Sources of nitrogen were reviewed to determine their significance in lake nitrogen budgets. Nutrients in rainfall were evaluated and found to be significant. Nitrogen and phosphorus budgets were calculated for 55 Florida lakes and critical loading rates established by comparing calculated budgets with data on trophic state. Nitrogen fixation by Cyanophyceae was studied in detail in two eutrophic Florida lakes for one year. Also a survey of fixation in Florida lakes was conducted and fixation found only in eutrophic lakes. Bacterial fixation was found to contribute significant nitrogen to the anoxic hypolimnon of a small stratified lake. Nitrogen fixation was found in both lacustrine and estuarine sediments. Sediments of 55 lakes were characterized chemically and results suggest that such sediment may act as an ammonia buffer, sorbing ammonia at high concentrations and releasing it to ammonia depleted water. Estuarine sediment sorbed ammonia strongly but failed to release it to overlying water. The acetylene reduction assay for nitrogen fixation was evaluated. Interferences in automated nutrient determinations due to organic color were studied and simple color correction found for nitrite, nitrate and orthophosphate but not ammonia as determined by the indophenol method. Amino acids also interfered with the ammonia analysis. (Modified author abstract)