SEA-WATER AS A FOOD SOLUTION 101 



long since have been poisoned by excess of nitrates derived 

 from the land. For the details of the investigations into the 

 activities of these bacteria the original papers should be 

 consulted. 



Determinations of the ammonia, nitrites, and nitrates in 

 surface water have been made. The Baltic and North Seas 

 analyses in 1904 gave on the average, for the Baltic, nitrogen 

 (as ammonia) O'o6i plus (as nitrites and nitrates) 0*134 = 0*195 

 milligram per litre ; for the North Sea (as ammonia) 0*058 plus 

 (as nitrites and nitrates) 0*152 =0*210 milligram per litre. For 

 warmer seas investigations have been made by Natterer, who 

 failed to find nitrates either in the surface or deeper waters of 

 the Mediterranean and Red Seas. Nitrites were found in quite 

 small quantities in the deeper layers, on the surface in traces 

 only (0*008 to O'ou milligram per litre). In the form of 

 ammonia, nitrogen was present on the average in 0*060 milli- 

 gram per litre, so that warm seas apparently contain only one- 

 third of the amount of nitrogen found in cooler waters of 

 Northern Europe. The warmer seas are therefore believed to 

 have nitrogen present in minimum amount, and consequently 

 they are poor in plankton. 



The " law of the minimum " above referred to applies to 

 other elements besides nitrogen, and it is quite possible that 

 some other essential food substance for plants, such as 

 phosphorus, is really that which regulates the amount of 

 plankton. Older determinations of phosphoric acid in sea- 

 water are unreliable, since they were not made on recently- 

 collected and properly-filtered samples. The death and decay 

 of planktonic organisms produces calcium phosphate, so that 

 unless the water is promptly filtered and analysed phosphorus 

 appears in excess. 



Here again the International investigations afford fresh 

 results for consideration. Baltic and North Sea water is found 

 to contain more phosphoric acid than nitrogen, but, even so, 

 less than i milligram per litre. There is a yearly fluctuation 

 in February and May from 0*14 to 0*25 milligram phosphoric 



