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NIELSEN 



[OHAP. 7 



Such a method can, of course, be employed only in the open sea well beyond the 

 influence of the coast and only as a first approximation. 



It must be emphasized that only the light absorbed by the photosynthetically 

 active pigments found in the algae is used in photosynthesis. In a hypothetic 

 ocean free of any particles and dissolved organic matter the depth of the photic 

 layer would be about 140 m (according to Jerlov in Steemann Nielsen and 

 Aabye Jensen, 1957). The lower boundary of the photic layer is ordinarily 

 considered to be the depth where 1% of the surface light is found. 



In the most transparent areas of the oceans — for example, in the centre of 

 the Sargasso Sea — the depth of the photic layer is about 120 m. It is obvious 

 that only a few algae can be present in this water, which has optical properties 



50 100 



Depth of photosynthetic layer, m 



Fig. 17. Depth of photosynthetic layer and maximum rate of photosynthesis per m 2 

 surface. (After Steemann Nielsen and Aabye Jensen, 1957.) 



resembling those of distilled water. Fig. 17 presents a curve showing the maxi- 

 mum rate of production below a surface unit as a function of the depth of the 

 photic layer. All stations from the Galatkea Expedition were divided into classes 

 according to the depths of the photic layer. From every class the station 

 showing the highest rate of production was selected. 



If the rate of production per square metre is definitely lower than the curve 

 values, the opacity of the water must be due primarily to causes other than 

 living planktonic algae, for example minerals brought out from land, or rem- 

 nants — including "yellow matter" — from a production which has taken place 

 previously in the water. In the latter case the result is that in oceanic waters 

 with considerable seasonal fluctuations in the size of the primary production, 

 such as in the temperate part of the North Atlantic, highly transparent water 



