Papers in Marine Biology and Oceanography, Suppl. lo vol. 3 of Dccp-Sea Research, pp. 451-464 



Factors affecting productivity in fertilized salt water* 



By W. T. Edmondson 

 Department of Zoology, University of Washington. Seattle 5, Washington 



Summary — The response of phytoplankton populations to added nutrients was studied by filling 

 large out-of-door concrete tanks with sea water, and adding phosphate and nitrate. In general 

 there was an immediate increase in photosynthesis and in growth of the population. In some cases. 

 but not all, addition of phosphate alone was ineffective. 



A large part of the variation in photosynthetic rate over a 40-day period is attributable to variation 

 in the amount of light falUng on the tanks, in the chlorophyll content of the phytoplankton. and m 

 the phosphate concentration in the water. Large variations in the qualitative composition of the 

 population had no apparent effect on this relation. The efficiency of the population in using light 

 varied directly with the amount of fertilizer added, but not in proportion to it. Addition of nutrients 

 to the bottles in which measurements of photosynthesis were made permitted evaluation of limiting 

 factors. 



The rate of phosphorus absorption, measured either as decrease of phosphate in the water or 

 increase of phosphorus in particulate form, was directly related to the rate of photosynthesis. 



INTRODUCTION 



The STUDY of phytoplankton production in fresh and salt waters has been hampered 

 by a paucity of experimental techniques applicable to large bodies of water; it is 

 difficult enough to make the necessary observations. The invention of the suspended 

 bottle method of measuring population growth and photosynthesis (Whippll, 1896. 

 Gaarder and Gran, 1927) was a long step forward, but it is desirable that techniques 

 be invented and improved to the extent that various population functions can be 

 effectively studied beyond the hmits set by observation alone. Knowledge of algal 

 physiology is at present too limited to permit widespread ecological prediction of 

 the detailed behaviour of populations. Enough is known of the complex relations 

 existing in natural habitats to make it clear that there is a need for developing methods 

 of assessment of the condition of natural populations as they occur, and of the opera- 

 tion of environmental factors. Since large bodies of water are difficult to experiment 

 with, save in the crudest way by mass fertilization, the most promising approach 

 seems to involve manipulation of samples of natural populations. For example. 

 Hutchinson (1941) was able to demonstrate that both nitrate and phospiiate were 

 acting as limiting factors to photosynthesis in Linsley Pond on several occasions by 

 enriching the bottles in which measurements of photosynthesis were bemg made. 

 Nelson and Edmondson (in press) used similar methods with photosynthesis and 

 population growth prdiminary to fertilizing a lake. Lund (1949; 1950) was able to 

 evaluate hmiting and controlling conditions in lakes in detail by such man.pulat,o.i. 

 Furthermore, while the physiological role of chlorophyll has bcc.i elaborately 

 studied, more information is needed on the ecological role of chlorophyll. Various 

 studies have shown that, in a variety of aquatic populations, the rale ot photosynthesis 

 was closely related to the chlorophyll content of the plankton population. .1 proper 



* Contribution No. 781 from the Woods Hole Oceanographic Institution. 



451 



