This study investigated the consumption of oxygen (O₂) in 11 European lakes ranging from 48 m to 372 m deep. In lakes less than ~ 100 m deep, the main pathways for O₂ consumption were organic matter (OM) mineralization at the sediment surface and oxidation of reduced compounds diffusing up from the sediment. In deeper lakes, mineralization of OM transported through the water column to the sediment represented a greater proportion of O₂ consumption. This process predominated in the most productive lakes but declined with decreasing total phosphorous (TP) concentrations and hence primary production, when TP concentrations fell below a threshold value of ~ 10 mg P m⁻³. Oxygen uptake by the sediment and the flux of reduced compounds from the sediment in these deep lakes were 7.9–10.6 and 0.6–3.6 mmol m⁻² d⁻¹, respectively. These parameters did not depend on the lake's trophic state but did depend on sedimentation rates for the primarily allochthonous or already degraded OM. These results indicate that in lakes deeper than ~ 100 m, mineralization of autochthonous OM is mostly complete by the time of sedimentary burial. This explains why hypolimnetic O₂ concentrations improve more rapidly following TP load reduction in deeper lakes relative to shallower lakes, where larger sediment‐based O₂ consumption by settled OM and release of reduced substances may inhibit the restoration of hypolimnetic O₂ concentrations.

We investigated radiatively driven under-ice convection in Lake Onego (Russia) during 3 consecutive late winters. In ice-covered lakes, where the temperature of water is below the temperature of maximum density, radiatively driven heating in the upper water column induces unstable density distributions leading to gravitational convection. In this work, we quantified the key parameters to characterise the radiatively driven under-ice convection: (1) the effective buoyancy flux, $B_{\ast }$ (driver), and its vertical distribution; (2) the convective mixed-layer thickness, $h_{CML}$ (depth scale); and (3) the convective velocity, $w_{\ast }$(kinematic scale). We compared analytical $w_{\ast }$ scaling estimates to in situ observations from high-resolution acoustic Doppler current profilers. The results show a robust correlation between $w_{\ast }$and the direct observations, except during the onset and decay of the solar radiation. Our results highlight the importance of accurately defining the upper limit of h_CML in highly turbid water and the need for spectrally resolving solar radiation measurements and their attenuation for accurate $B_{\ast }$ estimates. Uncertainties in the different parameters were also investigated. We finally examined the implications of under-ice convection for the growth rate of nonmotile phytoplankton and provide a simple heuristic model as a function of easily measurable parameters.

Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming‐induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2 m thick water layers at around 12 m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large‐scale phenomena like algal blooms.

In ice‐covered lakes, penetrative radiation warms fluid beneath a diffusive boundary layer, thereby increasing its density and providing energy for convection in a diurnally active, deepening mixed layer. Shallow regions are differentially heated to warmer temperatures, driving turbulent gravity currents that transport warm water downslope and into the basin interior. We examine the energetics of these processes, focusing on the rate at which penetrative radiation supplies energy that is available to drive fluid motion. Using numerical simulations that resolve convective plumes, gravity currents, and the secondary instabilities leading to entrainment, we show that advective fluxes due to differential heating contribute to the evolution of the mixed layer in waterbodies with significant shallow areas. A heat balance is used to assess the relative importance of differential heating to the one‐dimensional effects of radiative heating and diffusive cooling at the ice‐water interface in lakes of varying morphologies.

Thin high-gradient interfaces that occur naturally within double-diffusive staircases are used to estimate the response characteristics of temperature and conductivity microstructure sensors. The knowledge of these responses is essential for resolving small-scale turbulence in natural water bodies and for determining double-diffusive fluxes of heat and salt. Here, the authors derive microstructure sensor responses from observed differences in the statistical distributions of interface thicknesses at various profiling speeds in Lake Kivu (central Africa). In contrast to the standard approach for determining sensor responses, this method is independent of any knowledge of the true in situ temperature and salinity structure. Assuming double-pole frequency response functions, the time constants for the Sea-Bird Electronics SBE-7 conductivity sensor and the Rockland Scientific International FP07 thermistor are estimated to be 2.2 and 10 ms, respectively. In contrast to previous assumptions, the frequency response for the SBE-7 is found to be substantial and dominates the wavenumber response for profiling speeds larger than 0.19 m s−1.