520 CYCLES OF ORGANIC AND IXORCAXIC SUBSTANCES 



simplicity must be regarded as a first approximation of unknown 

 extent, and a main present purpose of laboratory experiment and 

 sea sampling must be to find out how and where it breaks down. 



As I have said, this apparent simplicity is imposed by the 

 methods of sampling. Plant pigments, especially chlorophyll a, 

 are measured by the methods of Richards (1952) and others. The 

 chlorophyll \'alues are obviously related to the rate of gro\\th of 

 the plants as well as being an index of the amount of plant carbon 

 piesent. The ratio of certain of the pigments may also provide an 

 indication of the physiological state of the plants, particularly with 

 respect to nitrate deficiency (Yentsch and \accaro, 1958). But 

 certainly one of the most important of the new methods is that 

 introduced by Steemann Xielsen (1952), which provides a meas- 

 ure of production by using radioactive carbon. Its widespread 

 use was described in a symposium held by the International 

 Council for the Exploration of the Sea at Bergen in 1957. 



Yet these methods raise many problems since their interpreta- 

 tion is often not so exact as the measurements themselves. Thus 

 the conversion from a pigment measurement to an estimate of 

 plant biomass in terms of organic carbon has very wide limits 

 since both high light intensities anfl low nutrient concentrations 

 tend to decrease the chlorophyll to carbon ratio. In particular 

 there is a daily periodicity in photosynthesis (Doty and Oguri, 

 1957) which in the tropics can amount to a factor of five between 

 the morning maximum and afternoon niinimum. This periodicity 

 seems to correspond to diurnal changes in chlorophyll content 

 (Yentsch and Ryther, 1957; Shimada, 1958). 



The principal discussion, however, has been over the inter- 

 pretation of C'^ values. This C^* method is important because it is 

 at the focus of the diverse interests I have mentioned, and so the 

 interpretation of its results involves knowledge of many of the 

 factors affecting plant growth. Because of its sensitivity, it has 

 revealed previously unconsidered complexities in these relations. 

 The original argument, whether the method measured gross or 

 net production, that is, whether it measured photosynthesis or 

 photosynthesis less respiration, has led to questions and experi- 

 ments about the effects of light inhibition, the mode of nutrient 



