VERTICAL MIGRATION RECONSIDERED 



337 



We have suggested that the cause might be one which varies during the twenty-four 

 hours, bringing about a deeper vertical migration of the zooplankton organisms during 

 the day in regions of denser phytoplankton, but allowing the organisms to return to the 

 upper layers during the night. Legendre (1922) has studied the changes in pU values 

 throughout the day in the coastal waters of France, showing that they may be consider- 

 able, and linking them with the photosynthetic activity of the littoral algae. At 0800 

 he found a/>H value of 7-99, at 1500 one of 8-19 and at 1900 one of 8-oi. He comments 

 upon the fact that such diurnal changes in the open ocean due to the photosynthetic 

 activities of the phytoplankton have not been found to be anything like so great. We 

 have at present only two records of the changes in pH during the day at fixed points 

 within our area, that at St. 41 when observations were taken at 1300, 1715 and 2105, 

 and at St. 393 in a later survey when observations were taken at 1320, 1847, 2248 and 

 0255. The data from these two stations are given in Tables LXXIV and LXXV. The 

 changes recorded are very slight indeed. 



In Table LXXVI we show the range of pH values down to 200 m. at the stations in our 

 survey for which such data are available. The stations are arranged in order of phyto- 

 plankton values. The pH values of 8-30 and over are printed in heavy type, those below 

 8-io are printed in italics. The night stations — those taken between sunset and sunrise- 

 are shown at the foot of the table by the letter N. The higher alkalinity of the upper 

 photosynthetic layers is clearly seen; but whilst there is in general a slight tendency 

 for the higher pU. values to extend lower in the water in regions of high phytoplankton, 

 it is not sufficiently pronounced to offer a reasonable explanation for the zooplankton 

 remaining high in the water in the regions of lower phytoplankton. At St. 134, which 

 is one of very low phytoplankton and at which Euphamia saperba and Parathemisto 

 were taken at the surface in numbers, the upper pR values are the same as those at 

 St. 124, the highest phytoplankton station for which we have pH data. 



Table LXXIV 

 Repeated pH observations at St. 41 



Table LXXV 

 Repeated pH observations at St. 393 



