54 MEASURING THE PRODUCTIVITY OF THE SEA 



they obviously have no means of existence in the oceans, where the average 

 depth is more than 4,000 metres. 



But if the fauna of the oceans was obhged to hve on plants which 

 grow in the narrow coastal fringes marine animals would be rare, at least 

 at some distance from the coast. Yet animals occur in large numbers all 

 over the seas, even in the middle of the oceans. It follows that there must 

 be other marine plants than the ones which we observe by the shores. 



If you were to row out from the coast on a summer's day and haul in 

 a bucket of water, it might surprise you to learn that it contained several 

 million plants. This is no exaggeration, and the plants are real ones if, 

 of course, very minute. Their average diameter is one-hundredth of a 

 millimetre. The microscope reveals them as an infinite variety of diminu- 

 tive green, brown, and golden plants to which we give the same plankton 

 algae. The word "plankton" comes from a Greek word meaning "wan- 

 dering", and it aptly describes these plants which are found drifting 

 all over the upper reaches of the seas. But it is only in the upper reaches 

 that they occur, for only there is there adequate light to supply the energy 

 which they require for converting inorganic into organic matter. 



Sunlight in penetrating the sea is absorbed both by the water itself and 

 by what it contains, including plankton algee. We shall be returning to 

 this question later in the chapter. 



To supply their requirements of light energy plants need at least one 

 per cent, of the light which strikes the surface. Thus only a thin layer 

 of the total ocean masses is able to produce vegetable matter. The upper- 

 most 100 metres, in fact, provides all organic matter on which all marine 

 animals live, including those which inhabit the abyss at 10,000 metres. 



The existence of oceanic plankton algae has been known since the latter 

 half of the nineteenth century, but it is only in the present century that 

 we have gained any real knowledge of them. Their production of organic 

 matter — the basis of all marine life — was first measured by the Galathea 

 Expedition, employing radioactive carbon. 



In a few coastal waters with a high volume of plankton algas the 

 amount of organic matter formed by them had previously been measured 

 by normal chemical means. When plants assimilate carbon dioxide they give 

 off an equivalent amount of oxygen, and there is a sensitive chemical me- 

 thod by which the amount can be determined. The principle is very simple. 

 Samples of water are taken at various depths and measured for oxygen. 

 Then, in glass bottles, they are lowered to the depths from which they 

 were taken and kept suspended there for 24 hours. They are hauled up 

 again and then measured for oxygen content. Given the difference in 



