284 POPULATIONS OF THE SEA 



at different depths were carried out by Marshall and Orr (1928) 

 and by Jenkin (1937). These classic investigations demonstrated 

 clearly the important feature that, in the diatoms which they 

 used in their experiments, an inhibition of photosynthesis oc- 

 curred near the surface, the depth for the photosynthesis optimum 

 varying according to time of day and season. Marshall and Orr 

 found that the winter species Coscinosira polychorda and a sum- 

 mer species of Chaetoceros had the same optimum light intensity 

 but that inhibition at the surface was more pronounced in the 

 Chaetoceros species. The latter result was unexpected, and the 

 authors suggest that it might be due to the fact that the Chaetoceros 

 species had been grown in weak light. Jenkin worked on the dia- 

 tom Coscinodiscus excentricus and to a lesser extent on Biddiilphia 

 regia, which are both winter forms. They were found to have 

 similar optimum light intensities. 



Jenkin concluded that the greatest scope for future work would 

 seem to lie in the laboratory, but during the twenty years which 

 have elapsed since her paper was published, only incidental ob- 

 servations of this kind have been made. Mjaaland (1956) ob- 

 served excellent growth in the coccolithophorid Coccolithus huxleyi 

 even at as strong light as 50,000 lux while Nordli (1957) in his 

 experiments on ceratia, observed an optimum between 2,500 and 

 5,000 lux. He concluded that his results were at variance with 

 the vertical distribution pattern of these species. Lanskaja and 

 Sivkov (1950), who studied growth of five diatoms, remark that 

 there is reason to believe that photosynthesis and also growth 

 may differ according to the state of the cells and their previous 

 history, a view also held by Marshall and Orr (1928). This is in 

 accordance with the behavior of other algae, and may explain 

 the extremely low optimum intensities obser\'ed by Schreiber 

 (1927) in the case of the two species of Biddiilphia. 



The general effect of light upon phytoplankton photosynthesis 

 and growth seems to be well established by studies reviewed by 

 Steemann Nielsen (1958b), and by Steemann Nielsen and Jensen 

 (1957), but it is obvious that too few investigations of the light 

 response of the various species are available for a comparison 

 with their distribution in the sea. Field observations indicate 



