Light Assimilation Curves of Surface Phytoplankton 253 



The water sample was also used for measurement of photo- 

 synthesis by means of the carbon fourteen isotope method. Be- 

 cause of lack of any appropriate non-metal closing sampler, 

 materials in the subsurface zone were not examined. The car- 

 bon^^ productivity experiments followed the method of Doty and 

 Oguri (2). A dark bottle and four light bottles each of 250 cc 

 capacity were placed in an assimilation box in the laboratory 

 on shipboard. These four light bottles received 10, 25, 50 and 

 100 per cent of light respectively in assimilation box. No 

 neutral filters were available on this cruise, so the bottles had 

 to be covered with one to three layers of metal sieves instead 

 of neutral filters. These possibly let the light pass through into 

 the bottles with minor change of wave length composition. The 

 bottles were kept at the temperature of the sea surface by run- 

 ning sea water around the bottles. Samples were taken from the 

 sea in the morning or at midday and they were exposed to the 

 light in the box for five hours from about 6:00 a.m. to about 

 11:00 a.m. or from about 1:00 p.m. to 6:00 p.m. For illumina- 

 tion in the assimilation box, two cool white fluorescent tubes of 

 20 watts were set at each side of the bottles. Each fluorescent 

 tube emitted as much as 4000 lux at 100 volts as measured with 

 a selenium photocell which is sensitive to 400-600 m/i wave 

 length. Therefore, the light intensity emitted by four tubes would 

 be roughly 16000 lux. However, unfortunately, the plastic wall of 

 the tank in which the bottles were set was not absolutely trans- 

 parent. It was found that the light was reduced by about one- 

 half after it had passed through a plastic plate and again a 

 glass plate having similar thickness to that of the experimental 

 bottle. Accordingly, the maximum intensity of light which reached 

 the sample water was supposedly only about 8000 lux. This in- 

 tensity was undoubtedly insufficient for the experiments; the 

 results obtained therefore are limited to a certain extent. 



Table 1 shows the total cell number and chlorophyll content 

 of phytoplankton, in the surface water at fifteen stations. Relative 

 abundance of cells of leading species in the population and 

 amount of carbon taken by plant organisms are also presented. 

 Despite the fact that the location of samplings in most cases was 

 far from land, most of the species contained in the samples were 



