Table 1. Availability of Nutrient Elements and of Oxygen in "Average" 

 Seawater (S=34.7°/oo; T=2°C) and the Ratios of Their Availa- 

 bility and Utilization by Plankton (after Redfield, et al., 1963) 



The oxygen content is of particular interest in terms of polluting the sea with 

 added nutrients. Since photosynthesis takes place in the near surface layers, the oxy- 

 gen produced is released to the atmosphere while the organic matter produced can 

 sink to deeper layers, isolated from the surface, where it is decomposed. Table 1 shows 

 a very small margin of safety in terms of preventing the complete removal of oxygen 

 from the water and the production of anoxic and objectionable conditions. 



The concentrations of nitrogen and phosphorus shown in Table 1 are characteris- 

 tic of the average deep water of the world oceans and are considerably higher than 

 the concentrations that would be found in surface layers of coastal or estuarine 

 waters. The oxygen content of the warm, shallow waters would also be less because 

 oxygen solubility decreases with an increase in temperature. Nevertheless, the num- 

 bers are useful to set limits that should not be exceeded for nitrogen and phosphorus 

 in order to avoid the development of anoxic conditions. 



The Limiting Nutrient 



These ratios do not represent the proportions in which the elements are available in 

 coastal or estuarine seawater but rather the ratios of change in their concentrations 

 that result from biological activity. The phytoplankton biomass will be limited by the 

 nutrient available in the environment in the smallest quantity relative to the require- 

 ment of the plants, provided all other factors such as light and temperature are favor- 

 able. It will be recognized that the element may not be limiting at the time of observa- 

 tion if all of the essential elements are present in excess. The limiting element will be 

 the first nutrient to become exhausted following growth of the phytoplankton popu- 

 lation. 



In freshwater, phosphorus is commonly the limiting nutrient, and Miller et al. 

 ( 1 975) have suggested that when freshwater contains a N: P ratio greater than 1 1 .3: 1 

 by weight (25:1 by atoms) the water may be considered phosphorus-limited while 

 water containing lower N:P ratios can be considered nitrogen-limited for algal 

 growth. In freshwater, consequently, removal of excess phosphorus from sewage 

 wastes has resulted in considerable improvement in the quality of the receiving 

 water. 



In contrast to this, the common limiting nutrient in marine waters is nitrogen 

 (Ryther and Dunstan. 1971; Eppley et al., 1971; Thomas et al., 1974; Doig and 

 Martin, 1974; Goldman, 1976). In marine waters a N:P atomic ratio lower than 15: 1 

 (6.8:1 by weight) implies nitrogen limitation of the phytoplankton production. 



In New England coastal waters, Ketchum et al. (1958) showed that the ratio of 

 change of nitrogen and phosphorus in the near surface waters was 15:1 by atoms but 



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