NATURE ADRIFT 



The basic sources in the sea, just as on the land, arc the green plants. They 

 alone can combine the inorganic nutrients — the nitrates, phosphates etc. — 

 with the dissolved carbon dioxide to form organic compounds that can be 

 used as animal food — the carbohydrates, proteins, oils and fats. To do this 

 they use the sun's energy and they need the green pigment chlorophyll to 

 act as a catalyst — a chemical the very nature of which enables the combina- 

 tion to occur and to go on occurring without itself changing. The food of 

 the green plant is therefore the dissolved salts and carbon dioxide, and plants 

 can thrive only in the light and as long as these nutrients last or are replenished. 

 This replenishment will depend on the regeneration ofnutrients from the w;^ste 

 products of animals, and from the decay of plant and animal tissue by the 

 help of bacteria. The distribution of the nutrients into the light zone is a 

 feature of the current systems, both in horizontal transport and in the vertical 

 turbulence of convection currents and upwelling. 



In addition to these main nutrients the plants require minute amounts of 

 metabolites — growth-promoting substances, vitamins and the like — some of 

 which are manufactured only by bacteria, others by the plants themselves. 

 Free bacteria in the sea are not very abundant but they do live in close associ- 

 ation with any solid surfaces including, of course, the plants and animals. It 

 is this necessity for metabolites that makes it difficult to grow things in arti- 

 ficial sea water, no matter how carefully oreparcd, unless some 'soil extract' 

 or other complex witch's brew is added. The bacteria are thus a very im- 

 portant link in the food chain. It should be emphasized, of course, that these 

 bacteria are not connected with any disease and that the number of beneficial 

 bacteria in the world in general, and the sea in particular, far exceeds the 

 harmful ones. 



To remain in the light zone the plants must be attached at the coast in 

 extremely shallow water, or else float; and as the coastal area is so hmited 

 compared with the total surface of the sea, it is the floating plants that really 

 matter in the marine food chain. Some types of these plants have been de- 

 scribed in Chapter 3, the most important being the very minute nanoplankton, 

 the diatoms and the dinoflagcllates. With the onset of stable conditions in the 

 spring (p. no) the nanoplankton and the diatoms start to multiply rapidly 

 by simple division, and as a generation is measured in minutes and hours it 

 does not take long to establish a population. The wag who said that political 

 troubles are the only things that can be multiplied by division was not a 

 biologist ! 



The dinoflagcllates follow later. The process of rapid reproduction, 

 reaching a peak and then dying down, is called 'blooming' or 'flowering'. 

 There is a succession, not only of these types but of the different species 

 of diatoms. This is only in part due to seasonal variation in the current 



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