Mineralization pathways in lake sediments with different oxygen and organic carbon supply

Abstract

The intensity and pathways of mineralization of sedimentary organic matter were investigated in eutrophic Lake Zug, Switzerland. In a depth transect (25–180 m) from oxic to anoxic bottom water we recorded in situ sediment pore‐water concentration profiles of O2, NO3−, and NH4+ with a benthic lander system equipped with both oxygen and ion‐selective electrodes. Anaerobic sedimentary mineralization ranged from 13.1 to 34.9 mmol carbon (C) m−2 d−1 and increased linearly with water depth, as determined from the NH4+ flux rates in the anoxic pore water and the molar C: nitrogen (N) ratio of the surface sediment. A parallel increase of the total organic carbon concentration of the sediment was attributed to lateral transfer of resuspended sedimentary matter. Denitrification was estimated from nitrate profiles and contributed only 1.5‐3.2% to the total organic carbon mineralization at any water depth. Aerobic respiration and oxidation of reduced compounds were calculated from O2 microprofiles and pore‐water data of dissolved Mn(II), Fe(II), S(‐II), and CH4. When the O2 concentration exceeded 0.15 mmol L−1 in the sediment overlying water, 41–58%, or 12.4–18.1 mmol C m–2 d−1, was mineralized aerobically, whereas at lower concentrations (<0.04 mmol L−1), >92% of organic carbon was mineralized anaerobically. Total benthic mineralization of organic carbon was 26.9–34.9 mmol C m−2 d−1. A budget including particulate as well as dissolved reduced compounds in the sediment indicated that >95% of the anaerobic mineralization was due to methanogenesis. Oxidation of CH4 consumed 39‐56% of the O2 at the sediment‐water interface. Oxygen exposure times for these sediments were estimated to be on the order of weeks to months. These time spans are too short to change the reactivity spectrum of sedimentary organic matter.