Benthic nitrogen cycle in a meromictic lake: nitrate delivered via sinking algae sustains denitrification under deep, anoxic waters

Temperate meromictic deep lakes are characterized by oxygen (O 2 ) depletion below the oxycline due to respiration and oxidation rates exceeding O 2 diffusion. The effects of meromixis on nutrient biogeochemistry are generally studied in the water column, whereas those produced in the benthic compartment are less explored. We expect marked effects of anoxia on benthic nitrogen (N) cycling as nitrification, the main source of nitrate (NO 3 − ) within sediments supporting denitrification and dissimilatory nitrate reduction to ammonium (DNRA), is suppressed. To test the hypothesis that meromixis depresses N loss and increases benthic internal N recycling and accumulation, we studied N dynamics in intact sediments collected from three zones (i.e., oxic, hypoxic and anoxic) of the 122-m deep Lake Idro, a meromictic Alpine Italian Lake. We measured nitrification, denitrification, and DNRA with 15 N-tracing techniques, together with dissolved inorganic N, reactive phosphorous, sulphate and sulfide and metals (iron and manganese) fluxes and porewater profiles. In Lake Idro nitrification decreased along with the O 2 gradient and was suppressed under anoxic conditions. Denitrification exhibited a progressive decline in sediment with increasing water depth and reduced O₂ availability, with rates of 45.5 ± 1.5, 31.0 ± 2.9, and 5.9 ± 0.9 µmol N m −2 h −1 measured in sediments with oxic, hypoxic and anoxic overlying water, respectively, and DNRA accounted for 14, 39 and 18% of total NO 3 − reduction. The highest share in the hypoxic zone was likely promoted by high organic carbon to nitrate ratio (OC:NO₃⁻). Notably, denitrification and DNRA were not suppressed under anoxic waters. NO 3 − supplied by settling phytoplankton with elevated intracellular NO 3 − concentrations (ICNO3) has the potential to maintain both processes active. We propose that in Lake Idro settling diatoms rich in ICNO3 maintain ecologically relevant rates of microbial nitrate reduction. We also propose that NO 3 − reduction is likely autotrophic, with largely available sulfides as electron acceptors. We suggest that this process could be present in other meromictic lakes sharing similar conditions as Lake Idro.