SECTION III 



INTRODUCTION 



Aquatic plants, both algae and angiosperms (macrophytes) , 

 are responsible for nuisance conditions when excessive 

 growths of the organisms develop in polluted lakes and 

 streams. One approach to the control of these undesir- 

 able growths is to reduce supplies of an essential 

 nutrient element to growth-limiting levels. 



Considerable evidence indicates that nitrogen and phos- 

 phorus are most likely to become limiting for plant 

 growth in aquatic environments, and most investigators 

 favor phosphorus as the primary limiting nutrient. 

 However, evidence obtained by Gerloff and Skoog (1957) 

 suggested that nitrogen rather than phosphorus limited 

 growth of the blue-green alga Microcystis aeruginosa in 

 several southern Wisconsin lakes. In a recent report, 

 (Ryther and Dunstan, 1971) nitrogen was considered to 

 become limiting for algae in coastal marine waters of 

 northeastern United States. Data obtained by Goldman (1960, 

 1964) indicated that in some lakes trace element supplies 

 limit aquatic plant growth. 



Reliable assays of nutrient supplies in lakes and streams 

 relative to plant needs would be highly useful to engineers, 

 water chemists, biologists, and others who must predict 

 and assess the effectiveness of suggested pollution control 

 measures. Various procedures have been proposed and used 

 for nutrient assay, for example chemical analyses of water 

 samples and enrichment bioassays involving growth or 

 photosynthesis of aquatic organisms following additions 

 to water samples of nutrient elements that might be 

 growth limiting (Gerloff, 1969). There are problems 

 associated with both approaches and neither has been 

 developed to a degree that has led to acceptance as a 

 general diagnostic procedure. For example, it is difficult 

 to interpret water analyses in terms of concentrations at 

 which specific elements become growth limiting (Lee, 1969; 

 Lund, 1969). Enrichment assays, in addition, are subject 

 to errors associated with extending data obtained with 

 isolated samples to evaluations of nutrient supplies in 

 large bodies of water (Gerloff, 1969). As an alternative, 

 a bioassay based on plant rather than water samples has 

 been developed (Fitzgerald, 1969). The amount of ortho- 

 phosphate extracted from plant samples with boiling water 

 and the uptake of NH^-N in the dark correlate with the 



