ZoBell — 18 — Marine Microbiology 



Although compounds of nitrogen occur in sea water only in very low 

 concentrations, they have received much attention because they are essen- 

 tial for plant growth. Moreover, the nitrogen content of sea water is a 

 useful indicator of the past history of the water. 



The nitrate-N content of sea water ranges from near zero to about one 

 milligram per liter. Since the concentration of nitrate like that of nitrite, 

 ammonium, and phosphate is relatively low, it is usually expressed as 

 milligrams per cubic meter (mgm./M.^) or as micrograms per liter 

 (/jLgm./L.). One milligram per liter is equal to looo mgm./M.^ or looo 

 /zgm./L. 



The nitrate content of sea water varies with season, depth, latitude, 

 distance from land, and other factors. As a rule water in the euphotic 

 zones contains less nitrate than that at greater depths. Thompson and 

 Robinson (1932) report that at a certain station in the Pacific Ocean 

 5 mgm./M.^ of nitrate-N was found in surface waters, 384 mgm./M.^ at 

 400 meters and 500 mgm./M.^ at 800 meters. In the euphotic zone the 

 nitrate-N content is generally less than 100 mgm./M.^ and often less than 

 10 mgm./M.^ 



The fact that deep water usually contains more nitrate than surface 

 waters is cited by certain workers as evidence that nitrate is formed in 

 deep water, but more likely this vertical distribution merely shows that 

 nitrate is being utilized by photosynthetic organisms. The effect of 

 microorganisms on the nitrate, nitrite, ammonium, and phosphate content 

 of sea water is discussed in Chapters XI and XIII. Additional information 

 on the nitrate content of sea water is given by Harvey (1926) and by 

 Thompson and Robinson (1932). 



The nitrite-N content of sea water ranges from near zero to 

 50 mgm./M.^ It is usually less than 10 mgm./M.^ and often less than 

 i.o mgm./M.^ Orr (1926) and Rakesteaw (1936) have summarized 

 the factors which influence the occurrence of nitrite in the sea. 



There is nearly always some detectable ammonia-N in sea water. The 

 concentration ranges from i to 350 mgm./M.^, usually 5 to 50 mgm./M.^ 

 The distribution of ammonia in the sea is somewhat irregular but the 

 ammonia content of water deeper than 500 meters is relatively uniform 

 and low (Rakestraw, 1936; Redfield and Keys, 1938), nearly always 

 being less than 10 mgm./M.^ The highest concentrations of ammonia are 

 found near shore particularly where there is much terrigenous pollution or 

 near the sea floor in shallow water. Seiwell (1931) found as much as 

 350 mgm./M.^ of ammonia in a bottom sample where the water depth was 

 around 50 meters, compared with o to 50 mgm./M.^ found in surface 

 waters. 



Ammonia is produced and consumed by microorganisms. The evi- 

 dence that ammonia is assimilated by marine plants, perhaps preferen- 

 tially, is reviewed by ZoBell (1935). 



There is a marked parallelism between the concentration of phosphate 

 and that of nitrate in the sea (Redfield, 1934; Rakestraw, 1936). Be- 

 cause it is an essential plant nutrient which sometimes becomes a limiting 

 factor in the productivity of the sea, the occurrence of phosphate has re- 

 ceived much attention by oceanographers and marine biologists. 



The concentration of phosphate in the sea generally increases with 

 depth. During the photosynthetic season, surface water in tropical and 

 temperate zones may contain from o to i mgm./M.^ of phosphate-P, 

 whereas water at depths exceeding 500 meters contains 5 to 200 mgm./M.^ 



