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Close coupling of N‐cycling processes expressed in stable isotope data at the redoxcline of the Baltic Sea
Author(s) -
Frey Claudia,
Dippner Joachim W.,
Voss Maren
Publication year - 2014
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1002/2013gb004642
Subject(s) - denitrification , anoxic waters , eutrophication , nitrification , isotopes of nitrogen , water column , stable isotope ratio , nitrate , environmental chemistry , nitrogen cycle , nitrogen , oceanography , chemistry , environmental science , geology , nutrient , physics , organic chemistry , quantum mechanics
Over the past decades, the hypoxic state of the central Baltic Sea has deteriorated because of eutrophication, but little is known about the extent to which related factors such as nitrogen removal have been altered. The Baltic Sea is a stratified semi‐enclosed basin with a large, anoxic bottom water mass in its central Gotland Basin and highly active microbial nitrogen transformation processes at the redoxcline, the interface between oxic and anoxic waters. In this study, we identified and quantified the dominant transformation processes of reactive nitrogen by exploiting fine resolution profiles of δ 15 N NO3 , δ 18 O NO3 , and δ 15 N NH4 through the pelagic redoxcline between 60 and 140 m. Our results showed increasing δ 15 N NO3 and δ 18 O NO3 values with decreasing nitrate concentrations, but the associated low apparent isotope effect ( ε = ~5‰), as inferred from a closed system Rayleigh model, was not consistent with the high ε (~25‰) characteristic of denitrification in the water column. These findings could be explained by substrate limitation. The observed δ 18 O NO3 : δ 15 N NO3 ratio of 1.38:1 rather than the usual 1:1 ratio typical for denitrification‐dominated systems could be explained by the occurrence of both nitrification and denitrification. We then developed a numeric reaction‐diffusion model, according to which a realistic denitrification rate of 14 nmol N L −1 d −1 was estimated, and a nitrification rate of 6.6 nmol N L −1 d −1 confirmed. Our study not only demonstrates the value of stable isotope data for investigating nitrogen transformation processes but also highlights that care is needed in interpreting systems with closely coupled processes such as those at ocean redoxclines.