390 MARINE DINOFLAGELLATES AND RED WATER CONDITIONS 



some 14 species between 18° and 25° C. Braarud and Pappas (1951) 

 noted a temperature optimum for Peridinium triquctrum at 18° C, 

 while Nordli (1953) found that Ceratium fiisiis and C. fiirca grew 

 most rapidly at temperatures of 15° and 20° C respectively. Provasoli 

 (personal communication) finds temperatures of 20-25° C most 

 suitable for growing Gyrodinium californicum. Although the preceding 

 experimental evidence is obviously insufficient to form a basis for 

 generalization, it is perhaps significant that all the above species, 

 despite their relatively high temperature optima for growth, were 

 isolated from temperate regions of the ocean. 



Salinity 



Dinoflagellates occur in fresh, brackish, and full sea water, and it 

 is again impossible to define optimal or limiting salinity conditions 

 for the group as a whole. The unarmored dinoflagellates appear to be 

 most abundant in the open ocean plankton, while the armored forms 

 are more typical of coastal and estuarine regions ( Fritsch, 1935 ) . This 

 suggests that the optimal growth conditions for the latter may occur 

 at somewhat lower salinities than are observed in the open ocean. 



Several plankton investigations have revealed that dinoflagellate 

 maxima may often be correlated with the seasonal or geographical 

 occurrence of relatively low salinity water (Gran, 1924; Marshall, 

 1947; Gaarder and Gran, 1927; Marshall and Orr, 1927; Gran and 

 Braarud, 1935). It is noteworthy that the majority of such studies 

 have been concerned principally with the armored species and have 

 largely neglected the smaller, naked dinoflagellates owing to the 

 difficulties involved in collecting and preserving them. 



Nordli (1953) found optimum salinities for the growth of three 

 species of Ceratium (C. furca, C. fusiis, and C. tripos) at 20-25 o/oo- 

 He was able to correlate the relatively low salinity and high-tempera- 

 ture optima of these fonns with similar conditions in the regions where 

 they occur along the Norwegian coast. NordH suggests that the so- 

 called Tripos-plankton region of Gran (1902) off the northern coast 

 of Norway may be a biogeographical area limited by high-tempera- 

 ture and low-salinity borders. 



- Table I gives a summary of the optimum and maximum range of 



