202 



ANALYSIS OF THE ENVIRONMENT 



two elements in marine plankton approxi- 

 mates the same ratio.* 



The negative correlation that was noted 

 for the amount of nitrates and the quantity 

 of dissolved oxygen in sea water becomes 

 more pronounced when phosphates and 



PO4 P|, mg/M' 

 31 62 9,3 



124 



1000 



:ooo 



3000- 



Fig, 46. Depth profiles of phosphate-phos- 

 phorus. ( Redrawn from Sverdrup, Johnson, and 

 Fleming. ) 



nitrates together are compared with oxygen 

 content. The seasonal distribution of phos- 

 phates resembles that of nitrates (Fig. 45), 

 except that the "regeneration" of phosphates 

 in surface waters takes less time; hence 

 the autumnal upswing starts earher than 

 does that of nitrates in the same waters. 

 This may mean simply that the release of 

 dissolved phosphates from dead organisms 

 is a relatively simple process, whereas the 

 similar nitrate release represents an end 

 stage in a longer series of changes (Harvey, 

 1928). 



" The nitrogen: phosphorus ratio in fresh 

 water is not necessarily the same as that in the 

 sea (Hutchinson, 1941a). 



There exist for restricted locations quan 

 titative data concerning cycles of abundance 

 of carbonates and sihcates as well as phos- 

 phates and nitrogen compounds, both as 

 mineral nutrients and at the organic level. 

 Often the peaks and depressions are corre- 

 lated with utilization of the given substance 

 by organisms or with their release after 

 death and decay, and these changes tend to 

 be associated with the local annual climatic 

 cycle. 



Although the amounts of nitrates and 

 phosphates, taken together, seem to be the 

 principal Umiting mineral nutrients in the 

 sea, at times the growth of populations of 

 unicellular algal cells, like diatoms, are at 

 a minimum when these two mineral con- 

 stituents are relatively plentiful, and there 

 must be other hmiting factors in addition to 

 these two important ones. 



LIMNOLOGICAL ASPECTS 



In fresh waters both nitrogen and phos- 

 phorus occur in small amounts and are sub- 

 ject to marked changes seasonally and ver- 

 tically. Phosphorus averages 0.05 mg./L. 

 or less; inorganic nitrates are usually less 

 than 0.5 mg./L., and nitrites less than 

 0.1 mg./L. Ammonia is equally scarce. 

 There is usually a seasonal maximum to- 

 ward the end of winter stagnation in ice- 

 covered ponds and lakes, followed by redis- 

 tribution in the vernal overturn that results 

 in uniformity. With the formation of the 

 thermocline and summer stagnation, uni- 

 form distribution is again lost, since the 

 flowering out of epilimnial phytoplankton 

 makes demands on these salts in the surface 

 waters. There is a steady drizzle of dead, 

 decomposing organisms down into the 

 hypolimnion, with a resulting increase of 

 the raw materials for protein synthesis in 

 these deeper waters. By the middle of sum- 

 mer stagnation, since the thermocline tends 

 to retard the free distribution of nitrogen 

 and phosphorus, these latter may be several 

 times more abundant in the hvpolimnion 

 than at the surface. In fact, diminishing 

 amounts of these elements may act as limit- 

 ing factors and retard diatom production, 

 although light intensity, temperature, and 

 other conditions are favorable. The autum- 

 nal overturn after the thermocline disap- 

 pears redistributes these vital elements, par- 

 ticularly by currents. During winter stagna- 

 tion, with low light intensity, photosynthe- 

 sis is notably reduced, and the water is be- 



