Nitrogen-15 Isotope Enrichment in Benthic Boundary Layer Gases of a Stratified Eutrophic Iron and Manganese Rich Lake |
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| Authors: | L K Pearson C H Hendy D P Hamilton W B Silvester |
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| Institution: | (1) Chemistry Department, University of Waikato, Private Bag 3105, Hamilton, 3240, New Zealand;(2) Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton, 3240, New Zealand |
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| Abstract: | The applicability of the natural abundance of nitrogen gas isotope ratios was used to indicate the spatial distribution of
nitrogen transformations in the water column and sediment pore waters of Lake Ngapouri, a small (area 0.19 km2), monomictic, eutrophic lake in the Taupo Volcanic Zone, North Island, New Zealand. Samples were collected from the epilimnion,
hypolimnion, benthic boundary layer and at 5-cm intervals from the sediment pore waters at monthly intervals for 1 year. Values
of δ15N N2] ranged from −1 to 0.28‰ in the epilimnion, −1.5 to 1.25‰ in the hypolimnion, −1.8 to 12.2‰ in the benthic boundary layer
and −0.7 to 3.5‰ in sediment pore waters. Values of δ15N N2] showed a strong seasonal pattern that was related to the loss of dissolved oxygen in the hypolimnion during seasonal stratification.
Increases in 15N-enriched dinitrogen take place in the benthic boundary layer during the periods of anoxia (taken to be dissolved oxygen
concentrations <6.3 μM) and may be related to abundant ammonium substrate (up to 275 μM) to support denitrification. Nitrate
concentrations increased up to 36 μM with increasing duration of anoxia. We hypothesise that an alternative electron acceptor
besides oxygen is required to support the nitrification needed for the production of nitrate. Iron and manganese hydroxides
and oxides from material sedimenting out of the water column may have induced chemo-nitrification sufficient to oxidise ammonium
in the anoxic benthic boundary layer. The nitrate formed would mostly be rapidly denitrified so that the δ15N N2] would continue to become enriched during the presence of anoxia, as observed in hypolimnion and benthic boundary layer of
Lake Ngapouri. The changes in δ15N N2] values indicate the potential use of isotope ratios to identify and quantify potential chemo-nitrification/denitrification
in the water column and sediment pore waters of lakes. |
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