392 MARINE DINOFLAGELLATES AND RED WATER CONDITIONS 



waters, follow the decline of the spring diatom flowerings and rela- 

 tively large populations often persist throughout the summer months 

 when the supplies of these nutrients are almost undetectable. Gran 

 (1926-27) has proposed that the dinoflagellates require less nutrition 

 for growth than the diatoms on account of their relatively low rate 

 of metabolism. 



There is some experimental evidence that dinoflagellates can utilize 

 nitrogen and phosphorus at rather low concentrations. Thus Barker 

 (1935) observed that increasing the nitrogen content of aged sea 

 water by 1000 to 10,000 times did not increase the growth rates of 

 Prorocentrum micans, P. gracile, or Peridiniwn sp. Similarly P. micans 

 grew equally well in K0HPO4 concentrations ranging from 5 X 10~^ 

 to 5 X \0~^%. King (1950) found that increasing nitrogen and phos- 

 phorus 1 to 200 times their concentrations in aged sea water did not 

 increase the growth rate of Gymnodinium simplex. 



There is no indication, however, that such low nutrient concentra- 

 tions are necessary for the optimum growth of these organisms. 

 Braarud ( 1945 ) found that maximum populations of several dino- 

 flagellates occurred in the regions of heaviest pollution in the Oslo- 

 fjord. Marshall (1947) observed a dense growth of P. triquetrum in 

 Loch Craiglin immediately following fertilization of the loch. In the 

 following section, a factor other than the nutritional physiology of the 

 dinoflagellates will be discussed as a possible explanation for their 

 occurrence and growth in nutrient-poor waters. 



As mentioned earlier, dinoflagellates as a group show a continuous 

 variation in their modes of nutrition from autotrophic to holozoic, 

 while many species are facultative, obtaining their food by either or 

 both methods (Kofoid and Swezy, 1921). We are concerned here 

 principally with the former type of nutrition. However, it is perhaps 

 questionable whether any of the dinoflagellates are completely auto- 

 trophic, in the literal sense of the term. 



For many years they have remained among the most difficult of 

 marine organisms to grow and maintain in culture. The author knows 

 of no case in which dinoflagellates have been grown in a completely 

 inorganic medium, either of the completely artificial or the enriched 

 sea water type. The few media which have been developed and used 

 successfully for growing these organisms include, almost without 



