SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 211 



the phytoplankton. The dominant organism throughout the year is Coccolithus 

 huxleyi, which does not require B12 (but requires thiamine), and the bloom of 

 diatoms occurs in April after the level of B12 has increased to 0.06-0.1 mp.g/1. 

 (a dozen species of diatoms isolated from the Sargasso require B12). 



A new series of enrichments of depleted Sargasso waters (at the end of the 

 diatom bloom) containing their natural populations shows clearly that the 

 relative proportions of various nutrients are directly responsible for species 

 composition (Menzel, Hulburt and Ryther, in press). The initial popula- 

 tion in the samples was dominated by C. huxleyi (90% numerically). One series 

 of enrichments was done in glass carboys, the other in polyethylene bottles. 



In glass carboys the enrichment with N + P caused dominancy of diatoms 

 (70% Skeletonema costatum + 25% of three species of Chaetocerus) ; N + P + Fe 

 produced a short, rich bloom of Chaetocerus simplex (later supplanted by 

 unidentified flagellates). In the polyethylene bottles N + P enrichment caused 

 a small, short-lived Skeletonema bloom followed by a large Coccolithus bloom ; 

 N + P + Fe favored preponderance of flagellates; N + P + Si favored a large 

 Nitzschia closterium bloom concomitant with a smaller Coccolithus bloom ; 

 N + P + Si + Fe caused a dense bloom of C. simplex. In an environment which is 

 almost stable for temperature and light, as the Sargasso Sea, the chemical 

 environment seems, more than any other factor, to be responsible for the 

 distribution and seasonal succession of forms. 



Menzel and Ryther (1961) confirm and extend the results of Ryther and 

 Guillard (1959). Iron is the most limiting factor for primary productivity of 

 the Sargasso Sea and iron deficiency is not removed by the addition of EDTA. 

 However, the addition of iron alone results in a short burst of growth, and 

 sustained productivity is achieved only by adding Fe + N + P ; evidently 

 N + P are also limiting. These data from enrichments fit into the ecology of the 

 Sargasso Sea: N and P are minimal and total Fe averages 10 [j.g/1. without 

 any seasonal peak ; C. huxleyi, the yearly dominant species, has an unusually 

 low requirement for Fe [growth for many serial transfers is not affected by lack 

 of Fe in the enrichment (Ryther and Kramer, 1961)]. 



From these results it is clear that the combining of chemical and bioassay 

 analysis of the waters with enrichment experiments and the determination in 

 vitro of the nutritional characteristics of the dominant species permits good 

 insight into the ecological events. 



References 



Adair, E. J. and H. S. Vishniac, 1958. Marine fungus requiring vitamin B12. Science, 127, 



147-148. 

 Allen, E. J., 1914. On the culture of the plankton diatom Thalassiosira gravida. J. Mar. 



Biol. Assoc. U.K., 10, 417-439. 

 Allen, M. B., 1956. Excretion of organic compounds by Chlamt/dotnonas. Arch. Mikrobiol., 



24, 163-168. 

 Allen, M. B. and E. Y. Dawson, 1959. Production of antibacterial substances by benthic 



tropical marine algae. J. Bad., 79, 459-460. 



