ESTIMATION OF N-2 FIXATION BASED ON DIFFERENCES IN THE NATURAL-ABUNDANCE OF N-15 AMONG FRESH-WATER N-2-FIXING AND NON-N-2-FIXING ALGAE

The dynamics of nitrogen supply was investigated for blue-green and gr een algae from Smith Lake and other freshwaters of subarctic and arcti c Alaska. The natural abundance of N-15 (defined as delta(15)N) of six N-2- fixing blue-green algae was 1.0+/-1.3% ((X) over bar+/-SE), indi cating supply of metabolic nitrogen from atmospheric N-2 (delta(15)N = 0.0%). The delta(15)N of six green algae showed an average of 6.6+/-4 .5%, which is significantly higher than delta(15)N of N-2 fixing blue- green algae from the same waters, reflecting the utilization of dissol ved inorganic nitrogen (DIN). Nitrogen-fixing algae also showed higher nitrogen content (7.l+/-2.1%) than non-N-2-fixing algae (2.9+/-1.5%). The delta(15)N of a bloom-forming species, Anabaena flos-aquae (Lyngb .) Breb. in Smith Lake showed no significant inter-annual variations d uring a three-year study period. The changes in delta(15)N during each bloom were probably due to variations in the N-15 composition of DIN and in the proportional uptake of DIN and Nz fixation. An estimation o f the fractional contribution of atmosphere-derived nitrogen (ADN) fro m delta(15)N indicated that A. flos-aquae obtained 58-75% of its nitro gen by N-2 fixation. This technique agreed with the result obtained us ing a N-15(2) enrichment method. The delta(15)N of the presumed N-2-fi xing terrestrial plant was similar to that of the atmospheric N-2, whe reas the delta(15)N of the presumed non-N-2-fixing terrestrial plants reflected their nitrogen sources.