200 WtOVASOLI [chap. S 



At present, with the knowledge acquired on the nutrition of marine algae, it 

 is possible to bioassay the biological properties for the phytoplankton of 

 different w r ater-masses. This possibility, though not fully exploited, has given 

 some extremely interesting results. 



Sea-water samples with their natural flora of living micro-organisms and 

 phytoplankton were enriched with nutrients and incubated in continuous light 

 for various periods. The effect of the various enrichments was gauged either 

 by the amount of growth elicited during a fortnight (the samples being observed 

 periodically during this period, Thomas, 1959; Johnston, in press) or by 

 measuring the uptake of 14 C03 after 24 h incubation in a light of 1500 ft candles 

 (Ryther and Guillard, 1959). The enrichments were : (a) single additions of N, P, 

 Si, trace elements, soil extract (Thomas); (6) a complete enrichment: N + P-l- 

 Si -I- trace elements -I- soil extract (Thomas); N + P + Si + thiosulphate + under- 

 chelated trace-metal mixture + vitamins (Ryther and Guillard); N + P + Si + 

 over-chelated trace-metal mixture (Johnston) ; (c) the complete enrichment 

 minus one of the components (Thomas ; Ryther and Guillard) ; complete 

 enrichment minus only the chelated trace metals (Johnston); (d) an equal 

 mixture of surface sea-water and deep water (100 or 1000 m) from the same 

 water column (Ryther and Guillard). Though the methods differ somewhat 

 and the samples are from very different areas, all the sea- waters supported good 

 growth only when enriched with trace metals ; the other enrichments were in- 

 different ; Si was stimulatory. Thomas treated only two samples : one from an 

 oligotrophic area west of Baja California, and one from a eutrophic area off 

 Central America. The oligotrophic sample had a natural population of 500 cells/1, 

 of diatoms and dinoflagellates ; none of these organisms grew in any enrich- 

 ment. This might be due to the scant natural inoculum more than to an adverse 

 (bad) type of sea-water. The eutrophic sample had 500,000 cells/1, of diatoms 

 and dinoflagellates. The only effective single additions were soil extract and, 

 in a minor way, the trace metals ; early growth of diatoms followed by late 

 growth of a Gymnodinium was favored by the complete enrichment and the 

 complete enrichment minus P or N ; the diatoms did not grow when Si or trace 

 metals were lacking from the complete enrichment but the Gymnodinium grew. 

 A similar contrast between the northern spring diatoms (Skeletonema, Chaeto- 

 ceros, Nitzschia, Thalassiosira) and dinoflagellates (mainly Ceratium) was 

 found also by Johnston : the type of enrichment he employed favored growth 

 of the spring diatoms in all samples of sea-water (the amount of growth depend- 

 ing upon the sample) while the dinoflagellates present in the natural inoculum 

 failed to grow. Omission of the chelated trace-metal mixture from the enrich- 

 ment resulted in no or poor growth, including the dinoflagellates, in all types 

 of w r aters (Johnston employed hundreds of samples collected in different 

 seasons and depths in the North Sea, North Atlantic, Faroe and Icelandic 

 waters). 



Ryther and Guillard treated seven samples of surface waters collected from 

 the continental shelf to the Gulf Stream and six samples in the Sargasso Sea ; 

 omission of the trace-metal mix from the complete enrichment had the most 



