position, occurring more numerously than at the previous station. The fact that its 

 maximum (35400 cells pr. litre) lies as deep as 80 mètres, while the surface only con- 

 tains 8720 cells pr. litre, shows that the time of active production for this vegetating- 

 period must be över. It looks as if the development of this species follows after that 

 of the spore-forming diatoms mentioned above. At this station Achnanthes taeniata and 

 Chaetoceras Wighami are about to disappear. It is not yet cleared up, as to whether 

 Thalassiosira baltica forms resting-spores and disappears from the plankton or if it is 

 found pelagically all the year round. 



Besides Thalassiosira baltica the diatom Melosira hyperborea with resting-spores and 

 the ciliat Mesodinium rubrum occurred in numbers worth mentioning. 



The following stations, F 41, F 50, F 61 and F 74 ly in a Une stretching from 

 east to west from the head of the Gulf of Finland to the most northerly part of the 

 Baltic. Some species, for instance Achnanthes taeniata, decrease westwards from station 

 to station, while others, such as Thalassiosira baltica, Gonyaulax catenata and Mesodi- 

 nium rubrum, occur fairly equally at ail the stations. 



N:o 5. 5 



