SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 37 



station in the Gulf of Maine is shown. In spring, when active growth com- 

 mences, most of the phosphorus is present in inorganic form ; particulate 

 phosphorus occurs chiefly in the photosynthetic zone (0-60 m) and very little 

 dissolved organic phosphorus is present. During the summer and autumn the 

 dissolved organic fraction becomes much greater and is distributed equally 

 at all depths, providing evidence that phosphorus is carried downward by the 

 sinking or migration of organisms. In winter the organic phosphorus decreases 

 as the regeneration of inorganic phosphorus proceeds and in this form phos- 

 phorus is returned to the surface layer by the mixing of the water. 



Measurements of the rate of phosphate regeneration in the laboratory 

 indicate that up to one-half the total content of decomposing plankton appears 

 in soluble form in the first day (Cooper, 1935; Seiwell and Seiwell, 1938). 

 Vinogradov (1953) estimates that about half the nitrogen and phosphorus in 

 algae is water-soluble. The remainder appears to exist in stable compounds 

 that are decomposed after longer periods. These compounds appear in the 

 water as the soluble organic fractions of phosphorus and nitrogen. 



In the upper layers of the ocean, where organisms are decomposing in 

 sufficient quantity, a substantial fraction of the phosphorus may be present in 

 organic combination. Ketchum, Corwin and Keen (1955) found that nearly 

 50% of surface samples from the North Atlantic contained more than 0.25 mg 

 atom/m 3 of dissolved organic phosphorus. As depth increases, however, the pro- 

 portions of samples in which statistically significant quantities of organic 

 phosphorus were present diminished and in waters deeper than 1000 m none 

 could be detected with certainty. 



Rakestraw (1947) found that when samples of water from the oxygen 

 minimum layer and from greater depths were left in the dark at the in situ 

 temperature for nearly two years the oxygen content did not change after the 

 first 50 days of storage. During the initial period oxygen was consumed, but 

 not in excess of a few per cent of the total content, so that evidently the frac- 

 tion of oxidizable organic matter in %he deep water is very small. 



These two lines of evidence justify the assumption that, in the deep ocean, 

 the oxidation of organic matter has gone so nearly to completion that the un- 

 oxidized residues may be ignored for many practical purposes. 



The regeneration of nitrate in sea-water is somewhat more complicated than 

 that of phosphate. Dissolved nitrogen compounds are present in sea- water, 

 Robinson and Wirth (1934) having found some 23 mg atoms/m 3 of organic 

 nitrogen in Puget Sound and 7 mg atoms/m 3 in the off-lying Pacific Ocean. 

 Nitrogen is released from organic combination as ammonia and is subsequently 

 oxidized to nitrite, and then to nitrate. The steps in this process have been 

 followed in laboratory experiments by von Brand and Rakestraw (1941). These 

 experiments showed the successive appearance and disappearance of the 

 fractions as decomposition proceeded. It required three or four months for 

 the regeneration of nitrate to be completed. 



In the sea. ammonia and nitrite appear as intermediate products at times 

 and places where organic matter is decomposing in quantity. Ammonia is the 



