38 Caroline Leegaard. 



These species, which are found over the wliole Finnish area, also occur in the saline 

 water of the Skager Rack and the North Sea. 



On the contrary Diatoma tenue for instance is restricted to the inner part of the 

 Gulf of Finland, where the water is only slightly saline; it is scarce or entirely lacking 

 at the other stations, and it is not found at the open ocean. 



[CE-CONDITIONS. 



From the maps of AVitting (Jahrbuch 1912, pl. .'! and 4) \ve see that all the sta- 

 tions except F 41 have probably not been affected by the compact ice. Station F 50 

 was free from dril't-ice on the 14 th of April and station F 41 on the 27 th of April. 

 Such observations are lacking from the other stations. 



At station F 41, where the winter-ice has lain, the diatoms have developed, when 

 the melting of the ice has produced vertical circulation, and the ice at the saine time 

 has become so thin as to let the light pass down to the assimilating algae. The flourishing- 

 period will liere at station F 41 appear later than for instance at station F 74, where 

 the water has been open ail the winter. We have seen, that station F 41 has its 

 maximum no doubt about the month of May, while the maximum of station F 74 very 

 probably can be found earlier in the year. 



Those places which are çovered with ice longest are situated nearest to land. On 

 that account they have a rieh supply of food from the shore during the period of thawing. 

 Tins fact has contributed to the rieh production of station F 41, which has been men- 

 tioned earlier (p. 5). 



At ail the other stations, where the conditions vary gradually from the surface to 

 the bottom, the cooling of the upper layers has produced vertical circulation, so that 

 the nutritive substances have risen to the upper layers, and the organisms have begun 

 to develop. 



It appears that the plankton-forms of the material examined from the month of May 

 for the most part are strongly decreasing. 



ARCTIC SPECIES. 



When comparing the Finnish plankton with that of the other oceans, it is very 

 remarkable, that so many species are common to the Arctic Ocean and the Finnish area. 

 On the other hand the Cattegat, the Skager Rack and the North Sea have other plankton- 

 forms, some few species excepted. On comparing the results of thèse examinations and 

 the data of the „Nordisches Plankton" (1908) and Meunier (1910) we find the following 

 species occurring in the Arctic Ocean as well as in the waters surrounding Finland: 



Achnanthes taeniata Chaetoceras Wighami 



Chaetoceras debile Coscinodiscus lacustris 



Tom. XLVIU. 



