Département Eaux de surface

Small boreal lakes are known to contribute significantly to global CH₄ emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water CH₄ concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emission potential, was low (< 0.5 µM). We studied the biogeochemical processes involved in CH₄ removal by chemical profiling and through incubation experiments. δ¹³C-CH₄ profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH₄ oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH₄ oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH₄ oxidation. We observed a distinct peak in CH₄ concentration at the chlorophyll-a maximum, caused by either in situ CH₄ production or other CH₄ inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in CH₄ oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH₄ oxidation. Surprisingly, nitrite seemed to inhibit CH₄ oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH₄ oxidation can be a key process in CH₄ removal in the water column of humic, turbid lakes, thereby limiting diffusive CH₄ emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen.

Macroinvertebrates are widespread in lake sediments and alter sedimentary properties through their activity (bioturbation). Understanding the interactions between bioturbation and sediment properties is important given that lakes are important sinks and sources of carbon and nutrients. We studied the biogeochemical impact of macrofauna on surface sediments in 3-month-long mesocosm experiments conducted using sediment cores from a hypoxic, macrofauna-free lake basin. Experimental units consisted of hypoxic controls, oxic treatments, and oxic treatments that were experimentally colonized with chironomid larvae or tubificid worms. Overall, the presence of O₂ in bottom water had the strongest geochemical effect and led to oxidation of sediments down to 2 cm depth. Relative to macrofauna-free oxic treatments, chironomid larvae increased sediment pore water concentrations of nitrate and sulfate and lowered porewater concentrations of reduced metals (Fe²⁺, Mn²⁺), presumably by burrow ventilation, whereas tubificid worms increased the redox potential, possibly through sediment reworking. Microbial communities were very similar across oxic treatments; however, the fractions of α-, β-, and γ-Proteobacteria and Sphingobacteriia increased, whereas those of Actinobacteria, Planctomycetes, and Omnitrophica decreased compared to hypoxic controls. Sediment microbial communities were, moreover, distinct from those of macrofaunal tubes or feces. We suggest that, under the conditions studied, bottom water oxygenation has a stronger biogeochemical impact on lacustrine surface sediments than macrofaunal bioturbation.