DISSOLVED SALTS AS LIMITING FACTORS 



201 



in the sea as largely unsolved. It is his be- 

 lief that, besides the soil-hke nitrifiers that 

 are commonly found in bottom deposits and 

 near land, "there are other marine nitrifying 

 organisms which have escaped detection." 



PHOSPHORUS 



Phosphates, hke nitrates, are brought by 

 rivers to the sea in large amounts. The 

 Mississippi River carries enough combined 



from the Mississippi River (Riley, 1937). 



Phosphates show much the same distri- 

 bution in depth as do nitrates, and, in broad 

 outline, their seasonal and geographical 

 variation approaches that of the nitrates. 

 The depth profile is shown in Figure 46. 

 Phosphates are present in much smaller con- 

 centration than the nitrates and appear to 

 be of high importance as limiting factors 

 in the development of populations of plant 

 plankton. One miUigram of phosphate- 

 phosphorus in a M' of sea water is enough. 



40t 



30 



.20 



10 • 



O I 23456789 10 II 12 :l 23456789 10 II 12 



? 1925 !926 



°- MONTHS 



Fig. 45. The seasonal cycle of dissolved nitrates and phosphates in the English Channel. 



drawn from Harvey.) 



100 1- 

 90^ 



70 : 



•50 



UJ 



23456789 10 II 12 :l 23456789 10 II 12 



1925 1926 



40 9 



m 

 30 3 



20 £ 



Q. 



10 



ol 



80 J 



70 2 



Z 

 60 ~^ 



z 

 50 I 



UJ 



40 H 



cr 



aoH 



z 



20,n 



I- 



( Re- 



phosphorus to contribute 1 mg./M^ daily to 

 an area of 1000 square kilometers near its 

 mouth and to a depth of 50 meters. Growth 

 of phytoplankton is measurably aflFected in 

 a relatively narrow coastal zone for at least 

 four geographic degrees along the neigh- 

 boring Gulf coast. Although it is not certain 

 that phosphorus here acts as a hmiting fac- 

 tor under usual conditions, the zone of 

 greater density of phytoplankton approxi- 

 mates the area of increased phosphate con- 

 tent of the water produced by the inflow 



other things being equal, to allow the pro- 

 duction of a large population of diatoms. 

 The ratio of nitrate-nitrogen to phosphate- 

 phosphorus present in the sea roughly ap- 

 proaches a constant value of about 15:1 in 

 milhgram-atoms and is approximately 7:1 

 in weight (Cooper, 1938; Sverdrup, John- 

 son, and Fleming, 1942). Many variations 

 occur from any given mean value, but still 

 the idea of a rough ratio of about this order 

 of magnitude helps fix in mind their rela- 

 tive abundance. The proportion of these 



