Limitation by Nutrients in Nature 287 



act as limiting factors for the growth of aquatic plants. The availa- 

 bility of nutrients in fresh water and in the sea is complicated by the 

 following facts: (1) the supply and the availability of these materials 

 are involved with circulation and various chemical equilibria in the 

 water, (2) the amounts of the nutrients needed by the plants vary 

 according to their physiological condition, and (3) the growth of the 

 plants significantly depletes the supply. For a more detailed discus- 

 sion of these complicated interrelations than is possible here the reader 

 should refer to such treatments as those of Ruttner ( 1953 ) or Sverdrup 

 et al. (1942, Ch. 16). 



As would be expected, the amounts of nutrient salts in fresh water 

 vary widely from lake to lake and also may change greatly within 

 the same body of water from season to season, as rates of plant growth 

 and organic decomposition wax and wane. Differences in availabil- 

 ity of plant nutrients in the water is the principal criterion upon which 

 European limnologists have divided lakes into three major types: 

 oligotrophic, eutrophic, and dystrophic. Although difficulty exists in 

 applying this classification universally (Welch, 1952), the following 

 general distinctions can be made. Oligotrophic lakes, typically very 

 deep, are poor in phosphorus, nitrogen, and calcium; electrolytes and 

 organic materials are low, but oxygen is abundant at all depths and 

 in all seasons. Eutrophic lakes are relatively shallow, and are typi- 

 cally rich in plant nutrients and in organic materials; electrolytes are 

 variable in abundance; oxygen is depleted seasonally and may be en- 

 tirely absent in the hypohmnion. Dystrophic lakes, occurring prin- 

 cipally in bog surroundings or old mountains, are abundantly supplied 

 with phosphorus, nitrogen, calcium, and organic materials, but the 

 growth of most lake organisms is limited by the occurrence of high 

 concentrations of humic substances. Electrolytes are low, and 

 oxygen is almost or entirely absent in deeper water. 



In view of the stress laid upon the constancy of the ocean as an 

 environment in many respects including its saHnity, it might be sup- 

 posed that all nutrient salts in sea water are uniformly abundant. 

 Such is far from the truth, however. The salinity of the ocean is de- 

 termined almost entirely by the abundance of the salts listed in Table 

 3, and these are measured in parts per thousand. Phosphate and 

 nitrate are not found in this table at all— they occur in quantities of 

 the order of parts per billion. The paucity of these nutrients is known 

 to be frequently responsible for the curtailment of plant growth in 

 the sea. 



Other materials occurring in small or trace quantities may act as 

 limiting factors in the sea. Silicate has been reported upon occa- 



