the critical concentrations for nitrogen and phosphorus, 

 respectively. Concentrations of all the trace elements 

 also were relatively high. The data suggest it would be 

 necessary to remove considerable quantities of nitrogen, 

 phosphorus, or any other essential nutrient, from pollution 

 sources entering Lake Mendota to significantly reduce 

 nuisance macrophyte growths. 



Samples of 1971 



In Table 12, data are presented from the analyses of 

 Elodea and Ceratophyllum which had been in the assay 

 baskets for approximately two months, from late June until 

 early September. Samples also were collected in late 

 July from the natural populations in these lakes and 

 analyzed. Unfortunately, both species were not present 

 in all 5 lakes. The concentrations of nitrogen and 

 phosphorus were well above the critical concentrations of 

 1.60% nitrogen and 0.14% phosphorus in Elodea and 1.30% 

 and 0.10% in Ceratophyllum in all samples from the natural 

 populations. There was no indication either nitrogen or 

 phosphorus was growth limiting in early July. 



Comparisons of nitrogen and phosphorus concentrations 

 in natural populations of the two species obtained from 

 the same lake (Clear or Little John) are of interest. 

 Elodea produces roots which presumably are imbedded in 

 bottom sediments while Ceratophyllum has almost no roots. 

 Elodea , therefore, obtains nutrients from both the mud 

 and water; Ceratophyllum must rely on the water. Nitrogen 

 concentrations were much lotfer in Ceratophyllum than in 

 Elodea in both Clear and Little John lakes suggesting 

 the advantage of roots in drawing on bottom sediment 

 nitrogen supplies. There was little difference in the 

 phosphorus concentrations in the two species, indicating 

 an unusual capacity of Ceratophyllum to absorb phosphorus 

 from the very low concentrations in the water. The col- 

 lections of 1970 indicated Allequash to be a fertile lake. 

 This correlates with the lack of a difference in nitrogen 

 concentrations in Elodea and Ceratophyllum from this lake. 



Unfortunately, the Elodea and Ceratophyllum inoculated 

 into the baskets grew very poorly. There was hardly 

 enough tissue for nitrogen and phosphorus analyses by 

 early September when the baskets were removed from the 

 lakes. Whether this was due to the shock of transfer of 

 laboratory-grown plants to the lake environments, to 

 placement of the plants too near the surface, or to other 



34 



