452 W- T- Edmondson 



allowance was made for light intensity (Manning and Juday, 1941; review by Ed- 

 mondson, in press). 



The present paper is a contribution to the study of these matters. It briefly reports 

 some of the main results of an investigation of the effects of added nutrients upon 

 marine phytoplankton populations growing in large, out-of-door concrete tanks. 

 It is based on the results of three experiments, one in 1947, two in 1948, and reference 

 will be made to them by chronological number. Although several aspects of popula- 

 tion biology were studied, the present paper is limited to a discussion of the effects 

 of added nutrients upon the rate of photosynthesis, and relations with certain factors 

 known to affect this rate. Since the work was undertaken with particular reference 

 to the effect of fertilization on productivity of small bodies of salt water, high con- 

 centrations of nutrients were added. Nevertheless, the general approach is applicable 

 to all aquatic communities including the ocean. The work followed the prehminary 

 experiments by Edmondson and Edmondson (1947). 



This is one of a series of investigations carried out at the Woods Hole Oceano- 

 graphic Institution, Massachusetts, under the general supervision of Dr. George L. 

 Clarke. The author expresses his indebtedness to Dr. Clarke and to Dr. C. O'D. 

 ISELiN, then Director of the Institution, for making available an institutional fellow- 

 ship and for support for the work. Thanks are due to Miss Rosemary Hewlett who 

 made all the counts of phytoplankton. Help with the work was received from a 

 number of other persons, particularly Miss Jean Keen, Dr. B. H. Ketchum, Dr. D. 

 M. Pratt, Dr. W. S. von Arx, Mr. Alburt Rosenberg, and my wife, Yvette H. 

 Edmondson. Financial assistance for some of the computation was received from 

 the State of Washington Research Fund in Biology and Medicine (Initiative 171). 

 A grant from the Agnes Anderson Fund of the University of Washington is grate- 

 fully acknowledged. 



It is a pleasure to dedicate this paper to Professor Henry B. Bigelow, founder and 

 former director of the Woods Hole Oceanographic Institution. 



PROCEDURE 



The tanks and most of the procedure and methods have already been described (Edmondson and 

 Edmondson, 1947; Pratt, 1950). In the present experiments, one large concrete tank was divided 

 by wooden partitions into four sections, each containing five cubic metres of water. The tanks were 

 shaded by a whitewashed glass roof in order to reduce the inhibitory effect of full sunlight. 



The tanks were filled by pumping sea water through two layers of #10 bolting silk to strain out 

 the larger animals. After varying periods, sodium nitrate or dibasic potassium phosphate or both 

 were added as concentrated tap water solutions, while the tanks were vigorously stirred with an oar. 

 Subsequently, almost daily determinations were made of phytoplankton population, phosphate, 

 oxygen, chlorophyll, light income, rates of photosynthesis and phosphate assimilation. Before each 

 sampling, usually made between 7 and 9 a.m., the tanks were stirred. The morning temperature of 

 the tanks was found to vary little over the entire period of time, and most of the temperatures were 

 between 20 and IT. 



Usually there was no attempt to dislodge material from the sides. It was realized that periphyton 

 would grow, but felt that, to achieve interpretable results, most of the work should be limited to the 

 organisms which naturally would be found free in the water, rather than a heterogeneous mixture 

 of plankton and dislodged periphytic organisms. In Experiment 3, the sides of one of the tanks 

 were scraped down daily prior to sampling. This procedure dislodged sessile organisms, and had a 

 profound effect on the phosphorus metabolism of the tank, as described by Pratt (1950). 



Since the details of the fertilization are important, the schedule of additions of nutrients is shown 

 (Table I). It will be noticed that in some cases phosphate was added before nitrate, and various time 



