The world oceans have long been considered a valuable resource for the disposal 

 of human waste materials based on the philosophy that "the solution to pollution is 

 dilution." As Goldberg points out in this volume, it is critical to assess the assimila- 

 tive capacity of the marine environment in order to avoid making foolish mistakes. 

 To provide perspective on this problem, it can be estimated that it would take about 

 twenty thousand years to double the total available nitrogen content of sea water if all 

 of the sewage from today's human population were discharged into the ocean. This 

 assumes complete worldwide mixing and that there would obviously be local con- 

 centrations greatly in excess of the average where the material is discharged. It as- 

 sumes, furthermore, that there would be no biological adjustment of the system, such 

 as by the sequestering of some of the added materials to the sediment or by de- 

 nitrification. And, of course, the problem would be exacerbated by further growth 

 of the world population. 



The fact that there is a large capacity does not answer the fundamental question. 

 How much of this capacity can be safely used? Would doubling the available nitro- 

 gen content of the sea be acceptable? Would 1 percent? 1 percent? Would the added 

 nutrients in sewage stimulate marine productivity so that the harvest from the sea 

 could be increased? Could enough material be harvested to keep the system in a con- 

 tinuous steady-state balance?* How can local deleterious concentrations that upset 

 the natural ecosystem and do more damage than they do good be avoided 



THE NATURAL NUTRIENT CYCLE 



Nitrogen and phosphorus are the elements that most frequently limit plant growth 

 in aquatic ecosystems. For diatoms, silica, which is essential for the formation of the 

 shell, is also sometimes limiting. Various trace elements such as boron, cobalt, cop- 

 per, iron, molybdenum, and zinc are also required for plant growth but are rarely 

 limiting in sea water. Other elements such as carbon, potassium, and sulfur are essen- 

 tial components of living material, and they are present in excess quantities for plant 

 requirements in the sea. The scope of this paper will focus upon nitrogen and phos- 

 phorus as the most critical elements. + 



Normal Proportion of Elements 



In the production of organic matter in photosynthesis and the subsequent decom- 

 position of this material, the major elements are utilized or released in statistically 

 similar proportions to one another ( Redfield et al., 1963). In sea water, these propor- 

 tions are: 



-0:C:N:P = - 276:106:15:1 (by atoms) 

 or - 142:41:6.8:1 (by weight) 



The oxygen value is negative in the above relations because it is released in photo- 

 synthesis when the other elements are being absorbed, and it is utilized in decom- 

 position when the other elements are being released. Except for carbon, which is 

 present in sea water in considerable excess of the needs of the plankton, these propor- 

 tions are remarkably close to the concentrations available in seawater. The data in 

 Table 1 (Redfield et al., 1963) show a comparison between availability and utiliza- 

 tion. Except for carbon, which is available at nearly 10 times the amount required by 

 the phytoplankton, the other elements are present in average seawater in the approx- 

 imate proportions they are needed for the ecological cycle. 



•The present annual fish harvest of about 60 million metric tons removes about 1.3 « JO" kg N/yr- 1 assuming 120 kg protein 

 containing 17.8 percent N per ton. This is about 3 percent of the nitrogen in the human sewage of the present world popula- 

 tion. The importance of marine protein to human nutrition is discussed by Ketchum (1973). 



+ Phytoplankton production is. of course, also dependent on other environmental characteristics, especially the transparency of 

 the water, which determines the depth of the euphotic zone, the depth of the mixed layer, salinity, and temperature. 



69 



