280 



POPULATIONS OF THE SEA 



150% — 



100 V, 



50 Vo- 



Fig. 6. Asterionella japonica : Growth at different temperatures as percentage of 

 controls (Kain and Fogg, 1958). 



Before discussing how the results for Asterionella japonica and 

 Thalassiosira nordenskioeldi may be interpreted, we may look at 

 observations of representatives for the two other main groups. 



For dinoflagellates the growth-temperature relationship of a 

 number of species has been studied in nonbacteria-free strains 

 from the Oslofjord. The results are presented in Fig. 7. In all 

 these cultures optimum was observed in the region of 15-20°C, 

 which is the temperature range for summer, when these species, 

 with the exception of C. lineatum, are found in greatest abundance. 

 Ceratium lineatum, which Nordli (1957) in his experiments found 

 to have an optimum at 20°C also, has its main occurrence in the 

 Oslofjord at seasons when the temperature is lower. This species 

 shows, however, good growth at lower temperatures as well and 

 other ecological factors, such as the nutrient supply, may be re- 

 sponsible for its prevalence in seasons other than summer. The 



