Hidden source of nitrate fuel benthic denitrification in the anoxic monimolimnion of a deep meromictic lake.

Deep meromictic lakes are anoxic below the chemocline and accumulate chemically reduced solutes in bottom waters. In the 120 m deep meromictic Lake Idro (Italy), nitrate is almost completely consumed at the chemocline depth, yet measurable concentrations (≈ 3 μM NO₃⁻) are consistently found in the sulphide-rich bottom water and surficial sediment. Under strongly reducing conditions, nitrate is expected to be rapidly consumed as evidenced by measured denitrification rates, and its presence suggests the existence of a hidden source.

Two potential nitrate sources were hypothesized: (i) the oxidation of NH₄⁺ to NO₃⁻ via manganese oxides (MnOx) as the lake sediment is rich in Mn; and (ii) allochthonous inputs, supported by nitrate-rich sinking particles.

To test the first hypothesis, potential nitrification was measured in sediment slurry incubations amended with ¹⁵NH₄⁺ and MnOx. These experiments showed no clear evidence of NH₄⁺ oxidation, indicating that Mn-driven nitrification is unlikely to sustain the observed nitrate pool. The analyses of intracellular nitrate storage revealed nitrate concentrations an order of magnitude higher than the dissolved fraction, suggesting that sinking diatoms are potential nitrate sources for the benthic system.

Diatoms are well known for their ability to accumulate nitrate in vacuoles and to respire it under unfavourable or anoxic conditions. Lake Idro experiences frequent diatom blooms, and the sediment is enriched in diatom frustules, primarily from the genus Aulacoseira, which is capable of surviving in anoxic sediments for extended periods. These observations support the hypothesis that sinking diatoms may act as carriers of nitrate to the deep sediments of Lake Idro, fuelling benthic nitrogen transformations.

The application of the ¹⁵N isotope-pairing technique on intact sediment cores confirmed active ¹⁴N-NO₃^- denitrification in the monimolimnion sediment, with measured rates of 5.9 ±1.5 µmol ¹⁴N-NO₃^- m^-2 h^-1, accounting approximately for 25 % of total benthic denitrification in the entire lake. Dissimilative nitrate reduction to ammonium (DNRA) rates were also detected but were five times lower than denitrification. These findings demonstrate that diatom-mediated delivery of intracellular nitrate may represent a quantitatively significant and previously overlooked nitrate source to sediments in stratified lakes. Consequently, this mechanism represents an important sink for nitrate and must be considered in future nitrogen-cycle models of meromictic and stratified lakes.