Low concentrations of dissolved oxygen remain a global concern regarding the ecological health of lakes and reservoirs. In addition to high nutrient loads, climate‐induced changes in lake stratification and mixing represent additional anthropogenic menace resulting in decreased deepwater oxygen levels. The analysis of 43 years of monitoring data from Lake Geneva shows no decreasing trend neither in the areal hypolimnetic mineralization rate nor in the extent of hypoxia. Instead, hypoxic conditions are predominantly controlled by deep mixing in winter and much less by the trophic variations over the past decades. To reproduce winter mixing, the one‐dimensional hydrodynamic model SIMSTRAT was specially adapted to deep lakes and run for several climate scenarios. The simulations predicted a decrease in the maximum winter mixing depth from an average of ∼172 m for 1981–2012 to ∼136 m and ∼127 m in response to predicted atmospheric temperatures between 2045–2076 and 2070–2101 according to Intergovernmental Panel on Climate Change scenarios. Concurrently, events with complete homogenization of temperature and oxygen in winter will decrease by ∼50%. Consequently, the hypolimnetic oxygen concentrations will significantly decrease. These results demonstrate that changes in deep mixing can have stronger impact than eutrophication on the deepwater oxygen levels of oligomictic lakes.

Abstract. The flux of reduced substances, such as methane and ammonium, from the sediment to the bottom water (F red ) is one of the major factors contributing to the consumption of oxygen in the hypolimnia of lakes and thus crucial for lake oxygen management. This study presents fluxes based on sediment porewater measurements from different water depths of five deep lakes of differing trophic states. In mesoto eutrophic lakes F red was directly proportional to the total organic carbon mass accumulation rate (TOC-MAR) of the sediments. TOC-MAR and thus F red in eutrophic lakes decreased systematically with increasing mean hypolimnion depth (z H ), suggesting that high oxygen concentrations in the deep waters of lakes were essential for the extent of organic matter mineralization leaving a smaller fraction for anaerobic degradation and thus formation of reduced compounds. Consequently, F red was low in the 310 m deep mesoeutrophic Lake Geneva, with high O 2 concentrations in the hypolimnion. By contrast, seasonal anoxic conditions enhanced F red in the deep basin of oligotrophic Lake Aegeri. As TOC-MAR and z H are based on more readily available data, these relationships allow estimating the areal O 2 consumption rate by reduced compounds from the sediments where no direct flux measurements are available.

Low oxygen concentrations in lakes and reservoirs are an ongoing environmental concern, particularly in light of increasing anthropogenic activity and climate change. Oxygen depletion processes in lakes are still not completely understood and a variety of models have been proposed based on limited field observations. Here, we present field measurements of oxygen depletion processes in a deep lake, Lake Geneva (Switzerland). The aim of this study was to quantify three basic processes controlling hypolimnetic oxygen depletion and their relative contribution to the total oxygen depletion (TOD) rate. Sediment oxygen uptake (SOU) and the flux of reduced substances were estimated based on oxygen microprofile measurements and sediment core data of reduced substances. Acoustic Doppler current profiler measurements and hydrodynamic modeling were used to ensure that SOU was measured under typical hydrodynamic conditions. Comparison with long‐term monitoring data allowed for an estimate of the relative importance of SOU and water column mineralization (WCM). Results show a decrease in both SOU and WCM down to mid‐depth which could not be explained by changes in hydrodynamic conditions or temperature. Below mid‐depth, TOD increased due to an enhanced sediment area to water volume ratio (α). This vertical pattern of oxygen depletion is driven by (1) lake morphometry paired with increasing α, and (2) decreasing organic matter mineralization in the water column with depth. The findings are explained by a model which separates the oxygen depletion into an exponentially decreasing component, representing the fast‐decaying fraction of the organic matter, and a constant background component.

Areal oxygen (O2) consumption in deeper layers of stratified lakes and reservoirs depends on the amount of settling organic matter. As phosphorus (P) limits primary production in most lakes, protective and remediation efforts often seek to reduce P input. However, lower P concentrations do not always lead to lower O2 consumption rates. This study used a large hydrochemical dataset to show that hypolimnetic O2 consumption rates in seasonally stratified European lakes remain consistently elevated within a narrow range (1.06 ± 0.08 g O2 m−2 d−1) as long as areal P supply (APS) exceeded 0.54 ± 0.06 g P m−2 during the productive season. APS consists of the sum of total P present in the productive top 15 m of the water column after winter mixing plus the load of total dissolved P imported during the stratified season, normalized to the lake area. Only when APS sank below this threshold, the areal hypolimnetic mineralization rate (AHM) decreased in proportion to APS. Sediment trap material showed increasing carbon:phosphorus (C:P) ratios in settling particulate matter when APS declined. This suggests that a decreasing P load results in lower P concentration but not necessarily in lower AHM rates because the phytoplankton community is able to maintain maximum biomass production by counteracting the decreasing P supply by a more efficient P utilization. In other words, in-lake organic matter production depends only on APS if the latter falls below the threshold of 0.54 g P m−2 and correspondingly, the atomic C:P ratio of the settling material exceeds ~155.