Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

Abstract

We evaluated the seasonal variation of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the sediments and the integrative N (and O) isotopic signatures of dissolved inorganic nitrogen (DIN) compounds in the overlying water column of the monomictic Lake Lugano south basin. Denitrification was the dominant NO−3 reduction pathway, whereas the contribution of anammox and DNRA to total benthic NO−3 reduction was < 6% and < 12%, respectively. Sedimentary denitrification rates were highest (up to 57.2 ± 16.8 µmol N m−2 h−1) during fully oxic bottom water conditions. With the formation of seasonal bottom water anoxia, NO−3 reduction was partitioned between water column and sedimentary processes. Total benthic NO−3 reduction rates determined in 15N‐label experiments and sediment—water interface N2 fluxes as calculated from water column N2 : Ar gradients revealed that sedimentary denitrification still accounted for ∼ 40% of total N2 production during bottom water anoxia. The partitioning between water column and sedimentary denitrification was further evaluated by the natural abundance stable N isotope composition of dissolved NO−3 in the water column. With anaerobic bottom water conditions, water column NO−3 concentrations gradually decreased, paralleled by an increase in δ15N— and δ18O— NO−3 from approximately 7‰ to 20‰ and from 2‰ to 14‰, respectively. Using a closed‐system (Rayleigh) model, the N and O isotope effects associated with community NO−3 consumption were 15ε ≈ 13.7‰ and 18ε ≈ 11.3‰, respectively. With the assumptions of a relatively low net N isotope effect associated with sedimentary denitrification (i.e., 15εsed = 1.5–3‰) vs. a fully expressed biological N isotope fractionation during water column denitrification (i.e., 15εwater = 20–25‰), our results confirm that 36–51% of reduction occurred within the sediment. The general agreement between the indirect (isotopic) approach and the flux and rate measurements suggests that water column nitrate isotope measurements can be used to distinguish between benthic and pelagic denitrification quantitatively.