PLANKTON AS FOOD 



first in the terms of phosphate content wc learn from the marine chemist 

 that the summer level in the sea is about 0-2 microgram atoms of phosphate/ 

 phosphorus per litre. Translated into terms of sodium phosphate, and con- 

 sidering only the top 50 metres of water (the photic depth, at which there is 

 enough light for the plants to grow) this works out at 1-64 grams per square 

 metre of surface, or 1,406 lb per acre, or about 4 tons per square mile. The 

 summer level of phosphate in the sea is the depleted figure after the plants 

 have had their spring growth, and we should want to double this figure to 

 give any real increase in phosphate content. After the summer growing 

 season, and after many of the plants and animals have died off there is a 

 regeneration of nutrients from their decay and with the winter mixing 

 (p. III). This results in a winter level of about four times the summer level 

 so that doubling the summer level would not be at all excessive. Not only 

 do we want to double the summer level of phosphate, but we want to main- 

 tain it at the higher level so that a single fertilization is grossly inadequate. 

 To maintain the level we should need to add our fertilizer about every two 

 weeks during the spring increase of plant growth and monthly thereafter for 

 about four months only, a total of about ten applications per year. 



Using superphosphate, which has only about one-quarter of the phos- 

 phate content of sodium phosphate, but which is the usual form of fertilizer, 

 we would need sixteen tons per square mile at each application. We would 

 also need to add other fertilizers in proportion, especially nitrate, and the 

 total tonnage needed would be about four times the superphosphate. It would 

 be pointless to apply one without the others as the plankton growth would 

 be limited by whichever substance was in shortest supply. Basing our calcula- 

 tions on superphosphate at -^15 per ton, and sodium nitrate at ^25 per ton, 

 this works out at -^1,440 per square mile for each application, or ^14,400 

 per year without any allowances for transport and labour in distributing it. 

 The North Sea has an area of about 125,000 square miles! The annual yield 

 offish from the North Sea is in the order of ^50 million or an average of 

 ^400 per square mile, though certain parts will yield much less than the 

 average and other parts considerably more, as much as jC, i ,000 per square 

 mile. But is it worth spending ^14,000 to double the plant production even 

 in the best areas, when double the yield would only give an extra -^1,000? 



More than that, doubling the plant production would be no guarantee 

 at all of doubling the yield offish; there are so many stages in the food chain, 

 as well as losses at each stage, and things can go wrong at any of them. One 

 example of this was given on page 138 in reference to 'red tides'. Here an 

 abnormal increase of fertilizer brings about the sudden increase of a toxic 

 dinoliagellate instead of the normal plankton, and this results in wholesale 

 destruction offish and the ruination of tourist trade to popular seaside resorts. 



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