306 EMERY, ORR, RITTENBERG 



CONCLUSIONS 



Fixed nitrogen and phosphorus in their various forms are present in 

 the ocean in quantities amounting to 100 times their estimated annual 

 use by phytoplankton. The annual use by phytoplankton, however, far 

 exceeds the annual contribution to the ocean of nitrogen and phosphorus. 

 Accordingly, the major proportion of the nutrients used by phytoplankton 

 must be regenerated from organic debris settling through the photo- 

 synthetic zone. Though most of the regeneration occurs in and just 

 below the photosynthetic zone, some occurs at greater depths and even 

 within bottom sediments. This has led to the accumulation of large nutri- 

 ent reserves at depths too great for depletion by photosynthesis. 



It is of interest that the renewal times (or the number of years that 

 would be required for dissolved nutrients in rivers, plus rain in the 

 case of nitrogen, to build up the nutrients dissolved in the ocean water 

 to their present concentrations in the absence of withdrawal) are similar: 

 12,000, 60,000, and 27,000 years for nitrogen, phosphorus, and silicon, 

 respectively. All are close to the renewal time for the water itself, 50,000 

 years (volume of ocean divided by annual volume of river flow). For 

 phosphorus, which has the longest renewal time, desorption of phosphate 

 ions on clays carried by rivers to the ocean would reduce the required 

 time. Similarly, solution of minerals may reduce the renewal time for 

 phosphorus and silicon. Nitrogen, with the shortest renewal time, would 

 be completely transferred from the atmosphere to the ocean in only 

 fifty million years if denitrification did not recycle much of it back to 

 the atmosphere. The brevity of these renewal times and the character 

 of the geological record, suggesting that the total life in the ocean has 

 been more or less constant over long periods, indicate that steady state 

 conditions exist. 



Under steady state conditions as much nitrogen, phosphorus, and 

 silicon must be deposited annually in sediments, or otherwise lost from 

 the ocean, as are contributed to the ocean from non-marine sources. Total 

 fixed nitrogen is contributed in far greater quantity than is lost to sedi- 

 ments (Table 1) ; we attribute this difference to denitrification. On the 

 other hand, both phosphorus and silicon are brought to the ocean and 

 deposited on its floor in approximate balance. Because the contribution 

 of each is dominantly in the form of mineral grains, the percentages of 

 phosphorus and silicon in the ocean bottom sediments are not materially 

 different from their percentages in stream-borne sediment. In summary, 

 we conclude that nitrogen, phosphorus, and silicon exist in steady state 

 conditions. Bruyevitch (1953), on the basis of similar calculations for 



