62 MEASURING THE PRODUCTIVITY OF THE SEA 



are virtually unproductive deserts and that the total sea area is about 

 two and a half times the land area. 



The Galathea set out at the right moment. Two years earlier and an 

 adequate quantity of C^^ would have been unobtainable. A few years later 

 and others might have forestalled us in this work. 



Oceanographers have many reasons for studying the penetration of 

 light in the sea. The physicist seeks information about heating, because 

 absorption of the sun's visible and invisible rays causes the water masses 

 to rise in temperature. The zoologist is interested in the effects of light 

 penetration on the conditions governing animal life at various depths. The 

 illumination of the sea is of particular interest to the botanist because it 

 is the factor which determines where plants can live. 



On the Galathea Expedition measurements of light penetration were 

 absolutely essential because the measurement of production at the various 

 levels was dependent upon them. Many measurements were therefore 

 made, and as previous measurements of light penetration had been on 

 a small scale we were able to provide a good deal of new information. 



Up to 25 years ago we had to use photographic plates, and the 

 method was both laborious and of doubtful accuracy. The invention of 

 the photo-electric cell considerably facilitated the work, and when in the 

 nineteen-thirties a type of cell was developed which dispensed with the 

 need to amplify the electric current the measuring of light penetration 

 in the sea began to make headway, though measurements were still 

 largely confined to coastal regions. Most of the oceanic measurements, 

 and the best, were made by the Swedish Albatross Expedition of 1947^ — • 

 48. Even so, they were made at only 26 positions scattered over the 

 Atlantic, the Pacific, the Indian Ocean, and the Mediterranean. The 

 number of stations was deliberately restricted because, though the expe- 

 dition was equipped with all the latest devices, it was desired to confine 

 operations to days when conditions were perfect; in calm weather with- 

 out appreciable swell, and in clear sunshine with the sun near zenith. 

 As a result the measurements are of the highest possible quality. 



We employed a different procedure, taking as many measurements as 

 possible, sometimes when conditions could not give high quality. In 

 scientific work, however, accuracy must always be adapted to the object 

 in view, and our main object was to procure information for use in 

 estimating planktonic production. We did not need the same accuracy as 

 purely physical tests demand. 



In studying heat the fact that the sun's radiation is a sum of rays of 

 varying wavelengths can be disregarded. Not so when we study the in- 



