presently sparse weed beds in Salsich Lake. 



Because of inadequate plant tissue, it was impossible to 

 run molybdenum analyses on all samples from the 9 lakes. 

 The range of molybdenum in 7 samples analyzed was 0.50 to 

 0.73 ppm, in the terminal one-inch, with an average of 

 0.57 ppm. Even the lowest concentration was 3 to 4x the 

 critical concentration of 0.15 ppm presented in Table 6, 

 thus eliminating the need to consider molybdenum as a 

 possible growth-limiting factor. 



The copper analyses reported in Tables 8 and 9 are of 

 interest because some are sufficiently low to suggest Cu 

 deficiency. This cannot be verified until the copper 

 critical concentration is firmly established in laboratory 

 experiments. However, comparisons with critical concentra- 

 tions determined for agricultural and horticultural 

 terrestrial species support the indicated copper deficiency. 

 For example, a critical copper concentration of 5 ppm 

 has been reported for corn, alfalfa, and soybeans and 7 ppm 

 for alfalfa (Melstead, Motto, and Peck, 1969) . Less than 

 4 ppm Cu in the leaves was associated with copper deficiency 

 in citrus (Chapman, 1961) . Several of the copper con- 

 centrations in Elodea from Salsich and all of the values 

 from Little John were less than 4 ppm. For a number of 

 samples the copper data reported in Tables 8 and 9 were 

 checked by atomic absorption analysis. The atomic absorp- 

 tion values were consistently higher. However, the copper 

 contents obtained by atomic absorption still were below 

 the 4-7 ppm critical concentrations for economic crops and 

 confirm that either Elodea was copper deficient in Little 

 John Lake, or that it has an unusually low requirement 

 for that element. 



Although the critical copper concentration has not as 

 yet been determined, it has been possible to grow Elodea 

 severly deficient in copper. Yields from cultures to 

 which copper was not added were only 25 percent (0.909 g) 

 of yields (3.636 g) with copper added. The copper 

 concentration in the first two inches of the copper 

 deficient plants was 1.57 ppm; in normal plants, it was 

 17.7 ppm. From experience with the other trace element 

 cations, it can be anticipated that the critical copper 

 concentration will be at least double the 1.57 ppm in the 

 deficient plants. This supports the view that copper 

 supplies did become growth-limiting in some of the lakes 

 sampled. 



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