234 DISCOVERY REPORTS 



The greatest vertical mixing between the two currents also takes place between 38 

 and 43° S, so that the phosphate is not lost from the Antarctic Zone but is returned to 

 it in the warm deep current. 



North of the Rio Grande ridge, where there is a sharper discontinuity between the 

 Antarctic intermediate water and the warm deep water, the surface of the warm deep 

 water is not enriched as much as it is farther south. 



In the Tropical Zone the surface stratum is depleted of phosphate, but lower down 

 the phosphate content of the water increases with depth to a maximum of about 120- 

 140 mg. PjOs/m.^ in Antarctic intermediate water. 



There is a minimum phosphate content at the level of greatest oxygen content in 

 the North Atlantic deep water, and it is probable that the water at this level has 

 most recently been at the surface. In Antarctic bottom water the content increases to 

 90-100 mg. PaOs/m.^ 



The distribution of nitrate which is also shown in Plate IX is similar to that of 

 phosphate. The greatest concentration is found in 38-43° S between the warm deep 

 water and the sub-Antarctic and Antarctic intermediate waters. Nitrate is returned to 

 the Antarctic Zone in a cycle similar to that which returns phosphate. 



The vertical distribution of nitrate shows that the regeneration of nitrate from de- 

 composition products takes place principally south of the Rio Grande ridge : the nitrate 

 content of the water north of the ridge is much less than that of the water south of 

 the ridge. 



The surface water in the Tropical Zone is almost depleted of nitrate. Most is found 

 in the Antarctic intermediate layer, and a smaller concentration in the North Atlantic 

 deep water. The bottom water is richer in nitrate than the deep water. 



The examination of the nitrite content of sea water has shown that in the Antarctic 

 Zone there can be as much as 6-7 mg. nitrite Ng/m.^ at the surface and 8 mg. nitrite 

 Na/m.^* at a depth of 80-100 m.; but as soon as a discontinuity appears in the water, 

 which makes vertical mixing with the surface water difficult, no nitrite is found below it. 

 None was ever found below 150 m. 



Nitrite was found in sub-Antarctic water in amounts decreasing from 5-5 to 3-5 mg. 

 nitrite N./m.^ towards the north. North of the sub-tropical convergence there were 

 only small amounts of nitrite; but farther north, just below the sharp discontinuity at 

 the bottom of tropical water, amounts as large as 30 mg. nitrite Na/m.^* have since been 

 found. Nitrite is a stage in the formation of nitrate from animal decomposition products, 

 but we have rarely found it at a depth from which vertical mixing with the surface water 

 was difficult. The absence of nitrite from the deep layers in which there are the greatest 

 phosphate and nitrate contents is surprising. 



