MEASURING THE PRODUCTIVITY OF THE SEA 



59 



nitrates and phosphates, constituents which often determine the size of 

 land crops. Both nitrates and phosphates are required for building up in 

 plants important substances like protein. The intensity of light might 

 have been thought the main conditioning factor. This may be so in the 

 dark months of a Danish winter, but even in Denmark light ceases to 

 be the major factor by March, when nutrient salts become more impor- 

 tant. 



Where do they come from? A small quantity is carried out to sea from 

 the land, but on the whole this is extremely little. We found practically no 

 inorganic phosphate in the water of the Congo River, the only estuary 

 which we studied, though it is true that some rivers, such as the Elbe, 

 carry fairly large quantities of nitrate and phosphate out to sea. 



In the open ocean supplies from land have no signifiance at all. The 

 nutrient salts there originate from the deeper water layers, where they are 

 found in fairly large quantities. There is a steady sinking of organic mat- 

 ter in the sea and it is re-converted in the ocean depths into nutrient 

 salts by bacterial decomposition. In the abyss these salts cannot be utihzed 

 by plants as there is no light, but they can be if the deep water rises 

 to the surface, where light is plentiful. 



All the ocean regions where organic production is high are notable for 

 the upwelling of water from deeper layers to the surface, though the 

 means by which the water rises may vary. Let us take as an example the 

 eastern side of the South Atlantic along the coast of South Africa. Here 

 there is nearly always an off-shore wind which takes the surface water 

 along with it. There cannot, of course, be a vacuum, and so the water 

 blown away is replaced by water from below. 

 It is not surprising that the greatest production 

 measured on the Galathea Expedition was in 

 this very region (Fig. alongside). Similar re- 

 gions along the west coast of South America 

 and the west coast of Australia are probably 

 equally productive. 



There are various other means by which 

 deep water is brought to the surface, one being 

 the formation of eddies near the boundaries of 

 different systems of currents. There is a 

 typical example in the Pacific south of 

 Hawaii, where the counter-current meets 

 the North and South Equatorial currents 

 (see map, p. 6i ). 



Food production ojf the west coast 

 of South Africa is exceptionally 

 high. Grams of carbon per square 

 metre of surface per day. 



