rinsed in distilled water, placed in nylon bags, and dried 

 to constant weight at 65-70°C. 



Total nitrogen analyses on all field samples were by a 

 semimicro Kjeldahl procedure. In addition, all samples 

 were analyzed for 10 elements by the Jarrell-Ash Multichannel 

 Emission Spectrometer at the Wisconsin Alumni Research 

 Foundation Laboratories. In all lakes in which there were 

 indications from the emission spectrometer analyses that 

 a specific element had become limiting, or was close to 

 limiting, for plant growth, analyses were verified by the 

 quantitative procedures used in establishing critical 

 concentrations in the laboratory. This included analyses 

 for calcium, phosphorus, and copper. 



Samples analyzed by the Jarrell-Ash Spectrometer were 

 ground in a Wiley Mill equipped with a stainless steel 

 screen. Samples analyzed by conventional quantitative 

 procedures were ground with an agate mortar and pestle to 

 minimize trace element contamination. 



Some additional samples were obtained from natural macro- 

 phyte populations in Wisconsin lakes during the summer of 

 1971. However, emphasis was shifted to another use of 

 the plant analysis technique. The Elodea bioassay 

 organism was placed in porous, inert containers near the 

 surface in lakes in which nutrient supplies were to be 

 evaluated. Samples of the first and second one-inch index 

 segments were to be removed periodically, analyzed for 

 elements suspected to limit growth, and evaluated by 

 comparisons with the critical concentrations. The 

 anticipated advantage of this procedure was that the 

 assays would primarily reflect nutrient availability in 

 the water layer and would be relatively uninfluenced by 

 the direct abosrption of nutrients from sediments. 



Cylindrical baskets 8-1/2 inches in diameter and 12 

 inches long were constructed of 1/4 inch mesh polyethylene 

 screen. The cylinders were closed at the ends with the 

 polyethylene mesh and were reinforced with plastic rings 

 1/4 inch wide. After the baskets were anchored in 

 relatively sheltered areas of lakes in the vicinity of 

 macrophyte beds where the water was no more than 5 feet 

 deep, an inoculum of laboratory-grown Elodea or 

 Ceratophyllum was placed in each. The baskets were 

 floated near the water surface by two empty, sealed 

 one-gallon polyethylene bottles. One basket of each 

 of the two macrophytes was placed in 5 northern 

 Wisconsin lakes. 



26 



