Organic carbon mass accumulation rate regulates the flux of reduced substances from the sediments of deep lakes

Thomas Steinsberger,Bernhard Wehrli,Alfred Wüest,Robert Schwefel,Martin Schmid,Beat Müller

doi:10.5194/bg-14-3275-2017

Abstract

Abstract. The flux of reduced substances, such as methane and ammonium, from the sediment to the bottom water (Fred) 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 Fred was directly proportional to the total organic carbon mass accumulation rate (TOC-MAR) of the sediments. TOC-MAR and thus Fred in eutrophic lakes decreased systematically with increasing mean hypolimnion depth (zH), 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, Fred was low in the 310 m deep meso-eutrophic Lake Geneva, with high O2 concentrations in the hypolimnion. By contrast, seasonal anoxic conditions enhanced Fred in the deep basin of oligotrophic Lake Aegeri. As TOC-MAR and zH 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.

Highlights

Hypolimnetic oxygen (O2) depletion is a widespread phenomenon in productive lakes and reservoirs
Fluxes measured at the same locations on up to five different dates between March and October varied by 34 % in Lake Baldegg and by 84 % in Lake Aegeri for CH4, as well as by 80 % in Lake Baldegg and by 65 % in Lake Aegeri for NH+4
We demonstrate that the areal oxygen consumption in lakes caused by reduced compounds diffusing from the sediment, from the sediment to the bottom water (Fred), is strongly related to the local mass accumulation rate of organic carbon (OC) (Fig. 2)

Summary

Introduction

Hypolimnetic oxygen (O2) depletion is a widespread phenomenon in productive lakes and reservoirs. An intuitive explanation for the lack of recovery of O2 consumption is a delay caused by the mineralization of the large amount of organic carbon (OC) deposited in the sediments during hypertrophy, generating reduced species such as NH+4 , CH4, Mn(II), Fe(II) and S(-II). By reacting with O2 and other electron acceptors (directly or via microbial pathways), these reduced species contribute to the hypolimnetic O2 consumption. Matzinger et al (2010) demonstrated that sediment deposits older than 10 years contributed only ∼ 15 % to the areal hypolimnetic mineralization rate (AHM), putting the magnitude and timescale of the “sediment memory effect” into perspective

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