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Organic Carbon Mass Accumulation Rate Regulates the Flux of Reduced Substances from the Sediments of Deep Lakes

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Organic Carbon Mass Accumulation Rate Regulates the Flux of Reduced Substances from the Sediments of Deep Lakes Research Collection Journal Article Organic carbon mass accumulation rate regulates the flux of reduced substances from the sediments of deep lakes Author(s): Steinsberger, Thomas; Schmid, Martin; Wüest, Alfred; Schwefel, Robert; Wehrli, Bernhard; Müller, Beat Publication Date: 2017-07-10 Permanent Link: https://doi.org/10.3929/ethz-b-000191117 Originally published in: Biogeosciences 14(13), http://doi.org/10.5194/bg-14-3275-2017 Rights / License: Creative Commons Attribution 3.0 Unported This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Biogeosciences, 14, 3275–3285, 2017 https://doi.org/10.5194/bg-14-3275-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Organic carbon mass accumulation rate regulates the flux of reduced substances from the sediments of deep lakes Thomas Steinsberger1,2, Martin Schmid1, Alfred Wüest1,3, Robert Schwefel3, Bernhard Wehrli1,2, and Beat Müller1 1Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland 2Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland 3Physics of Aquatic Systems Laboratory, Margaretha Kamprad Chair, École Polytechnique Fédérale de Lausanne, Institute of Environmental Engineering, 1015 Lausanne, Switzerland Correspondence to: Beat Müller ([email protected]) Received: 31 January 2017 – Discussion started: 17 February 2017 Revised: 23 May 2017 – Accepted: 2 June 2017 – Published: 10 July 2017 Abstract. The flux of reduced substances, such as methane hypolimnetic O2 consumption (Livingstone and Imboden, and ammonium, from the sediment to the bottom water (Fred/ 1996; Hutchinson, 1938; Cornett and Rigler, 1980), yet the is one of the major factors contributing to the consumption key processes are still debated. Much to the irritation of lake of oxygen in the hypolimnia of lakes and thus crucial for managers, decreasing phosphorus (P) loads to lakes has of- lake oxygen management. This study presents fluxes based ten not resulted in a decrease in O2 consumption in the hy- on sediment porewater measurements from different water polimnion, and O2 consumption even increased in artificially depths of five deep lakes of differing trophic states. In meso- aerated lakes (Müller et al., 2012a). An intuitive explanation to eutrophic lakes Fred was directly proportional to the to- for the lack of recovery of O2 consumption is a delay caused tal organic carbon mass accumulation rate (TOC-MAR) of by the mineralization of the large amount of organic carbon the sediments. TOC-MAR and thus Fred in eutrophic lakes (OC) deposited in the sediments during hypertrophy, gener- C decreased systematically with increasing mean hypolimnion ating reduced species such as NH4 , CH4, Mn(II), Fe(II) and depth (zH/, suggesting that high oxygen concentrations in S(-II). By reacting with O2 and other electron acceptors (di- the deep waters of lakes were essential for the extent of rectly or via microbial pathways), these reduced species con- organic matter mineralization leaving a smaller fraction for tribute to the hypolimnetic O2 consumption. As direct mea- anaerobic degradation and thus formation of reduced com- surements of reduced substances are rare, several modeling pounds. Consequently, Fred was low in the 310 m deep meso- approaches have investigated the sediment oxygen demand eutrophic Lake Geneva, with high O2 concentrations in the related to the formation of reduced substances (Di Toro et al., hypolimnion. By contrast, seasonal anoxic conditions en- 1990; Soetaert et al., 1996). Further, Matzinger et al. (2010) hanced Fred in the deep basin of oligotrophic Lake Aegeri. As demonstrated that sediment deposits older than 10 years con- TOC-MAR and zH are based on more readily available data, tributed only ∼ 15 % to the areal hypolimnetic mineralization these relationships allow estimating the areal O2 consump- rate (AHM), thus putting the magnitude and timescale of the tion rate by reduced compounds from the sediments where “sediment memory effect” into perspective. no direct flux measurements are available. Müller et al. (2012a) proposed two key factors to be re- sponsible for the AHM: (i) the diffusion controlled O2 con- sumption by the mineralization of freshly settled OC at the sediment surface and (ii) the O2 consumed by the oxidation 1 Introduction of reduced substances diffusing from the sediment (Fred/. The flux of O2 from the bottom water to the sediment sur- Hypolimnetic oxygen (O2/ depletion is a widespread phe- face is a first-order process with respect to the concentration nomenon in productive lakes and reservoirs. Considerable of O2 and hence lakes with a large hypolimnion volume can work has been done to identify parameters responsible for Published by Copernicus Publications on behalf of the European Geosciences Union. 3276 T. Steinsberger et al.: TOC MAR regulates Fred from lake sediments sustain a larger O2 flux and increase the fraction of aerobi- cated in an agricultural area dominated by pig farms. After cally mineralized OC. As a consequence, AHM systemati- 34 years of artificial aeration and mixing, it is still eutrophic −3 cally increases with mean hypolimnion depth (zH/ of pro- with total phosphorus (TP) concentrations of ∼ 25 mgP m . ductive lakes. This relationship suggested a constant O2 con- Lake Hallwil is the shallowest of the investigated lakes sumption rate of the sediments, which agreed with the few (48 m) and is presently recovering from its eutrophic past available estimations from direct sediment porewater mea- (TP ∼ 12 mgP m−3/ after 30 years of artificial aeration. Lake C ∼ −3 surements of reduced compounds. The fluxes of NH4 , CH4, Aegeri is oligotrophic (TP 6 mgP m / and located in a Fe(II), and Mn(II) from eutrophic lakes determined from catchment dominated by pastures and forests. Lake Geneva porewater concentration profiles (summed up and expressed is the largest lake in central Europe by volume. It is still −3 in O2 consuming equivalents) were in a surprisingly nar- meso-eutrophic (TP ∼ 20 mgP m / and the areal O2 con- −2 −1 row range of 0.36 ± 0.12 gO2 m d (Müller et al., 2012a). sumption rate is among the highest measured in Swiss lakes This can be a substantial fraction of total AHM, especially in (Müller et al., 2012a; Schwefel et al., 2016). Lake Zug lakes with a small hypolimnion volume. Matthews and Ef- (197 m) is eutrophic, permanently stratified below ∼ 100 m −3 fler (2006) showed the importance of Fred for sediment O2 depth (meromictic), and has a TP value of ∼ 30 mgP m demand in Onondaga Lake. Further, Fred was responsible for in the productive epilimnion. In Lake Zug, only one set of up to 42 and 86 % of the total AHM in the Pfäffikersee and cores for porewater analysis, CH4 analysis, and bulk sedi- C Türlersee (Switzerland), respectively, where NH4 and CH4 ment parameters was collected from the permanently oxic −1 fluxes represented up to 90 % of Fred, while Fe(II) and Mn(II) part (> 4 mgO2 L throughout the year) at 62 m water depth. fluxes played only a minor role (Matzinger et al., 2010). Depending on the sedimentation regime and bottom-water 2.2 Sediment sampling and porewater analysis O2 availability, Fred is expected to vary spatially. Carignan and Lean (1991) documented that porewater fluxes varied Sediment cores were retrieved with a Uwitec gravity corer with lake depth and increased with increasing sedimenta- equipped with a PVC tube (6.5 cm inner diameter, 60 cm tion rate in a mesotrophic but seasonally anoxic lake. They length). The PVC tube has pre-drilled holes (diameter 2 mm) demonstrated the focusing of labile particulate OC as the at 5 mm intervals. The holes were sealed with adhesive tape cause for the depth dependence. In lakes Baldegg and Sem- prior to sampling. Sediment cores were taken along a depth pach, increasing Fe(II) and Mn(II) fluxes with lake depth gradient (Table 1). Porewaters were sampled on site immedi- were attributed to geochemical focusing (Urban et al., 1997; ately after retrieval. A total of 10–50 µL of sediment porewa- Schaller et al., 1997). In consequence, extrapolating mea- ter was retrieved by punctuating the adhesive tape and hor- surements performed at the deepest sites of lakes to the entire izontally inserting a MicroRhizon filter tube (1 mm diame- hypolimnion area can significantly overestimate the contri- ter, 0.20 µm pore size; Rhizosphere Research Products, Wa- bution of reduced sediment compounds to AHM. Hence, the geningen, Netherlands). The sampling resolution was 5 mm aim of this study is to systematically extend the knowledge for the first 5 cm of sediment, ≤ 1 cm between 5 and 10 cm of sediment flux measurements of reduced compounds and of sediment, ≤ 2 cm between 10 and 20 cm of sediment, and to identify a common driving factor of their creation. At least ≤ 3 cm below 20 cm of sediment. The porewater retrieval three sampling depths were selected in each of the five lakes time was between 10 and 30 s and samples were immedi- investigated to gain information on the spatial distribution of ately analyzed to minimize oxidation. Each porewater sam- fluxes of reduced substances. The combination of porewa- ple was analyzed once with two capillary electrophoresis ter sampling and on-site analysis with two portable capillary devices each equipped with a capacitively coupled contact- electrophoresis systems allowed a high sample throughput less conductivity detector (CE-C4D) (calibrated for anions and the acquisition of an unprecedented dataset of porewater and cations) directly at the lake shore. Full separation of C 2− − − concentration profiles. Based on observations from 45 cores, ions of interest (NH4 , Mn(II), Fe(II), SO4 , NO3 , NO2 / this paper assesses the constraints of fluxes of reduced com- was achieved within 6 min by applying a voltage of 15 kV C pounds (CH4, NH4 , Mn(II), and Fe(II)) from the sediments and a current of 0.5 µA.
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