274 



POPULATIONS OF THE SEA 



10 20 30 40%. 10 20 30 40%o 



Fig. 2. Dinoflagellates: growth at different salinities, as percentage of maximal 

 growth rate (Braarud 1951 ; Braarud and Rossavik, 1951 ; Xordli, 1957). 



even at 5%c (Braarud and Pappas, 1951). This explains how this 

 species is able to thrive as far into the Baltic as in the harbor of 

 Helsingfors, where the salinity, according to Valikangas (1926), 

 may vary between 3.5 %o and 6%o- The Ceratiiim species also show 

 differences in this respect, C. tripos being the only one in Nordli's 

 experiments (1957) which reproduced at 10%o (Fig. 2). This species 

 is known to penetrate farther into the Baltic than the other species 

 of this genus, although it does not go as far in as Peridiniiim 

 triquetrum. This feature in its distribution finds a reasonable ex- 

 planation in the response observed in the salinity experiments. 



