TISSUE ANALYSIS AS A MEASURE OF NUTRIENT AVAILABILITY FOR THE GROWTH OF ANGIOSPERM AQUATIC PLANTS 1

A tissue analysis technique was used to evaluate nitrogen and phosphorus supplies in natural waters for the growth of angiosperm aquatic plants. Tissue content of nitrogen and phosphorus was employed as an index of element availability in lakes from which plants were collected. This required establishment in the laboratory of the critical level for each element, that is, the minimum tissue content associated with maximum growth. To establish critical levels, a system was developed for culturing algae-free plants in a synthetic nutrient medium. The critical nitrogen content for the several species studied was approximately 1.3%; the comparable phosphorus value was 0.13%. The nitrogen and phosphorus contents of 13 species of aquatic plants collected during the summer from nine lakes were compared with the critical concentrations. In nine samples obtained mostly during periods of heaviest plant growth, the phosphorus content was at or below the critical level; in no sample was the nitrogen content less than the critical concentration. The results indicated that in all but one of the lakes, phosphorus supply was more likely to limit higher aquatic plant growth than was nitrogen. The primary importance of this study is as an initial step in the development of a satisfactory technique for evaluating nutrient supplies in natural waters.