Nitrogen cycling across the sediment-water interface in an eutrophic,artificially oxygenated lake |
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| Authors: | P Höhener R Gächter |
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| Institution: | (1) Limnological Research Center, Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-6047 Kastanienbaum, Switzerland;(2) Present address: Institute of Terrestrial Ecology, Soil Biology, Swiss Federal Institute of Technology (ETHZ), Grabenstr 3, CH-8952 Schlieren, Switzerland |
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| Abstract: | Processes controlling the nitrogen (N) exchange between water and sediment in eutrophic Lake Sempach were studied using three different independent methods: benthic flux chambers, interstitial water data and hypolimnetic mass balances. The sediments released NH
4
+
(1.1–16.1 mmoles m–2 d–1), NO
2
-
(0.1–0.4 mmoles m–2 d–1) and dissolved organic N (<0.25 mmoles m–2 d–1). A net NO
3
-
consumption (2.4–11.1 mmoles m–2 d–1) related to the NO
3
-
concentrations in the overlying water was observed in all benthic chamber experiments. The flux of the reactive species NO
3
-
and NH
4
+
was found to depend on hydrodynamic conditions in the water overlying the sediment. For this reason, benthic chambers overestimated the fluxes of inorganic N compared to the other methods. Thus, in short-term flux chamber experiments the sediment may either become a sink or a source for inorganic N depending on the O2 concentration in the water overlying the sediment and the stirring rate. As demonstrated with a15NO
3
-
experiment, nitrate-ammonification accounted for less than 12% of the total NO
3
-
consumption. After six years of artificial oxygenation in Lake Sempach, a decrease in hypolimnetic total inorganic nitrogen (TIN) was observed in the last two years. The occurrence of dense mats of H2S-oxidizingBeggiatoa sp. indicated micro-aerobic conditions at the sediment surface. Under these conditions, a shorter distance between the ecological niches of nitrifying and denitrifying bacteria, and therefore a faster NO
3
-
-transport, can possibly explain the lowering of TIN by enhanced net denitrification. |
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| Keywords: | Nitrogen cycling sediment-water interface sediment fluxes denitrification nitrification diffusive boundary layer Beggiatoa artificial oxygenation of lakes Lake Sempach |
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