Spatiotemporal redox dynamics in a freshwater lake sediment under alternating oxygen availabilities: combined analyses of dissolved and particulate electron acceptors

Abstract

Environmental contextAt sediment surfaces, the availability of oxygen is controlled by its downward transport from the water surface and its consumption in microbial metabolism. Microorganisms can also consume substances other than oxygen to dispose of the surplus charge that is generated during microbial metabolism. We investigate the complex dynamics of these other substances when the oxygen availability fluctuates, and thereby contribute to the mechanistic understanding of oxygen-consuming processes in aquatic environments. AbstractBenthic mineralisation in lakes largely controls the availability of oxygen in the water column above the sediment. In stratified lakes with anoxic hypolimnetic waters, mineralisation proceeds by anaerobic respiration using terminal electron acceptors (TEAs) other than O2. In past work, hypolimnetic oxygen consumption has been estimated from vertical concentration profiles of redox-active dissolved species in the water column and the underlying sediment. Electron transfer to and from particulate mineral and organic phases in the sediments was, however, not accounted for, mainly because of methodological constraints. In this work we use an electrochemical approach, mediated electrochemical analysis, to directly quantify changes in the redox states of particulate geochemical phases in a lake sediment. In mesocosm incubations, sediments were subjected to shifting oxygen availability similar to conditions during and after lake overturn events. The temporal redox dynamics of both dissolved and particulate phases in sediments were monitored at a high spatial resolution. We used a combination of experimental and modelling approaches to couple the observed changes in the redox state of dissolved and particulate species in the sediment to the oxygen turnover in the overlying water column. For the studied freshwater sediment, the amount of O2 consumed during the re-oxidation of these phases in the top 21mm of the sediment after switching from hypoxic to oxic conditions corresponded to ~50% of the total sediment oxygen consumption that was estimated from in-lake measurements after the onset of summer stratification. We found that solid phases in the sediments play a more profound role in electron accepting processes than previously considered. Based on these results, we propose that the herein presented analytical method offers the possibility to constrain parameters in theoretical models that simulate benthic redox dynamics including the electron transfer to and from geochemical phases in the sediments.