394 MARINE DINOFLAGELLATES AND RED WATER CONDITIONS 



In the open ocean, the presence of free organic compounds is more 

 difficult to account for. In this case it is perhaps the metabohtes or 

 decomposition products of other plankton organisms which create the 

 necessary growth conditions for the dinoflagellates. 



It has been pointed out that many of the dinoflagellates found in 

 temperate waters have relatively high-temperature optima, a fact 

 which may explain their paucity in the winter plankton community 

 and the spring blooms. However, there are certainly many species 

 which are able to show some growth in temperatures of 0° C and 

 below (Braarud, 1935). Yet even these are conspicuously absent from 

 the usual spring flowering of diatoms. Thus temperature alone may 

 not be sufficient to account for the seasonal periodicity of the dino- 

 flagellates. It would appear that, in many cases, the growth of these 

 organisms may be also dependent upon, or at least benefited by, the 

 previous existence of a flowering of diatoms. 



The underlying cause for this type of succession is probably nu- 

 tritional, but its exact mechanism is obscure. The diatoms may reduce 

 the concentrations of one or more of the nutrients or trace metals to 

 a level favorable for the growth of the dinoflagellates. This type of 

 relationship was proposed as an explanation of the succession of 

 plankton elements in freshwater by Pearsall (1932) and Hutchinson 

 ( 1944 ) . A somewhat different type of relationship has been suggested 

 by Lucas ( 1947, 1949 ) , who has proposed the production of external 

 metabolites or "ectocrines" by one group of plankton organisms which 

 may have a beneficial effect upon the succeeding population and an 

 inhibitory effect upon other competing organisms. This has been 

 demonstrated in fresh water by Rice ( 1954 ) . 



Ecological Significance of Motility in Dinoflagellates 



The presence of motility in the dinoflagellates, and its absence in 

 the diatoms, may have an important bearing in the relationships of 

 these organisms with such environmental conditions as temperature, 

 salinity, and the nutrient concentration of the water. This is the 

 hypothesis of Gran ( 1926-27 ) and is supported by Braarud ( 1935 ) . 



The diatoms are dependent upon vertical mixing and their natural 

 buoyancy for remaining in the upper, photosynthetic zone of the 

 ocean. They may derive some benefit from the synthesis of fats and 



