The purposes of this study were (1) to determine the feasibility of using isolated columns of water as an efficient way to obtain a natural series of similar bodies of water that are subject to identical climatic and environmental conditions, and (2) to determine the effect of fertilization and the fertilization rate that will give optimum and/or maximum phytoplankton production. Twelve open-ended transparent polyethylene tubes 4' in diameter and 9' in length, were used to isolate vertical "transects" of water in the study pond. Measurements of oxygen, pH, water temperature, and turbidity were made to determine any physical or chemical changes which may have been attributable to the tubes. The affect of fertilization was studied by applying three rates of single analysis of commercial fertilizer to the test tubes. The three experimental rates chosen were equivalent to 50, 100, and 150 pounds of 20-20-5 analysis fertilizer per surface acre of water. The C14 method of Goldman and Wetzel (1963) was used to determine the rate of carbon uptake by the phytoplankton. The data revealed that the physical and chemical characteristics of the water within the tubes did not vary appreciably among the tubes or between the tubes and the open lake water. These uniform conditions prevailed within the test tubes for sufficient time to allow comparison of fertilized tests. Of the three seasonal fertilization tests made, the response to the added nutrients was greatest in the spring. The highest carbon assimilation rate (300.1 mg C/m3 per hour) occurred one week after fertilization in water fertilized at the 100 pounds per surface acre equivalent. Generally, the data indicated that the greatest production occurred in the water of the test lake when fertilized in at the rate of 100 pounds per surface acre. Higher and lower rates of fertilization gave lower carbon assimilation values.
