NUTRIENT BUDGETS 301 



scant measurements presented by Cooper (1937). The two recent 

 estimates are two to three orders of magnitude less than the total of 

 other constituents of the oceanic reserve and thus may be neglected in 

 this discussion. 



The annual use of nitrogen by phytoplankton can be calculated from 

 the annual production of organic matter. The most recent figure for 

 organic production, that of Steeman-Nielsen (1954), is 42 gm carbon/ 

 m^/year, a value based upon carbon- 14 uptake under laboratory condi- 

 tions. This is only about 1/lOth of the estimates prepared by Trask 

 (1939) and Riley (1944) that were derived from oxygen production 

 under laboratory conditions, supported by field evidence based on oxygen 

 gain and nutrient depletion in the photosynthetic layer of the ocean. 

 Until the present uncertainty is resolved, an intermediate value of 150 

 gm/m^/year will be used in the following computations. With a 0.18 

 ratio of nitrogen to carbon in plankton (Sverdrup, Johnson, and Fleming, 

 1942, p. 929), 150 grams of carbon assimilated annually per square meter 

 corresponds to about 9.6 x 10° tons of nitrogen uptake for the whole 

 ocean (an area of 3.61 x 10^* m^) per year. The quantity of nitrogen 

 used by the plants is, thus, about 1% of the total reserve in the ocean; 

 however, it is probable that the circulation of the ocean water is not so 

 rapid that as much as 1% of the water comes within the photosynthetic 

 zone each year. Sources of nitrogen other than the general oceanic reserve 

 must therefore be available. 



The sources of new supply of nitrogen to the ocean are the land 

 and atmosphere from which fixed nitrogen is carried by rivers and rain. 

 Clarke (1924, pp. 63, 120) reported that of the 2.735 x 10° tons of 

 dissolved substances annually carried to the ocean, 0.90% is nitrate, 

 corresponding to 5.5 x 10*^ tons of nitrogen per year. The amount of 

 nitrogen carried to the sea in dissolved organic matter is less well known. 

 Clarke (1924, pp. 110, 119) and Hutchinson (1944) accepted John 

 Murray's average organic content of river water as 10% of the dis- 

 solved solids. The average nitrogen content of the organic matter may 

 be expected to be less than 7.6%, which is the nitrogen content of dry 

 marine plankton (Sverdrup, Johnson, and Fleming, 1942, p. 929). 

 Studies of Birge and Juday (1934) on lake waters gave an average of 

 3%. The latter figure, which was accepted by Hutchinson in his calcula- 

 tions, appears to be the best estimate now available and on this basis 

 8.2 x 10*^ tons of dissolved organic nitrogen are carried annually to the 

 ocean. Direct analysis for organic nitrogen in the Mississippi River, 0.35 

 mg/L (Riley, 1937), and the mixed river waters of Los Angeles Metro- 

 politan Water District, 0.28 mg/L, lend support to the above figure. If 



