SECTION V 



EVALUATION OF PLANT ANALYSIS FOR THE 



ASSAY OF NUTRIENT SUPPLIES IN WISCONSIN LAKES 



After critical concentrations of the essential elements in 

 Elodea occidentalis and Ceratophyllum demursum were 

 established in laboratory experiments, plant analysis was 

 used to evaluate nutrient supplies and growth-limiting 

 nutrients in Wisconsin lakes. This involved collecting 

 samples of the first and second one-inch segments of the 

 Elodea and Ceratophyllum from lakes known to vary consider- 

 ably in fertility. Samples were obtained at intervals 

 throughout the growing season, were analyzed for the 

 essential elements, and the values were compared with the 

 critical concentrations. Concentrations of an element 

 consistently above the critical level were interpreted 

 to indicate the element was in abundant supply and not 

 limiting plant growth; a concentration at or below the 

 critical level indicated supplies of that element had 

 become limiting at the time of sampling. In this case, 

 restriction of entry of the growth-limiting element into 

 the specific body of water could be expected to reduce 

 growths of nuisance macrophytes. Conversely, additions of 

 the element probably would increase macrophyte growth. 



EXPERIMENTAL PROCEDURE 



During the summer of 1970, samples of Elodea occidentalis , 

 the primary assay organism, were routinely collected from 

 9 Wisconsin lakes. All of the lakes sampled except one 

 are located in or near Vilas County in northern Wisconsin 

 in non-agricultural areas. They are relatively infertile. 

 Water hardness varied from 14 to 65; the pH of the water 

 was in the range of 7.1 to 8.0 (Black, Andrews, and 

 Threinen, 1963) . 



Plant samples collected from lakes were lifted from the 

 'water with a garden rake and taken to the laboratory while 

 immersed in lake water. Within several hours, sufficient 

 first and second inch segments to represent 3-4 g of oven- 

 dry material were cut from the main branches and laterals 

 of healthy, green plants. After repeated rinsing in 

 lake water to eliminate debris, the samples were soaked 

 30-45 seconds in . 2N HC1, rinsed with lake water, 



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