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

• Published in: International Review of Hydrobiology
• Main Author: Gu, Binhe
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2012
• Subjects: Smith Lake; Subarctic; Alaska
• Online Access: http://dx.doi.org/10.1002/iroh.201111466; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Firoh.201111466; https://onlinelibrary.wiley.com/doi/pdf/10.1002/iroh.201111466

Abstract Nitrogen stable isotopes (δ 15 N) of dissolved inorganic nitrogen (DIN = NH 4 + and NO 3 – ), 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 δ 15 NH 4 + , δ 15 NO 3 – and δ 15 N PON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, 15 N depletion in all three pools was closely related to the occurrences of a N 2 ‐fixing cyanobacterial bloom ( Anabaena flos‐aquae ). In summer, δ 15 N PON increased as phytoplankton community shifted to use NH 4 + and decreased as a brief N 2 ‐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 NO 3 – and NH 4 + in the annual cycle. This was followed by denitrification that led to the highest 15 N enrichment in NO 3 – . A peak of NH 4 + assimilation by phytoplankton along with the elevated δ 15 N PON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ 15 N DON 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 δ 15 N PON , and a negative correlation between NH 4 + and δ 15 N PON , suggesting that δ 15 N PON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)