Factors affecting productivity in fertilized salt water 453 



intervals elapsed between additions. The only method uhcd not adequately described in cxisiing 

 publications is that used for measuring light. 



An integrating photometer was used. The instrument was designed by Dr. W. S. von Arx and 

 built by Mr. Robert Walden. it consists essentially of a photo-tube mounted under a flat sheet of 

 flashed opal glass and fed by a constant-voltage power supply. The output, which vanes linearly 

 with the incident light, is fed into a condenser. The condenser, upon reaching a pre-detcrmmcd 

 voltage, is discharged by a thyratron tube, actuating a relay which operates a counter. The photo- 

 lube used is RCS #926, which has its maximum sensitivity at 425 ma and is relatively insensitive to 

 red. The condenser was chosen to have a capacity such that the discharge interval is slightly less 

 than one second in the brightest sunlight encountered in Woods Hole during the summer. 



The instrument was calibrated in 1947 by comparison with an Fpplcy pyrhcliomctcr mounted on 

 the roof of the Oceanographic Institution at Woods Hole, and early in the summer of 1948 by 

 similar comparison at the Blue Hill Meteorological Observatory. I am indebted to Mr. I. F. Hand 

 for making the latter comparison possible, and Messrs. von Arx, Andrew Bunker, and Kenneth 

 McCasland for the former. Values are given as visible light on the basis that one half the value gi\en 

 by the Eppley pyrheliometer is visible (see Edmondson, in press, for further discussion). 



Reference will be made in this paper to the rate of photosynthesis; what was actually measured 

 is the rates of oxygen change in illuminated and covered bottles suspended in the tanks for two-day 

 periods. 



Table I 

 Schedule of fertilization in the three experiments. Quantities of nitrogen and phos- 

 phorus are given as microgram atoms per litre 



June 28 Tanks filled 



July 2 



July 6 



5P 5P 5P 5P 



80N 80N 80N 80N 



1948 Experiment 3 



Augus,6 5P 5P 5P.80N 5P. 80N- 



August 23 SON 80N 



* Walls of tank 4 were scraped daily. 



EFFECT OF FERTILIZATION ON PHOTOSYNTHESIS 

 Bottles were filled and set out for the measurement of photosynthests just before 

 and just after addition of fertilizer. The two sets of bottles were 'h^-^f-; "P-"' 

 ,0 almost identical conditions, the only difference be.ng ,n '^c amount o nut cm. 

 .^s already reported (1947) the addition of such nutr.ents to "j"""' P^P"'' ""^ "^ 

 creases the rate of photosynthesis greatly, as determtned tn '"'^'^^ ""^ ^Z 

 present work, large increases in oxygen production were observed, up to an .n.r.nsc 



