Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

Abstract

We present 42 dual-isotope nitrate analy- ses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured d 15 N-NO3 - and d 18 O-NO3 - values corre- late negatively, while d 18 O-NO3 - displays no nega- tive correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signa- ture in the St. Lawrence River. In addition, d 18 O- NO3 - is negatively correlated with the seasonally variable d 18 O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrifi- cation). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of d 15 N-NO3 - and d 18 O-NO3 - in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regenera- tion of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).