"July 1973." "Project officer, Dr. Charles F. Powers, National Eutrophication Research Laboratory, Pacific Northwest Environmental Research Laboratory." U.S. Environmental Protection Agency Research Grant No. Contract Number: 16010-DCK. Includes bibliographical references (pages 141-157).

1. Conclusions -- 2. Recommendations -- 3. Introduction -- 4. Review of the nitrogen cycle in natural waters -- 5. Transport of nitrogen into lakes -- 6. Nitrogen fixation as an in situ nitrogen source for natural waters. I, Algal fixation in lakes -- 7. Nitrogen fixation as an in situ nitrogen source for natural waters. II, Bacterial fixation in lakes and sediments -- 8. Other in situ nitrogen sources and sinks -- 9. Analytical investigations -- 10. Acknowledgements -- 11. References -- 12. Publications and patents -- 13. Appendix. Sources of nitrogen were reviewed to determine their significance in lacustrine budgets. Nutrients in rainfall were found significant although their variability obviates precise conclusions. Using literature values for nutrient export from various land uses, nutrient budgets were calculated for 55 Florida lakes. Critical N and P loading rates (above which eutrophication is likely) were estimated from the calculated budgets and lake trophic conditions. Algal fixation in two eutrophic Florida lakes was studied in detail; the total amount of N fixed and factors affecting the occurrence of fixation were evaluated. A survey of fixation in 55 Florida lakes showed significant fixation only in eutrophic lakes. Bacterial fixation in the anoxic hypolimnion of a small lake contributed substantial nitrogen to the lake, and N fixing activity was found in both estuarine and lacustrine sediments. The acetylene reduction assay for N fixation was evaluated; short incubations were found essential. Reduction was light-dependent and N2 acted as a competitive inhibitor. A preliminary experiment suggested that lacustrine sediments act as ammonia buffers; estuarine sediment sorbed ammonia strongly with little tendency to release ammonia to the water. Interferences from high organic color were evaluated for automated inorganic N and P analytical methods. Various amino acids were also shown to interfere with the indophenol ammonia procedure.