Stable isotopes as indicators for seasonally dominant nitrogen cycling processes in a subarctic lake

Abstract

AbstractNitrogen stable isotopes (δ15N) of dissolved inorganic nitrogen (DIN = NH4+ and NO3–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were measured in Smith Lake, Alaska to assess their usefulness as proxies for the biological nitrogen cycling processes, nutrient concentration, and lake productivity. Large seasonal variations in δ15NH4+, δ15NO3– and δ15NPON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, 15N depletion in all three pools was closely related to the occurrences of a N2‐fixing cyanobacterial bloom (Anabaena flos‐aquae). In summer, δ15NPON increased as phytoplankton community shifted to use NH4+ and decreased as a brief N2‐fixing bloom (Aphanizomenon flos‐aquae) occurred in August. In early and mid‐winter, microbial nitrogen processes were dominated by nitrification that resulted in the largest isotope fractionation between NO3– and NH4+ in the annual cycle. This was followed by denitrification that led to the highest 15N enrichment in NO3–. A peak of NH4+ assimilation by phytoplankton along with the elevated δ15NPON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ15NDON displayed little temporal and spatial variations. This suggests that the DON pool was not altered by biological transformations of nitrogen as the results of its large size and possibly refractory nature. There was a positive correlation between Chl a concentration and δ15NPON, and a negative correlation between NH4+ and δ15NPON, suggesting that δ15NPON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)