Estimation of N2 fixation based on differences in the natural abundance of 15N among freshwater N2-fixing and non-N2-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 15N (defined as δ15N) of six N2-fixing blue-green algae was 1.0±%o(X±SE), indicating supply of metabolic nitrogen from atmospheric N2 (δ15N=0.0). The δ15N of six green algae showed an average of 6.6±4.5%o, which is significantly higher than δ15N of N2-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-N2-fixing algae (2.9±1.5%). The δ15N 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 δ15N during each bloom were probably due to variations in the 15N composition of DIN and in the proportional uptake of DIN and N2 fixation. An estimation of the fractional contribution of atmosphere-derived nitrogen (ADN) from δ15N indicated that A. flos-aquae obtained 58-75% of its nitrogen by N2 fixation. This technique agreed with the result obtained using a 15N2 enrichment method. The δ15N of the presumed N2-fixing terrestrial plant was similar to that of the atmospheric N2, whereas the δ15N of the presumed non-N2-fixing terrestrial plants reflected their nitrogen sources.