64 MEASURING THE PRODUCTIVITY OF THE SEA 



part of the spectrum; but since the downwardly scattered portion is not 

 lost and the absorption of blue rays is very slight, it is blue light which 

 penetrates furthest in pure water. Green light comes next. 



In very clear oceanic water, where plankton production is small, one 

 per cent, of the blue light penetrates to a depth of about 130 metres, one 

 per cent, of the green to about 80 metres, and one per cent, of the red 

 to about 15 metres. 



In water masses containing a larger amount of organisms or particles 

 it is the green rays which penetrate furthest. This is the case in coastal 

 regions and very productive oceanic regions. 



The variations are so great that no universal rule can be stated. In 

 coastal regions, however, one per cent, of the green light very rarely 

 penetrates deeper than 50 metres and usually much less than that. In 

 Walvis Bay off the west coast of South Africa we traced one per cent, of 

 the green light at a depth of 70 centimetres. In this water we found a 

 small plankton alga in such large quantities that it turned the water 

 brownish-red. This alga, which will be described by a Norwegian specialist 

 on the basis of data collected by the expedition, has been named after 

 the Galathea. It appears to secrete toxic matter which is lethal to fish. 

 Large numbers of fish had died just before our arrival in the bay. 



On the Galathea we made our light measurements from the stern. A 

 photo-electric cell was enclosed in a watertight case and glass filters of 

 various colours could be placed over the window, which faced upwards. 

 The photometer was affixed to a wire which passed over a block sus- 

 pended from a gallows. By means of an electric cable the photo-electric 

 cell was also connected with a sensitive meter on the deck. Another photo- 

 electric cell mounted on the deck was used for taking simultaneous read- 

 ings of the variations in the light which fell on the sea surface. 



