Organic carbon mass accumulation rate regulates the flux of reduced substances from the sediments of deep 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 meso- to 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 meso-eutrophic Lake Geneva, with high O₂ 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 O₂ consumption rate by reduced compounds from the sediments where no direct flux measurements are available.