production (that part of the plant's production in 

 excess of its respiratory requirements) at approxi- 

 mately 100 g/m /year or 3.6 x 10l3kg/year for the 

 oceans as a whole. 



Returning to our earlier consideration of land 

 plants, it was estimated that the category of agri- 

 cultural crops, grasses, etc. produce about 1000 

 g/m /year or 3.0 x lO^^y^g/year over its alloted 

 30 X lO^km^. Making another bold assumption that 

 the average forest tree is 50 years old (actually, 

 this could be halved or doubled without greatly af- 

 fecting the end result) the crop of 1 . 1 x lO^^kg of 

 trees would yield an annual production of 2 .2 x 

 lO^^kg/year. This is equivalent to about 500 

 g/m /year, roughly the lower limit of the values 

 obtained by Ovington and Pearsall (1956) for 20 - 

 40 year-old forests . 



Summarizing the production estimates, we 

 have: 



Ocean 

 Land 



Wasteland 



Crops, grasses, etc. 



Forests 



3 .6 X lO^^i-g/year 





 3.0 X ^_ _ 



2.2 X 10^ -^kg/year 



10^ ^kg/year 



Calculations of this second type have been 

 made before, but it is of some interest and value 

 to repeat them using independent methods and data. 

 Schroeder's (1919) value for the land (3.8 x 10^3 

 kg/year) and Steemann Nielsen and Jensen's (1957) 

 value for the oceans (2.4 - 3.0 x 10-^3)^g/year) are 

 both somewhat lower than those reported above, but 

 are nevertheless in surprisingly good agreement 

 with them . 



The interesting fact which emerges from all 

 this is that the annual rate of organic production 

 on land and in the sea is about the same despite 

 the fact that the latter is accomplished by a flora 

 less than one thousandth the biomass of the ter- 

 restrial vegetation. The explanation for this is 

 that most of the bulk of land plants is in the form 

 of slowly growing, non-photosynthetic structural 

 tissue. If we were to consider the relative quan- 

 tities of photosynthetic pigments in the two en- 

 vironments, the results would be quite different. 

 In fact, it has been proposed by Gessner (1949) 

 that the highest concentration of chlorophyll that 

 can be attained in nature (1-2 g/m ) is actually 

 the same for both water and land. What does this 

 mean in terms of the photosynthetic potential of 

 the land and sea? This depends not upon the 

 amount of chlorophyll per se , but upon the amount 

 of sunlight absorbed by this pigment. Steemann 

 Nielsen (1957) correctly points out that the chloro- 

 phyll which lies below the illuminated or "eu- 

 photic" zone of lakes or oceans has no bearing on 

 the productive potential of the area. In this con- 

 nection, however, it would be difficult to estimate 

 the chlorophyll in a comparable "euphotic zone" of 



a forest, where all of the pigment is probably never 

 Illuminated at any one time. 



This difficulty can be circumvented if we look 

 at the problem from another viewpoint and consider 

 a situation in which all of the sunlight falling on a 

 unit of the earth's surface is effectively absorbed 

 by photosynthetic pigments. Stipulating these con- 

 ditions in both a terrestrial and aquatic environ- 

 ment , how much of the solar energy will be con- 

 verted to organic matter in each case? This, of 

 course, depends upon the efficiency of the photo- 

 chemical process, the quantum yield of photosyn- 

 thesis . 



The quantum yield of photosynthesis has been 

 one of the most thoroughly studied aspects of plant 

 physiology. Reviews by Rabinowitch (1951) and 

 others reveal that, under similar conditions, ap- 

 proximately the same number of quanta of light are 

 required to reduce one mole of CO2 to carbohy- 

 drate by a wide variety of plant types; the process, 

 in other words, seems to be largely species in- 

 dependent . 



We have recently attempted to calculate the 

 quantum yield of photosynthesis under completely 

 natural conditions , considering the efficiency of 

 utilization of light falling on a square meter of the 

 earth's surface (Ryther, in press). The assumption 

 was made that all of the light, except that lost by 

 reflection and back scattering , was absorbed by 

 photosynthetic pigments and that all other condi- 

 tions for photosynthesis were optimal. In this 

 treatment it was necessary to take into account the 

 spectral composition of daylight and the photosyn- 

 thetic utilization of light of this mean spectral 

 composition. Particularly important was a consid- 

 eration of the effects of light intensity. Photo- 

 synthesis is proportional to intensity up to about 

 1,000 foot candles, approximately 10% of full sun- 

 light. Above this, the process becomes light satu- 

 rated, and at still higher intensities may be actu- 

 ally depressed. Obviously, photosynthetic effi- 

 ciencies decrease rapidly as plants are exposed to 

 increasing intensities above the saturation point. 

 At the same time, however, the higher intensities 

 are effective in illuminating organisms deeper in 

 the water of a planktonic community or the leaves 

 farther down in a thick forest. Thus, while effi- 

 ciencies are decreasing, production continues to 

 increase with higher intensities of solar radiation. 

 Individual algal cells or leaves become light satu- 

 rated but entire plankton communities or forests do 

 not. Figure 1 shows the striking similarity between 

 plankton algae (Ryther, 195 6a) and pine trees 

 (Kramer and Clark, 1947) in this respect. 



The resulting efficiencies which were ob- 

 tained after corrections for respiratory loss, were 

 equivalent to a yield ranging from 8-19 grams of 

 dry orgaruc matter/m^/day for radiation values of 

 200 - 400 langleys/day (the range normally encoun- 

 tered over most of the earth) . These theoretical 



73 



