reasons must be determined in future studies. The low 

 concentrations of both nitrogen and phosphorus in the Elodea 

 suggests this was due to poor physiological condition 

 induced by factors other than nutrient deficiencies. In 

 the Ceratophyllum samples, nitrogen was proportionally 

 closer to the critical concentration (1.30%) than was 

 phosphorus (0.10%). 



The similarity in nitrogen and phosphorus concentrations 

 in the Ceratophyllum from the natural populations and the 

 baskets suggests the absence of roots might make Cerato- 

 phyllum a more suitable assay organism than Elodea for 

 evaluating nutrient supplies for all non-rooted green 

 plants including algae. 



DISCUSSION 



The results obtained suggest plant analysis can be a 

 useful relatively simple procedure for nutrient assay in 

 aquatic environments, and that it is a procedure which 

 offers several advantages. Plant analysis minimizes the 

 difficulties associated with obtaining representative 

 samples of the aquatic environment. In this procedure, 

 plants become the sampling device, and a single analytical 

 value provides an integrated expression of all the factors 

 which affected the availability of an element in the micro- 

 environments to which the plants were exposed during 

 growth. Chemical analyses of water samples must be inter- 

 preted in terms of fractions available and unavailable to 

 plants and concentrations and quantities which become 

 limiting for growth. These problems are reduced in the 

 plant analysis procedure, because plant analysis is based 

 on values which have been correlated with plant growth and 

 yield responses. 



Two aspects of the field data seem worthy of comment. 

 First, although nitrogen and phosphorus are the elements of 

 most interest in practical pollution control, the data 

 indicated that neither element was a general limiting 

 nutrient for Elodea in the lakes sampled. In some lakes, 

 nitrogen or phosphorus was near the critical concentration. 

 In these ecosystems, reduction in available supplies of 

 nitrogen or phosphorus, whichever was limiting, probably, 

 would reduce macrophyte populations; also, further 

 eutrophication with these elements would accentuate 

 nuisance growths. A second point of interest, and 



36 



