334 University of California Publications in Zoology [Vol.22 



differences in distribution occur without appreciable difference in 

 immediate environmental conditions. 



Another point of interest concerns the relative amounts of morn- 

 ing and evening catches. In the following table, record is given 

 merely of the number of times in the month that morning or evening 

 catches were the larger. The results shown by this table have been 

 cheeked up by a table showing the number of times in each month 

 that a morning catch was higher or lower than both the evening catch 

 preceding it and the evening catch following it. and by a similar table 

 for evening catches. The substantial agreement in the three tables 

 makes it fair to state that this table correctly indicates distribution in 

 morning and evening catches. 



Morning and Evening Maxima .* 



Diatoms Dinoflagellates 



September September 

 Maxima at 8 a.m. 14 (5catehesnearmean) MaximaatSA.M. 12 (3 catches near mean) 

 Maximaat8p.M. 16 (4 catches near mean) Maximaat8p.M. 18(2catchesnearmean) 



October October 



Maximaat8A.M.23 MaximaatSA.M. 12 (1 catch near mean) 

 Maxima at 8 p.m. 8 (2 catches near mean) Maxima at 8 p.m. 19 (2 catches near mean) 



November November 

 Maxima at 8 a.m. 24 (3 catches near mean) Maxima at 8 a.m. 20 (3 catches near mean) 

 Maximaat8p.M. 6 Maximaat8p.M. 10 (3 catches near mean) 



December December 



Maxima at8 a.m. 13 MaximaatSA.M. 15(5catchesnearmean) 

 Maxima at 8 p.m. 18 (1 catch near mean) Maxima at 8 p.m. 16(1 catch near mean) 



It is very plain that no conclusions can be drawn from evidence 

 so limited and contradictory, but it may be permissible to make 

 suggestions. It seems strongly probable that there is a tendency to 

 a diurnal rhythm in productivity, a rhythm which might be expected 

 from what we know concerning fresh-water plants and land plants. 

 If, according to such expectation, the phytoplankton is mainly active 

 in photosynthesis by day and mainly active in growth and reproduc- 

 tion by night, we should get the results shown for diatoms in October 

 and November, i.e., larger numbers in the morning catches. 



Considering the multiplicity of factors involved, the large number 

 of component species in the samples taken, and the variable speed and 

 direction of water currents traversing the fixed point of collection, it 

 is rather more surprising to find such a strong indication of rhythm 



* I.e., one catch larger than the other. In a few cases the differences in 

 numbers were very slight, but usually there was considerable difference at 

 the two times of day. 



