Estimation of N fixation based on differences in the natural abundance of N among freshwater N-fixing and non-N-fixing algae.

The dynamics of nitrogen supply was investigated for blue-green and green algae from Smith Lake and other freshwaters of subarctic and arctic Alska. The natural abundance of N (defined as δN) of six N-fixing blue-green algae was 1.0±%(X±SE), indicating supply of metabolic nitrogen from atmospheric N (δN=0.0). The δN of six green algae showed an average of 6.6±4.5%, which is significantly higher than δN of N-fixing blue-green algae from the same waters, reflecting the utilization of dissolved inorganic nitrogen (DIN). Nitrogen-fixing algae also showed higher nitrogen content (7.1±2.1%) than non-N-fixing algae (2.9±1.5%). The δN of a bloom-forming species, Anabaena flos-aquae (Lyngb.) Breb. in Smith Lake showed no significant interannual variations during a three-year study period. The changes in δN during each bloom were probably due to variations in the N composition of DIN and in the proportional uptake of DIN and N fixation. An estimation of the fractional contribution of atmosphere-derived nitrogen (ADN) from δN indicated that A. flos-aquae obtained 58-75% of its nitrogen by N fixation. This technique agreed with the result obtained using a N enrichment method. The δN of the presumed N-fixing terrestrial plant was similar to that of the atmospheric N, whereas the δN of the presumed non-N-fixing terrestrial plants reflected their nitrogen sources.