200 



ANALYSIS OF THE ENVIRONMENT 



process of nitrogen regeneration often 

 reaches a peak at a depth of some 500 to 

 1500 meters. Turbulence, convection, and 

 diffusion carry the nitrates back toward the 

 surface. Nitrates from still deeper water are 

 brought up to the lighted zone of utiUzation 

 in regions of oceanic upwelling such as oc- 

 cur, for example, oflE the coast of Peru and 

 help produce the profusion of hfe found in 

 these localities. 



Vertical profiles of the amount of nitrate- 

 nitrogen in different oceans are given in 

 Figure 44. The values are suggestive rather 

 than exact.' The maximum that is often 



NO3 N,mg/M^ 

 140 280 420 



1000 



Fig. 44. Depth profiles of nitrate-nitrogen; 

 data from expeditions by the Atlantis and the 

 Dana. (Redrawn from Sverdrup, Johnson, and 

 Fleming. ) 



" Quantitative data given here in milligrams 

 per cubic meter of water can be converted into 

 mygram-atoms ((xg-atoms) per liter by divid- 

 ing by the gram-atomic weight of the element 

 and adjusting to the appropriate weight and 

 volume units. One gram-atom equals 10' milli- 

 gram atoms or 10" mygram-atoms. Mygram- 

 atoms per liter is the unit proposed by the 

 International Association of Physical Oceanog- 

 raphy. 



found in intermediate depth is shown in 

 this Figure only for the Atlantic Ocean. 



It is characteristic that the nitrate-nitro- 

 gen should approach exhaustion at the sur- 

 face in summer. This indicates its effective- 

 ness as a limiting factor. The depth profile 

 for the Atlantic, when compared with simi- 

 lar profiles for oxygen (Fig. 42), allows an- 

 other generahzation to the effect that there 

 tends to be an inverse correlation between 

 the amount of dissolved oxygen and that of 

 nitrate-nitrogen. This relationship is most 

 apparent during the growing season for 

 phytoplankton and is according to expecta- 

 tion if nitrogen is a limiting factor for the 

 growth of plant plankters in the well-hghted 

 surface zone. The depth profiles of Figure 

 44 also show correctly that decided geo- 

 graphic variations occur in nitrate-nitrogen; 

 such variations are known within a given 

 ocean as well as between oceans, and for 

 inshore waters as well as for the open sea. 



Seasonal variations in nitrates also occur. 

 Figure 45 illustrates the amount of this 

 variation in the English Channel. Similar 

 seasonal variations, though with different 

 quantities of nitrogen, are known for other 

 waters, especially for the Gulf of Maine 

 (Rakestraw, 1936) and Friday Harbor 

 (Phifer and Thompson, 1937). 



The vertical distribution of nitrites (NO2) 

 differs from that of nitrates. The amount 

 present is always small, and the maximum 

 is often found in a rather narrow layer near 

 the surface; they are also more abundant 

 near the bottom than in the intermediate re- 

 gion. The seasonal variation runs roughly 

 opposite to that of nitrates with the mini- 

 mum late in winter and the maximum late 

 in summer or early autumn. Ammonia also 

 is present in small, variable amounts; it ap- 

 parently has a rather uniform distribution 

 with depth. Common hmits for the three 

 forms of fixed nitrogen' are: 



NO3-N 1.5-600 mg/M' 

 NO2-N 1.5- 40 mg/M' 

 NH3-N 5-50 mg/M' 



Some nitrite and ammonia are produced 

 on the sea floor, but most recent data in- 

 dicate that ammonification occurs mainly 

 near the upper layers of the sea. Nitrite for- 

 mation appears to be confined to the upper- 

 most 200 meters of water. ZoBell (1946, 

 p. 153) regards the problem of nitrification 



"Based on estimates given by Sverdrup, 

 Johnson, and Fleming (1942). 



