important source of nitrogen is the atmosphere. Nitrogen enters the sea 

 with rain in the form of ammonia and nitrates (Vaccaro, 1965), and is 

 also fixed by blue-green algae (Carpenter, 1973; Mague et al . , 1974). 



It is generally acknowledged (Ketchum et al., 1958) that nitrogen 

 and phosphorus rise into the layer of photosynthesis in the same ratio 

 (N:P = 15) that they are found in the phytoplankton. In the photic 

 layer, due to the different rates of regeneration and loss of these 

 elements, their ratio varies--nitrogen becomes relatively scarcer than 

 in the deeper water. The lower the significance of the upward flow in 

 the supply of alagae with elements of mineral nutrition in comparison to 

 regeneration of these elements in the photosynthetic layer, the stronger 

 the depression of the N:P ratio. Therefore, this ratio is a good index 

 of the degree of limitation of primary production by conditions of 

 mineral nutrition (Ketchum et al . , 1958; Maksimova, 1973). 



Vedernikov (1976a) compared the concentration of nitrates and 

 phosphates in most regions of the World Ocean (Fig. 8). The N:P ratio 

 was almost never normal (although if ammonia nitrogen is included in the 

 analysis, such cases might be more frequent). Furthermore, when the 

 concentration of nitrates decreased to less than 1 ug -atom ?/ 'i, the 

 content of phosphates remains at the same level (approximately 0.1- 

 0.2 ug-atom ? / I) . This level should not lead to extreme phosphorus 

 "starvation" of marine planktonic algae, since the limiting con- 

 centrations of phosphates for the growth of cultures are usually < 0.15- 

 0.55 pg«atom ?/i (Ketchum, 1939; Goldberg et al . , 1951; Strickland, 

 1965; Thomas, Dodson , 1968). A concentration of nitrates of 

 < 1 Mg*atom N/z , however, should cause significant nitrogen "starvation 

 of marine planktonic algae (see Fig. 9). 



Analyzing the data on the concentration of dissolved phosphates and 

 nitrates, we can conclude that nitrogen plays a primary role as the 

 limiting factor of the existence of marine planktonic algae. Enrichment 

 experiments performed in various regions have shown that the limiting 

 elements are most frequently nitrogen, phosphorus and iron, and in 

 certain cases, silicon and trace elements (Co, Mn , Mo, Zn , Cu). 



However, these experiments have shown that the influence of 

 nutrients is different in short-term and in long-term experiments. 

 Several hypotheses have been proposed to explain this, but the data 

 presently available indicate only that long-term enrichment experiments 

 are more reliable than short-term experiments in determining the 

 elements which limit production. We can conclude that in most regions 

 of the World Ocean, nitrogen and, less frequently phosphorus, regulate 

 the level of primary production. Exceptions are regions of intensive 

 upwelling, where the main factor limiting production may be the shortage 

 of forms of iron which can be utilized by the phytoplankton and of trace 

 elements. In 2-5-day experiments performed in the "juvenile" waters of 

 upwellings, the addition of iron and trace elements, chelates or their 

 mixtures led to a significant increase in production (Barber, Ryther, 

 1969; Barber et al . , 1971). Barber et al . , assume that natural organic 

 chelators, liberated by the organisms as the surface water "ages," might 

 be partially responsible for the increase in the growth rate of phyto- 

 plankton to the north and south of the equator in the eastern Pacific. 



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