Understanding the biogeochemical controls on the partitioning between nitrogen (N) removal through denitrification and anaerobic ammonium oxidation (anammox), and N recycling via dissimilatory nitrate (NO 3 −) reduction to ammonium (DNRA) is crucial for constraining lacustrine N budgets. Besides organic carbon, inorganic compounds may serve as electron donors for NO 3 − reduction, yet the significance of lithotrophic NO 3 − reduction in the environment is still poorly understood. Conducting incubation experiments with additions of 15 N-labeled compounds and reduced inorganic substrates (H 2 S, Fe 2+ , Mn 2+), we assessed the role of alternative electron donors in regulating the partitioning between the different NO 3 −-reducing processes in ferruginous surface sediments of Lake Lugano, Switzerland. In sediment slurry incubations without added inorganic substrates, denitrification and DNRA were the dominant NO 3 −-reducing pathways, with DNRA contributing between 31 and 46% to the total NO 3 − reduction. The contribution of anammox was less than 1%. Denitrification rates were stimulated by low to moderate additions of ferrous iron (Fe 2+ ≤ 258 µM) but almost completely suppressed at higher levels (≥1300 µM). Conversely, DNRA was stimulated only at higher Fe 2+ concentrations. Dissolved sulfide (H 2 S, i.e., sum of H 2 S, HS − and S 2−) concentrations up to ∼80 µM, strongly stimulated denitrification, but did not affect DNRA significantly. At higher H 2 S levels (≥125 µM), both processes were inhibited. We were unable to find clear evidence for Mn 2+-supported lithotrophic NO 3 − reduction. However, at high concentrations (∼500 µM), Mn 2+ additions inhibited NO 3 − reduction, while it did not affect the balance between the two NO 3 − reduction pathways. Our results provide experimental evidence for chemolithotrophic denitrification or DNRA with Fe 2+ and H 2 S in the Lake Lugano sediments, and demonstrate that all tested potential electron donors, despite the beneficial effect at low concentrations of some of them, can inhibit NO 3 − reduction at high concentration levels. Our findings thus imply that the concentration of inorganic electron donors in lake sediments can act as an important regulator of both benthic denitrification and DNRA rates, and suggest that they can exert an important control on the relative partitioning between microbial N removal and N retention in lakes.

Abstract. Lacustrine sediments are important sites of fixed-nitrogen (N) elimination through the reduction of nitrate to N2 by denitrifying bacteria, and they are thus critical for the mitigation of anthropogenic loading of fixed N in lakes. In contrast, dissimilatory nitrate reduction to ammonium (DNRA) retains bioavailable N within the system, promoting internal eutrophication. Both processes are thought to occur under oxygen-depleted conditions, but the exact O2 concentration thresholds particularly of DNRA inhibition are uncertain. In O2 manipulation laboratory experiments with dilute sediment slurries and 15NO3- additions at low- to sub-micromolar O2 levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O2-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O2 concentrations (≥70 µmol L−1), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O2 tolerance. Nevertheless, differential O2 control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 µmol L−1 O2, denitrification was favoured over DNRA, while DNRA was systematically more important than denitrification at higher O2 levels. Our results thus demonstrate that O2 is an important regulator of the partitioning between N loss and N recycling in sediments. In natural environments, where O2 concentrations change in near-bottom waters on an annual scale (e.g., overturning lakes with seasonal anoxia), a marked seasonality with regards to internal N eutrophication versus efficient benthic fixed-N elimination can be expected.

<p><strong>Abstract.</strong> Lacustrine sediments are important sites of fixed nitrogen (N) elimination through the reduction of nitrate to N<sub>2</sub> by denitrifying bacteria, and are thus critical for the mitigation of anthropogenic loading of fixed N in lakes. In contrast, dissimilatory nitrate reduction to ammonium (DNRA) retains bioavailable N within the system, promoting internal eutrophication. Both processes are thought to occur under oxygen-depleted conditions, but the exact O<sub>2</sub> thresholds particularly of DNRA inhibition are uncertain. In O<sub>2</sub>-manipulation laboratory experiments with dilute sediment slurries and <sup>15</sup>NO<sub>3</sub><sup>−</sup> additions at low- to sub-micromolar O<sub>2</sub> levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O<sub>2</sub>-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O<sub>2</sub> concentrations (≥ 70 µmol L<sup>−1</sup>), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O<sub>2</sub> tolerance. Nevertheless, differential O<sub>2</sub> control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 µmol L<sup>−1</sup> O<sub>2</sub>, denitrification was favored over DNRA, while DNRA was systematically more important than denitrification at higher O<sub>2</sub> levels. Our results thus demonstrate that O<sub>2</sub> is an important regulator of the partitioning between N-loss and N-recycling in sediments. In natural environments, where O<sub>2</sub> concentrations change in near bottom waters on an annual scale (e.g., overturning lakes with seasonal anoxia), a marked seasonality with regards to internal N eutrophication versus efficient benthic fixed N elimination can be expected.</p>

Pockmarks are crater-like depressions formed by upward fluid flow (gas and/or liquid) through the unconsolidated sediment column on the floor of oceans and lakes. While pockmarks are well described in the marine realm, they have essentially been overlooked in lacustrine settings, likely due to a lack in economic interest to apply high-resolution hydroacoustic techniques in lakes. A swath-bathymetry survey on Lake Thun, Switzerland, revealed the existence of three pockmark systems. One pockmark (110 m in diameter) was discovered near a big karst system at Beatenberg at a water depth of ~217 m. Its activity is probably associated with episodic groundwater seepage induced by earthquakes, floods and snowmelt. At another site, Daerligen, we detected at ~60 m water depth the presence of multiple smaller pockmarks (~1.5 to 10 m in diameter) that seem to be active, continuously liberating CH4 gas by bubble ebullition. The CH4 displayed a biogenic carbon isotopic signature, however, the exact origin of the gas remains unknown. The third site, Tannmoos (~35 m water depth), comprises two large pockmarks (20–43 m in diameter) connected to a karst system in gypsum-carrying bedrock. One of these pockmarks is constituted of several unit pockmarks (e.g., sub-pockmarks; 0.3 to 0.8 m in diameter). While strong evidence is still lacking, we suggest that groundwater discharge occasionally occurs through these unit pockmarks during periods of intense precipitation. Hence, this study reveals the existence of three pockmark systems of variable morphology and mechanisms of formation within the same lake, reflecting different hydrological and biogeochemical regimes. Moreover, it underscores the potential importance of pockmarks in influencing hydrological and CH4 budgets in lakes. Clearly more work on quantifying seasonal fluxes of groundwater and CH4 release via lacustrine pockmarks is required, and it needs to be seen whether the observations made in Lake Thun are universal and apply also to many other lacustrine environments worldwide.