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Problems arising from excessive growths of phytoplankton in fresh water lakes have become intensified through the discharge of nutrients in sewage effluents and industrial wastes into these bodies of water. It has been proposed that the amount of growth might be reduced by removal of critical essential elements before the effluents are released. The effectiveness of removing a particular essential element will depend on the extent to which it is normally a limiting or critical factor for algal growth, as obviously removal of a nutrient that is already present in a lake in abundant supply cannot produce the desired restilts. Reliable measurements on the degree to which a specific element is actually limiting the growth of algae in nature are difficult to obtain. They must be based on determinations of the availahility of the element to individual species in relation to their quantitative nutritional requirements. Granting that algae are capable of absorbing elements from water to the limits of detection by chemical procedures, still the simple method of estimating nutrient supplies by analysis of water collected from the area in which the algae are present may be of little value in this connection for the following reasons: (1) the total supply of a nutrient element is dependent on the total volume as well as the concentration of the solution from which it is absorbed, and this effective volume will vary with the extent to which the algae are moved about and come in contact with different volumes of water. This fact becomes particularly important when it is considered that the composition of the water may vary greatly front the top to bottom layers, especially in lakes that stratify, in a particular area and from one area to another, and that the algae collected in one place may be derived from areas scattered widely with respect to both depth and surface location. (2) The concentration of nutrients in the water 1 This work was supported in part by a research grant from the Division of Research Grants and Fellowships of the National Institute of Health, United States Public Health Service, and in part by the University Research Committee on funds from the Wisconsin Alumni Research Foundation. The authors wish to thank Miss Betty Robinson, Technical Assistant, and Mrs. Margaret Hirozawa, Technical Assistant, for their aid in carrying out these experiments may not reflect the total supply in that water over a period of time but rather the level reached as a result of continuous withdrawal by organisms and renewal by inflow and release from less available forms, or, in the case of nitrogen, perhaps also from biological fixation by certain organisms. These difficulties in determining available supplies of nutrients for algal growth are essentially similar to those encountered in determining nutrient availability to crops in terms of soil analyses but in some respects are accentuated by the high degree of independent movement of both the organisms and the water. In recent years, methods of evaluating nutrient availability to plants in terms of tissue composition have been developed (Lundegatrdh, 195 1; Ulrich, 1952; Thomas, 1945; Goodall and Gregory, 1947) and have been extensively employed for crop production. It has been found that, regardless of the composition of the external medium and other factors which may affect the uptake of a certain element, a fairly definite concentration (critical level) of that element must be maintained in the tissue to permit optimum growth. The tissue content of the element may continue to increase far above this critical level when it is in abundant supply, but the excess (luxury consumption) has little effect on growth. This paper presents a comparable procedure for evaluating the supply of nitrogen and phosphorus, the elements generally considered most likely to limit phytoplankton growth, to Microcystis aeruginosa in lake waters and includes preliminary results obtained in its application.