148 NIELSEN [CHAP. 7 



surface of membrane filters after filtration. As it is difficult to predict the 

 presence of such fragile flagellates it would be desirable always to take some 

 precaution to minimize the loss. Among such precautions (cf. Guillard and 

 Wangersky, 1958) may be mentioned: (a) filtration of at the maximum 100 ml 

 (less, if the plankton concentration is very high) and (b) the use of a reduced 

 vacuum, when using thin, fast Millipore filters. 1 If the thick, slow filters from 

 the Membranfiltergesellschaft, Gottingen, are used, a higher vacuum may be 

 used. Owing to the long pores, the active suction becomes much less. Duursma 

 (1960) has shown that in certain northern waters the content of dissolved 

 matter may vary considerably between winter and summer. This does not 

 necessarily mean that the organic matter is primarily excreted during the 

 growth of the algae. It is very likely that most of the dissolved organic matter 

 originates either from algae only partly digested by the herbivores or from 

 dying algae (cf. Guillard and Wangersky, 1958). As mentioned on page 132, 

 during the spring outburst of algae the herbivores are often unable to eat all 

 the algae produced. 



H. Different Modifications of the Carbon-14 Method 



For the determination of the production of matter under 1 m 2 of surface, 

 three different modifications of the method may be employed. In principle the 

 best modification is the in situ method resembling Gaarder and Gran's oxygen 

 method. Instead of measuring the oxygen metabolism in the bottles suspended 

 at the different depths, the rate of photosynthesis is determined by the tracer 

 technique. To estimate the total photosynthesis below 1 m 2 of surface of the 

 sea, the measurements from the individual depths are presented graphically 

 (Fig. 9). The area to the left of the curve represents the photosynthesis below 

 the surface. This modification of the method is particularly suitable for measure- 

 ments in coastal waters. In Danish waters, the lightships have been found ideal 

 for such work (Steemann Nielsen, 1958). It is possible to adapt the technique 

 in such a way that the crew on the lightship is able to do the field work 

 adequately. A simple device for filtering the water samples is particularly 

 important (Fig. 10). 



It must be borne in mind that the conditions for the algae in the bottles are 

 only approximately the same as in the sea, where vertical currents ordinarily 

 prevent the algae from remaining constantly at the same depth. As very high 

 light intensities have the effect of reducing the rate of photosynthesis, and, as 

 it takes hours before this effect has disappeared, the lack of vertical movement 

 of the algae during an experiment may affect the result. Rodhe (1958) has 

 shown that in certain circumstances the yield of a 24-h experiment may be 

 21% or less than the sum of short-time experiments lasting a total of 24 h. The 

 present author considers a 24-h experiment to be too prolonged for ordinary 

 purposes (cf. page 140). Experiments from sunrise to noon or from noon to 

 sunset must be considered more suitable. 



In order to compute the rate of the production for a whole day, it is necessary 



1 Personal communication by Dr. G. Anderson. 



