352 



[chap. 17 



water, exclusive of local polluted areas, contains about 60 [xg atoms/1, or 

 0.00005% nitrogen, four orders of magnitude less than fertile land. A cubic 

 meter of this sea-water could support a crop of no more than about 5 g of dry 

 organic matter. 



As discussed above, the maximum depth of the euphotic zone is about 100 m. 

 This surface layer is normally poor in nutrients relative to the deeper waters 

 and seldom contains more than 10-20 fjtg atoms/1, of available nitrogen (e.g. 

 Riley, 1951). A 100-m euphotic zone containing nitrogen at a concentration of, 

 say, 15 [xg atoms/1, represents a reservoir of 21 g of nitrogen in a 1-m 2 column. 

 However, as phytoplankton grows in this water column, the organisms them- 

 selves absorb more light, the euphotic zone becomes progressively shortened, 

 and the reservoir of nutrients available to the plants is correspondingly dimin- 

 ished in size. As the plant population grows, the organisms not only consume 

 nutrients but also shut themselves off from their supply. Table I illustrates this 



Table I 



Relationships between Chlorophyll, Standing Crop of Organic Matter, Trans- 

 parency, Organic Production and Nitrogen Availability and Requirement 

 under Conditions Stipulated in Text 



by showing the relationship between the standing crop of phytoplankton, the 

 rate of organic production, water transparency and the daily requirement and 

 availability of nitrogen in a hypothetical situation where the water initially, 

 with no phytoplankton present, contained 15 [jLg atoms/1, of nitrogen. The table 

 is based upon the relationship between chlorophyll and transparency proposed 

 by Riley and discussed above. It also assumes that dry phytoplankton contains 

 1% chlorophyll and 10% nitrogen. Production is calculated from chlorophyll 



