Organic matter mineralisation in the hypolimnion of an ... - Springer Link

Aquat. Sci. 70 (2008) 225 – 237 1015-1621/08/030225-13 DOI 10.1007/s00027-008-8008-2 Birkhuser Verlag, Basel, 2008

Aquatic Sciences

Research Article

Organic matter mineralisation in the hypolimnion of an eutrophic Maar lake Sven Fahrner1, Michael Radke2, Dorothe Karger3 and Christian Blodau1,* 1

Limnological Research Station and Department of Hydrology, University of Bayreuth, D-95440 Bayreuth, Germany 2 Department of Hydrology, University of Bayreuth, D-95440 Bayreuth, Germany 3 Biological Research Station Mosenberg, University of Koblenz-Landau, D-56070 Koblenz, Germany Received: 26 April 2007; revised manuscript accepted: 12 March 2008

Abstract. Constraints on hypolimnetic methane production in productive lakes are of interest owing to the importance of methane emissions from lakes for the atmospheric methane burden. We studied carbon fluxes and terminal electron accepting processes in the hypolimnion of eutrophic Meerfelder Maar, Germany. Carbon fluxes from epilimnion and sediments were estimated using sediment traps, porewater analysis and incubation techniques. The contribution of various redox processes to overall dissolved inorganic carbon (DIC) production was quantified from a seasonal hypolimnetic mass balance. Carbon sedimentation rates were 22 mmol m–2 d–1 from the epilimnion and 20 – 34 mmol m–2 d–1 from the hypolimnion. Anaerobic respiration accounted for a DIC production of 21 mmol m–2 d–1, and was thus capable of using a substantial part of primary production. The diffusive C flux from the sediment was small in comparison, although potential respiration rates in

incubation experiments reached 63 – 75 mmol m–2 d–1. Ebullition of methane occurred, but could not be quantified. Following depletion of dissolved oxygen (DO) and nitrate, iron and sulphate reduction and methanogenesis proceeded concurrently in the hypolimnion. Estimated hypolimnetic DIC production decreased in the order methanogenesis (2267 mmol m–2) > oxic respiration (880 mmol m–2) > sulphate reduction (575 mmol m–2) > denitrification (159 mmol m–2) > acetogenesis (157 mmol m–2) > iron reduction (19 – 144 mmol m–2). Methane production thus dominated respiration and was not inhibited by sulphate and iron reduction. It also strongly accelerated with increased carbon sedimentation rates in September, which apparently eased limiting factors on the process. Hypolimnetic methane production is thus likely an important process in similar lakes, even in the presence of other electron acceptors, and will contribute to methane emissions during fall turnover.

Key words. Eutrophic lake; fresh water; iron reduction; sulphate reduction; methanogenesis.

Introduction

* Corresponding author phone: +49-921-55-2223; fax: +49-921-55-2049; e-mail: [email protected] Published Online First: June 16, 2008

Organic matter decomposition in the tropholytic zone and sediments has profound ecological impacts on lakes, as carbon, nitrogen, and phosphorus fixed in the trophogenic zone are remineralized and strongly contribute to the productivity of these aquatic eco-

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systems (Caraco et al., 1992; Cole et al., 1989). In dimictic lakes typical of the temperate and boreal zone, organic matter decomposition during summer largely proceeds in a semi-closed compartment, the hypolimnion, and thus relies on the reservoirs of oxidants that are present at the beginning of stratification (Kelly et al., 1988; Mattson and Likens, 1993). In highly productive systems, organic matter deposition into this compartment typically leads to rapid oxygen depletion and the accumulation of reduced compounds, such as ammonium, ferrous iron, hydrogen sulphide, and methane, which are detrimental for recreational use, fisheries, and drinking water supplies (Cooke et al., 1993; Wu et al., 2003). In addition, methane production in lakes, which may stem from the sediments and the hypolimnion, has been identified as an important methane source to the atmosphere. It has recently been estimated that this source contributes 6 to 16 % to the global methane burden (Bastviken et al., 2004a). Of particular importance in this respect are the formation and release of methane bubbles, i.e. ebullition (Mattson and Likens, 1990; McGinnis et al., 2004; 2006), and the rapid release of hypolimnetic methane following ice out and fall turnover (e.g. Riera et al., 1999). The rates and regulation of organic matter decomposition in the hypolimnion of lakes are thus of considerable practical and scientific interest. Microorganisms use a range of electron acceptors for the decomposition of organic matter, including dissolved oxygen (DO), nitrate (NO3–), iron (Fe3+), manganese (Mn2+), sulphate (SO42–), and low-molecular-weight organic substances. In a homogeneous environment, the use of electron acceptors should follow the Gibbs free energy gradient of the involved redox processes, as long as the electron acceptor does not limit microbial metabolism. Two lines of support for this contention exist. First, half saturation constants for organic substrates have often increased with decreasing energy yield of terminal electron accepting processes (TEAPs) in experimental studies with cultured microorganisms (Lovley and Klug, 1983). Second, bacteria mediating TEAPs with larger energy yields can lower the concentrations of organic intermediates, in particular H2, to levels that do not allow bacterial metabolism of TEAPs with lower energy yields (Conrad and Klose, 1999; Lovley and Goodwin, 1988). In all of these redox processes, CO2 is produced (Fenchel and Finlay, 1995; Madigan et al., 2003). Concentrations of reduced species can in principle be used to quantify the relative importance of redox processes for CO2 production (Mattson and Likens, 1993). In the hypolimnion of a lake, such processes can be further studied in-situ because the metalimnion

Organic matter mineralisation in a Maar lake

considerably slows turbulent transport and provides an upper boundary for the compartment (Ohle, 1956; Quay, 1980). This approach further relies on the assumption that changes in concentration as well as sources and sinks of the reduced species can be quantified. It must be considered that CO2 consuming processes, such as hydrogenotrophic methanogenesis, may result in an underestimation of gross CO2 production. Several studies have balanced organic carbon oxidation with reduction of electron acceptors in the hypolimnion of freshwater lakes (Jones and Simons, 1980; Kelly et al., 1988; Mattson and Likens, 1993). Both Jones and Simon (1980) and Mattson and Likens (1993) found an unexplained production of dissolved inorganic carbon (DIC) of 14 % and 24 – 66 %, respectively. Kelly et al. (1988) found a surplus in DIC production of 16 %, and suspected methanogenesis to be the cause. In light of such results, Jones and Simon (1980) and Mattson and Likens (1993) suggested to examine fermentation as a potential source of additional CO2. During fermentation, carbon from the same organic compound is partially oxidized and partially reduced (Madigan et al., 2003). Low-molecular-weight fatty acids (LMWFA), and acetate (CH3 COO-) in particular, are the major fermentative products in freshwaters (Thauer et al., 1989; Fenchel and Finlay, 1995). For this reason, it was hypothesized that LMWFA concentrations can be used to assess DIC production by fermentation. In this study, we investigated carbon fluxes and decomposition processes in the hypolimnion of a small eutrophic maar lake during summer stratification. Cfluxes across the boundaries of the hypolimnion were estimated, concentration changes of electron acceptors and LMWFA recorded, and CH4 production in the hypolimnion and in the sediments estimated. In particular, we sought to (a) identify the spatiotemporal sequence of redox processes, (b) relate this sequence to the Gibbs free energy available for the processes, and (c) quantify the contributions of each redox reaction to the overall DIC-production and hypolimnetic carbon balance over time.

Material and methods Study site and sampling Meerfelder Maar is a small lake about 40 km northeast of the city of Treves in Rhineland-Palatinate, Germany. Allochthonous input of organic carbon is limited to inflow from a small catchment area of 1.53 km2 (Scharf and Bjçrk, 1992) and atmospheric deposition. Meerfelder Maar is eutrophic (Scharf, 1987; Scharf and Bjçrk, 1992). In 2007, yearly average

Aquat. Sci. Vol. 70, 2008

chorophyll a levels were 15 – 20 mg L–1 (max. 41 mg L–1), total phosphorus 50 – 80 mg L–1 (max. 360 mg L–1), and phophate 10 – 15 mg L–1 (max. 40 mg L–1) in the upper 8 m of the water column (Karger and Sinsch, unpublished data). An overview of the lake and sampling sites is presented in Fig. 1. Water samples were collected at sampling point II from 0 to 6 m at 2 m depth intervals, from 7 to 10 m at 1 m intervals and at 0.5 m intervals beneath 10 m on 26/05, 21/06, 06/07, 30/ 08, 24/09 and 11/10 in 2005. The depth intervals encompassed the hypolimnion, which was defined using temperature profiles taken on the same dates as the samples. Samples were stored refrigerated and analysed within 48 h. Additionally, water samples for the determination of dissolved hydrogen were collected at 1 m intervals on 30/08 and 11/10.

Figure 1. Cross-section of Meerfelder Maar in N-S-direction (distance 0 m= northern edge), including sampling sites that are marked with I, II, III, IV (upper diagram), and sedimentation rates of organic carbon at the shown locations (lower diagram). The interface between metalimnion and hypolimnion is marked for May and August. The installation depth of the sediment traps at location II is indicated with H (5 m) and L (15 m). n = 3 for each trap.

Organic carbon input from the epilimnion into the hypolimnion was quantified using sediment traps installed at 5 m depth at sampling sites I, II and III (traps I, II-H and III). Another trap (II-L) was installed at 15 m depth at site II to quantify losses through sedimentation across the lower boundary of

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the hypolimnion. Each trap consisted of 3 cylindrical tubes (i.d. 5 cm). To avoid resuspension, a height-todiameter ratio of 7.5 was chosen, which is well above the minimum ratio of 3 suggested by Pennington (1974). No preservatives were added to inhibit degradation since deployment times were between 12 and 39 days (see Kajak and Lawacs, 1971). Traps were sampled and immediately reinstalled. Samples were stored refrigerated and processed within 48 h. The supernatant was discarded and the material was dried at 50 – 60 8C until a constant mass was reached. Due to the loss of some traps during deployment, the sample series is incomplete. Samples of trap II (L) could only be analysed for two intervals (02/09 – 24/09 and 14/10 – 29/10). Sediment cores were extracted on 18/07 and 31/10 using a modified sediment gravity corer (UWITEC, Mondsee, Austria) equipped with a Plexiglas tube (i.d. 6 cm) along a transect at sampling points I, II, III and IV. At each point, 3 cores were taken, transported to the laboratory and sliced into three compartments (0 – 5 cm; 5 – 10 cm; 10 – 20 cm; 0 cm = sediment surface). Dry weight and bulk density were determined for each sub-sample. A pore water peeper (Hesslein, 1976) with 48 cells (10 mm distance between cells), which was already installed by scuba divers in July, was sampled on sampling point II on 8/10. The cells were initially filled with distilled water and coated with a celluloseacetate-membrane (0.2 mm; Schleicher & Schll, Einbeck, Germany). After extraction of the peeper from the sediment, the membrane was tightly coated with a thick plastic layer, transported to the laboratory in a water-filled box, and immediately processed. Analytical methods Temperature and the concentration of dissolved oxygen were measured in 0.5 m intervals using an Oximeter 197 probe (accuracy 0.5 % for DO and +/– 0.1 8C for temperature, WTW, Weilheim, Germany), and pH was determined with a Surveyor 3 probe (accuracy +/– 0.2 units, ECOTECH, Bonn, Germany). Ionic species. The methylene blue method (Cline, 1969) was used for the determination of dissolved sulphides (Luther et al., 2004) in the lake water. A 10 mL-sub-sample was mixed with an aqueous zincacetate solution (2 % by weight) immediately after sampling in vials pre-filled with nitrogen (purity 99.999 %) to prevent oxidation. Limit of quantification (LOQ) was 0.5 mM. Dissolved Fe2+ was analysed spectrophotometrically by the phenanthroline method (Tamura et al., 1974) after acidification with HCl (1M) to pH < 3. LOQ was 2.5 mM. Sub-samples for the determination of NO3– and SO42– were stored frozen and filtered (0.2 mm, nylon syringe micro filter,

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MILLIPORE, Schwalbach, Germany) prior to analysis by ion chromatography (METHROM IC-system, Filderstadt, Germany) column: Anion Dual 3 Peek column; chemical suppression; mobile phase: 2 mM NaHCO3 + 1,8 mM Na2CO3 + 15 %Vol acetone; flow rate: 0.8 mL min–1). LOQs for this method were 6 mM (NO3–) and 5 mM (SO42–), Limit of detection (LOD) was 4 mM for both substances. LMWFA (acetate, propionate, butyrate, valerate) were analyzed in sub-samples by GC-FID (Varian CP3800, Darmstadt, Germany; column: OPTIMAFFAP 0.25 mm 0.32 mm 30 m, MACHEREYNAGEL, Dren, Germany) using direct aqueous injection. A volume of 1 mL of filtered sample (0.2 mm, nylon syringe micro filter, MILLIPORE, Schwalbach, Germany) was mixed with 100 mL of a mixture of concentrated phosphorous acid and acetone (9:1, v/v). Injection volume was 1 mL, LOQ was 8 mM for acetate, 7 mM for propionate, 6 mM for butyrate, and 5 mM for valerate. DIC, CH4 and H2. Dissolved inorganic carbon (DIC) and CH4 were determined after transferring a 0.5 mL aliquot to a vial (2 mL) containing 20 mL HCl (1 M). Vials were stored upside-down for 24 to 36 h, and CO2 and CH4 analysed in the headspace using gas chromatography with TCD/FID detection (GC 6890, AGILENT, Waldbronn, Germany; injection volume: 50 mL) or with a combination of methanizer and FID (SRI 8610, SCHAMBECK, Bad Honnef, Germany; injection volume: 1 mL). Dissolved hydrogen was determined with a ta3000R gas analyzer (AMETEK, Meerbusch, Germany). Sub-samples of 35 mL volume were injected into evacuated and nitrogen (purity 99.999 %) filled crimp vials (50 mL). The remaining H2 background was determined for each vial. The bottles were equilibrated upside-down at 20 8C for 24 h for analysis of H2 in the headspace. The results were corrected for background concentrations. For all analytical methods, LOQ and LOD were determined on an operationally defined basis. Samples with concentrations below LOQ or LOD were assigned to 0.5 LOQ or 0.5 LOD. Diffusive fluxes. The minimum flux of CO2 and CH4 from the hypolimnion into the epilimnion was calculated according to Ficks first law (Stumm and Morgan, 1981), using the following diffusion coefficients: Coefficient values of 9.16 10–5 m2 d–1 for CO2 and 8.21 10–5 m2 d–1 for CH4, respectively, both at 5 8C (Lerman, 1988). Second, a coefficient value of 4.32 10–4 m2 d–1 for both CO2 and CH4 was used (Quay, 1980). The concentration gradient was defined according to the concentration difference between the lower epilimnion and the upper hypolimnion. How-


ever, the resolution of measurements allowed for calculation of the concentration gradient along a 0.5 m depth layer only. Diffusive CO2 and CH4 fluxes from the sediment into the hypolimnion were calculated based on the concentration profiles of the peeper and applying Ficks first law (Stumm and Morgan, 1981). Porosity was assumed to be 0.96, and diffusion coefficients for 5 8C were taken from Lerman (1988). Sediments. Sediment pore water samples were analysed for CO2, CH4, Fe2+, and SO42– as described above. The total carbon content in deposited material and sediments was determined using a VARIO EL instrument (ELEMENTAR ANALYSESYSTEME GmbH, Hanau, Germany). To differentiate between organic and inorganic carbon, the samples were additionally analysed after acidification to a pH < 3 using hydrochloric acid on one occasion (31/10). Sedimentation rates and total sediment pools of organic carbon were calculated down to 20 cm. To determine production rates of CO2 and CH4 from sediments, one sub-sample of each depth increment was incubated under anaerobic conditions (N2 atmosphere) at 5 8C in darkness following extraction. Concentrations of CO2 and CH4 in the gas phase were determined at intervals of 12 h over two days. Production rates were calculated as the linear increase in concentration with time. Assays with poor correlation of gas concentration with time (R2 < 0.94) were disregarded. Production rates of the increments were accounted for according to the layer thickness to calculate total production rates for the upper 20 cm of the sediment. Visualization of data. The software Surfer (Version 8.00, GOLDEN SOFTWARE, Golden, USA) was used for visualization of concentration data. A kriging procedure was applied for gridding, using an anisotropy ratio of 25 for the interpolation to mimic impacts of stratification properly. The calculated concentrations are assumed to be close to real concentrations due to the general slowness of redox reactions. Mass balancing and electron budget Hypolimnetic turnover rates for each redox species were calculated using mean concentrations of 1 m depth layers. A digital 3D-model (ArcGis 9.0. ESR INC., Ellicot city, USA) was used to quantify the volumes of the depth layers according to the bathymetric map of Hllenkrmer (1991). For each layer, turnover rates between two successive sampling events were determined. Volumes of the layers and time spans between sampling days were weighted and mean rates for each species calculated. To examine the assumption of spatial horizontal homogeneity, DO,

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Table 1. Terminal electron accepting processes (TEAPs) and Gibbs Free Standard Energies (DGR0). DGR0 for 20 8C are taken from Sigg and Stumm (1996) for reactions 1 – 3, from Hansen et al. (2001) for reaction 4, and from Madigan et al. (2003) for reaction 5. They were corrected for temperature, but not for pressure. DGR0 (KJ mol–1)

Reaction

TEAP

1: Fe3+ reduction 2: SO42– reduction utilizing H2 3: SO42– reduction utilizing CH3COO4: Methanogenesis utilizing H2 5: Methanogenesis utilizing CH3COO-

CH3COO– + 8Fe(OH)3 + 17H– = 2CO2 + 8Fe2+ + 22H2O þ 4H2 þ SO2 4 þ 2H ¼ H2 SðgÞ þ 4H2 O þ 2 CH3 COO þ SO2 þ 2H2 O 4 þ H ¼ 2CO2 þ S 4H2 þ HCO3 þ H þ ¼ CH4 þ 3H2 O CH3 COO þ H þ ¼ CO2 þ CH4

conductivity, pH, and temperature were determined along a north-south transect, and on one occasion additional measurements were carried out at sampling sites II and III (Fig. 1). Calculated t-Values from the paired t-test indicate that, at the 5 % level, there is insufficient evidence to suggest that the profiles differ (data not shown). Production of CO2 from redox processes was calculated from the use of organic matter of the Redfield formula (Redfield, 1958) in oxic respiration, denitrification to N2, iron reduction to Fe2+, fermentation to CH3COOH, sulphate reduction to H2S, and acetoclastic methanogenesis to CH4 and CO2, as described in Mattson and Likens (1993). As the mineral phase of sediments in Meerfelder Maar consists mainly of manganese-free minerals (olivine, pyroxene, carbonate, hornblende (Irion and Negendank, 1984), the contribution of manganese-oxide reduction to DIC production was neglected.

–676 –55 –59 –41 –27

As amorphous iron sulphide (FeS) is the most important precipitate in freshwater at neutral pH (Schwertmann, 1991; Cornell and Schwertmann, 1996), saturation indices (SI) for the precipitation of amorphous FeS were calculated, using the solubility product for 4 8C taken from Liden (1983) and concentrations corrected to activities based on ionic strength and the Davies equation (Stumm and Morgan, 1981).

Results

Carbon turnover and fluxes across hypolimnetic boundaries The diffusion of DIC through the thermocline ranged from 8 mmol m–2 d–1 (diffusion coefficient taken from Lerman, 1988) to 40 mmol m–2 d–1 (coefficient taken from Quay, 1980) in average (n = 4); the diffusion of CH4 was too small to be determined. In material deposited in sediment traps, CaCO3 accounted for Thermodynamic calculations Gibbs free energy of ferric iron reduction, SO42– 12.3 11.3 % of total carbon (n = 8). Assuming reduction and methanogenesis with H2 and constant CaCO3-content in the trap material, organic CH3COO– as the electron donor were calculated carbon input into the hypolimnion varied from 7 to 37 using Equation 1 (Stumm and Morgan, 1981). Standard mmol m–2 d–1 (Fig. 1) until September, peaking at the Gibbs Free Energies for the reactions are listed in Table beginning of October at up to 70 mmol m–2 d–1, before 1. They were corrected with the measured temperature, decreasing in October to 19 to 29 mmol m–2 d–1. The but due to small effects not corrected for pressure mean sedimentation rate of organic carbon was 22 (Benjamin, 2002). Reaction quotients were calculated mmol m–2 d–1. Sedimentation rates of POC were 1.3 to using activity coefficients determined with the Debye- 2.5 times higher at the bottom of the lake than at 5 m depth (Fig. 1). Hckel equation (Stumm and Morgan, 1981). CO2 production in incubation experiments with (1) sediment samples in July and October were similar at DGR ¼ DG0R þ R T ln Q 63 and 75 mmol m–2 d–1 in the upper 20 cm of the –1 DGR : Gibbs free energy change of reaction (kJ mol ) sediment and much higher than the CH4 production of 16 mmol m–2 d–1 at the same location (data not shown). 0 Production of both gases was more or less uniform DGR : Standard Gibbs free energy change or with time and depth. reaction (kJ mol–1) Pore water concentrations of Fe2+, SO42–, CO2 and –3 –1 –1 R: Gas constant = 8.314 10 kJ mol K CH4 are presented in Fig. 2. Iron(II) concentrations were constantly low in the hypolimnion and increased (Atkins, 1993) with increasing depth in the sediment. Sulphate concentrations declined strongly within the lowest T: Temperature (K) 30 cm of hypolimnetic water from 150 mM to < 40 mM towards the sediments, suggesting reduction of sulQ: Reaction quotient (-)

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Figure 2. Concentrations of Fe2+, SO42–, CO2 and CH4 at the sediment-water interface on 08/10. The sediment-water interface is at depth = 0 cm. Negative depths indicate overlying water.

phate above the sediment-water interface. Carbon dioxide and CH4 concentrations were highest in the sediment and decreased towards the interface. Diffusion from the sediment into the overlying water amounted to 0.9 mmol CO2 m–2 d–1 and 1.5 mmol CH4 m–2 d–1 on 08/10. The average organic carbon content in the solid phase of the sediments was 16 2 % (n = 36), resulting in an organic carbon pool of 162 24 mol C m–2 (n = 36) in the upper 20 cm. Water chemistry The lake had stratified by the end of May (Fig. 3). The metalimnion dropped steadily throughout the summer and fall and levelled out at 11 m depth. The stratification was followed by depletion of oxygen in the lower water column (Fig. 4). Nitrate concentrations were highest in June (6 – 42 mM) and rapidly decreased thereafter (Fig. 4). Ammonium concentrations were highest at the sediment-water interface, increased gradually with time, and reached a peak near the sediment in October (275 mM). Sulphate was initially consumed at lower depths and later throughout the hypolimnion. Concentrations reached a mini-

mum of 30 mM near the sediment-water interface in October. Concentrations of H2S increased steadily throughout the hypolimnion towards the end of the sampling period to a maximum of 50 to 60 mM at the sediment-water interface. Iron(II) concentrations (Fig. 4) peaked near the sediment water-interface and rose slightly at all depths to maximum concentrations of 30 mM in August before decreasing beneath a depth of 12 m. Acetate (CH3COO-) was the only detectable LMWFA. Concentrations ranged from 30 to 120 mM, varied little with depth (Fig. 4), and reached a minimum of 30 to 45 mM in July. Methane concentrations (Fig. 4) were already elevated beneath 12 m on the first sampling event in May at 10 to 70 mM, increased steadily at all depths, and peaked near the sediment-water interface at 1100 mM in October. Carbon dioxide concentrations ranged from 1.2 to 1.5 mM, with highest levels in the lowest strata. Maximum of hydrogen concentrations ranged from 3 to 23 nM on both sampling days. Values of pH were slightly basic and mostly ranged from 7.0 and 7.3 in the hypolimnion and higher values of 8.6 to 9.2 in the epilimnion.


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Figure 4. Concentrations of Fe2+, CH4, and CH3COO- in lake water. Julian days of measurement were 146 (26/05), 172 (21/06), 187 (06/07), 242 (30/08), 267 (24/09) and 284 (11/10). Fe2+ was not determined on the first sampling date, CH4 not on the last one. Consequently, x-axes show different time intervals.

Figure 3. Temperature, saturation of DO and concentrations of NO3–, NH4+, SO42– and H2S in lake water. Julian days of measurement were 146 (26/05), 172 (21/06), 187 (06/07), 242 (30/08), 267 (24/09) and 284 (11/10). H2S was not determined on the first sampling date. Consequently, x-axes show different time intervals.

Mass and electron flow Dissolved oxygen was depleted at the end of June and most nitrate from the middle of July onwards (Fig. 5). DO was consumed with a rate of –21.5 mmol m–2 d–2 before its final depletion, and this rate was much faster than the consumption rates of NO3–, SO42– and Fe(III) later. The NH4+ production rate exceeded the NO3– consumption rate, indicating NH4+ release from organic nitrogen. Iron(II) production was constantly low, probably due to limited availability of Fe(III). Sulphate was steadily consumed throughout the summer but not depleted. Production rates of H2S were lower than SO42– consumption rates, suggesting iron sulphide precipitation. Methane production was more or less constant until the end of August, and increased afterwards to 46 mmol m–2 d–1. Total anaerobic decomposition rates ranged from –6 to –50 mmol m–2 d–1 not accounting for changes in the concentration of CH3COO-. The CH3COO- pool decreased strongly in May and June at rates of –8 to –18 mmol m–2 d–1, remained constant until end of

August, and increased slightly afterwards at 0.4 to 4 mmol m–2 d–1. The inferred production of DIC is presented in Fig. 5. Dissolved oxygen reduction led to the production of 880 mmol m–2 DIC. During DO consumption none of the other decomposition processes produced significant amounts of DIC. Nitrate reduction and Fe2+-production contributed little to overall DIC production, equivalent to 159 and 19 mmol m–2, respectively. Net CH3COO- production was low and thus fermentation accounted only for little of DIC production at 157 mmol m–2. Sulphate reduction accounted for 575 mmol m–2 of DIC production. Methanogenesis was quantitatively most important among anaerobic respiratory processes at a calculated DIC production rate of 2267 mmol m–2. Precipitation of iron sulphides and thermodynamic calculations Positive SI-values show that hypolimnetic water became supersaturated with respect to amorphous FeS over the period of stratification. SI-values declined after 30/08, which indicates that precipitation of amorphous FeS exceeded production of Fe(II) and sulphide. The Gibbs free energies of the respiratory processes were generally negative and more or less uniform with depth for each reaction presented in

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Figure 5. Gain and loss of redox species in the hypolimnion as the change of the total quantity of relevant redox species in the whole hypolimnetic water volume during the stratification period (upper panels), and resulting cumulative DIC production. CH4 production for the period 24/09 – 11/10 was not measured, but assumed to be constant from 30/08 onwards.

Table 1. The sequence of free reaction energies for the five examined reactions was the same on all five sampling occasions: Fe(III) reduction was most favourable (DGR = –619 kJ mol–1), followed by SO42– reduction and methanogenesis using H2 (DGR = –54 kJ mol-1 and –36 kJ mol–1, respectively), followed by SO42– reduction and methanogenesis using CH3 COO- (DGR = –26 kJ mol–1 and –18 kJ mol–1, respectively). It must be considered, though, that Gibbs free energies of the hydrogenotrophic processes were possibly overestimated due to the H2 analysis providing only upper concentration estimates.

Discussion Redox process patterns A clear redox sequence only occurred in the hypolimnion with respect to the change from the use of oxygen and nitrate to processes with lower energy yields. At the beginning of the sampling period, before day 200, concentrations of DO and NO3– had already decreased near the sediment-water interface and rapidly decreased also in other parts of the hypolimn-

ion thereafter (Fig. 3). Reduction of SO42– subsequently became the predominant TEAP at the sediment-water interface. Ferrous iron was produced more slowly, probably due to the limited availability of ferric iron colloids. Concentrations of CH4 near the sediment water interface, however also increased during this period and finally dominated respiration and electron flow in the hypolimnion. This process is illustrated by the increase in CH4 concentrations compared to concentrations of other respiration products (Fig. 3, Fig. 4). In May and June, DO was consumed significantly faster than NO3–, Fe(III) and SO42– later, which is in agreement with the larger energy yield of oxic respiration compared to the other TEAPs and laboratory studies of aerobic versus anaerobic decomposition (Harvey et al., 1995; Sun et al., 1997). Once methanogenesis predominated, CH4 production rates exceeded the initial DO consumption by a factor of 2 (Fig. 6). It has been documented in bioassay experiments with aquatic DOM that bacterial growth and bioavailability of DOM is not always lower under anoxic conditions (Bastviken et al., 2004b). Considering that the organic carbon sedimentation from the


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Figure 6. Carbon fluxes in the hypolimnion and the upper 20 cm of the sediment in Meerfelder Maar. Negligible fluxes (see text) are not shown.

epilimnion and hypolimnion strongly increased in September (Fig. 1), this result is possible. Indeed, Kelly et al. (1988) reported similar data for experimentally eutrophied softwater Lake 226 in the Experimental Lakes Area, Canada. In this study, DO consumption was 5.9 mmol m–2 d–1, consumption of other TEAPs reached 0.64 to 1.7 mmol m–2 d–1, and CH4 production rates reached 5.0 mmol m–2 d–1. Part of the increase in CH4 concentrations may be attributed to production in the sediments and subsequent transport into the hypolimnion. Diffusive CH4 fluxes from the sediments were much smaller at 1.5 mmol m–2 d–1 on 08/10 than production in the hypolimnion at 46 mmol m–2 d–1. It cannot be assumed that the diffusion rate determined on that date was representative for the whole stratification period. However, the accumulation of methane in the lower hypolimnion was so fast in comparison that diffusion over the sediment-water interface was obviously of minor importance in the observed increase in CH4 concentrations. This finding suggests that at least a substantial portion of the methanogenesis occurred in the lower water column, and proceeded concurrently

with reduction of SO42– (Fig. 3). The potential CH4 production rates in the incubation experiments exceeded the diffusive fluxes, which is a common finding when using incubation techniques (Blodau and Moore, 2003). Ebullition clearly occurred but could not be quantified due to problems with iron sulphide precipitates on our sampling device. The ebullition flux was likely considerably higher than the diffusive flux from the sediments based on results from other studies. In a shallow eutrophic pond in the English Lake District, Casper et al. (2000) found ebullition contributing 96 % to the total methane flux half a meter above the sediment water interface. Across a range of lakes, Bastviken et al. (2004a) estimated ebullition to account for 40 to 60 % of the methane flux to the atmosphere. We cannot accurately constrain this flux but it unlikely exceeded potential methane production rates in the incubation experiments, which reached 16 mmol m–2 d–1. In fact, we believe that is was considerably smaller, as the incubation experiments probably overestimated in situ CH4 production. In the incubation experiments, CO2 was produced at 63 – 75 mmol m–2 d–1, which is

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much larger than the in situ diffusive DIC flux of 0.9 mmol m–2 d–1 that accounted for 98 % of the total in situ production in a previous study (Casper et al. 2000). In analogy, the in situ production of CH4 was probably also lower than production in the incubation experiments, and the sum of diffusion and ebullition thus smaller than the incubation CH4 production of 16 mmol m–2 d–1. Bubbles probably also by-passed the lower hypolimnion to some extent before equilibrating with the surrounding water (McGinnis et al., 2006). Thus, we conclude that ebullition from the sediment did not quantitatively account for the observed increase in CH4 concentrations in the lower hypolimnion. When the hypolimnion as a whole is considered, TEAPs probably partly coexisted because they dominated in different depth layers (Fig. 5). During May and June, when iron and sulphate were reduced and methane began to accumulate near the sedimentwater interface, DO and nitrate were still present in the upper part of the hypolimnion. Near the metalimnion, reduction of sulphate began only in July and methane concentrations remained low (Fig. 5). This pattern can probably be attributed to the organic matter input and accumulation in the lower part of the hypolimnion due to sedimentation and resuspension. Abundance of electron donors lead to a more rapid depletion of oxygen and nitrate and, later, possibly also of ferric iron colloids. Overall, the sequence of redox processes in the hypolimnion appeared to proceed gradually as function of electron donor availability. TEAPs and methanogenesis overlapped and successively involved an increasing volume of the hypolimnion. This pattern is similar as reported earlier for redox sequences in water logged soils and sediments (e.g. Canfield et al., 1993). Redox process rates Over the entire period, methanogenesis accounted for most of the inferred DIC production with 2267 mmol m–2 of DIC, followed by DO reduction with 880 mmol m–2 DIC, and SO42– reduction with 575 mmol m–2 DIC and NO3– with 159 mmol m–2 DIC. These numbers underestimate DIC production by DO consumption, as most DO had already been consumed at the first sampling event in May. Nevertheless, the results demonstrate the importance of methane production in the metabolism of Meerfelder Maar. Similar findings have been previously reported for other seasonally stratified and productive lakes (Jones and Simon, 1980; Ingvorsen and Brock, 1982; Kelly et al., 1988; Mattson and Likens, 1993). Iron reduction, which only accounted for 19 mmol m–2 was likely underestimated by measuring accumulation of Fe2+ over time, as Fe2+ precipitated. This


conclusion is supported by the consumption of Fe2+ near the sediment-water interface (Fig. 2), over-saturation of the hypolimnetic water with respect to amorphous FeS, and intense formation of black precipitates on sediment traps and ebullition funnels. The total reduction of ferrous iron is constrained by the difference between the reduction rate of sulphate and the formation of hydrogen sulphide, which was 144 mmol m–2. While ferric iron production was a process of importance in the overall electron budget, it was subordinate compared to the consumption of other electron acceptors. This finding is in agreement with results from softwater lakes studied by Kelly et al. (1988) and Mattson and Likens (1993). The calculated DIC production is affected by some uncertainty. As mentioned above, ferric iron reduction rates were likely underestimated. Uncertainty also arises from the nature of the mineralized organic matter and the exact pathways of fermentation and TEAPs that we could not determine. Fermentation, for example, can proceed through a multitude of possible pathways and acetate is not the only intermediate that can accumulate in anaerobic systems (Lovley and Goodwin, 1988; Zehnder, 1988; Conrad and Klose, 1999). Also, a larger contribution of H2 consumption to sulphate reduction and methanogenesis would reduce real DIC production, as seen in Table 1. In fact, thermodynamic calculations suggested that the reduction of CO2 by H2 was the energetically favoured pathway of methanogenesis. Consequently, the calculated contribution of methanogenesis (2267 mmol m–2) to the overall DIC production in the hypolimnion may be an overestimation. Previous studies have reported an unexplained gap of up to 34 % in the electron flow balance between consumption of known electron acceptors and production of DIC in lakes (Jones and Simon, 1980; Mattson and Likens, 1993). The authors suggested that the gap could be the result of an accumulation of LMWFAs such as acetate, propionate and butyrate, whose production also releases DIC (Conrad and Klose, 1999). Unfortunately, we could not balance the hypolimnetic DIC budget of Meerfelder Maar, since the accuracy of our DIC measurements in the lake water was not adequate to quantify changes in DIC storage. The results of LMWFA measurements still show that only acetate was present in relevant concentrations. Acetate also did not substantially accumulate over the period but rather reached a concentration minimum during summer (Fig. 5). Accumulation of LMWFAs thus played a subordinate role for the hypolimnetic accumulation of DIC.

Research Article Schafran and Driscoll (1987)4 Kelly et al. (1988)2 Kelly et al. (1988)2 Kelly et al. (1988)2 Jones and Simon (1980)1 Mattson and Likens (1993)3 This study 0 –60 –6 11 14 37 ND 100 160 106 88 86 63 ND ND 71 68 42 25 11 56 4 25 30 30 2 7 14 2

1

Sulfate reduction estimated by sulfide accumulation Data for the three lakes recalculated (for detailed information see Mattson and Likens, 1993) 3 Average for 3 three years for the 8 – 11 m hypolimnion 4 Methane not determined but believed to be negligible ND = not determined

0.2 3 2 6 ND 2 0.5 15 13 1 1 17 0 4 81 47 5.5 9 42 43 22 4.7 8.10 7.38 7.62 10.7 5.33 16 ND ND 21.0 ND 38.2 10.0 22 Darts L 226 N L227 L223 Blelham Tarn Mirror Meerfelder Maar

Reference Excess Total CH4 SO42– Fe (mmol m–2 d–1) (mmol m–2 d–1)

O2

NO3–

Contribution of e- -acceptor to DIC accum. (%) C- accum.

In comparison to other lakes, Meerfelder Maar was characterized by moderate to high organic carbon deposition fluxes, which surprisingly increased from the metalimnion to the lower hypolimnion. Rates of anaerobic redox processes were on the order of the organic carbon input. Summer stratification resulted in rapid depletion of oxygen and nitrate in the hypolimnion, concurrent reduction of iron and sulphate, and production of methane. The latter proc-

C-Input

Conclusions

Lake Name

Carbon budget Over the period of May to October, redox processes accounted for 27 mmol m–2 d–1 of DIC production, which exceeded the flux of 22 mmol m–2 d–1 of particulate organic carbon input from the epilimnion (Fig. 6). Anaerobic respiration in the hypolimnion was thus capable of fully oxidizing the particulate organic carbon input from the epi- and metalimnion. Even more organic carbon, 20 – 34 mmol m–2 d–1, was deposited in particulate form on the sediments (Fig. 6), and thus carbon was overall fixed in the hypolimnion and transferred to the sediments. This net-fixation of carbon is in agreement with chlorophyll concentrations peaking at depths of about 8 m in the upper hypolimnion (Blodau and Radke, unpublished data). Diffusive CO2 and CH4 fluxes from sediments were low in comparison, not accounting for ebullition. Diffusive fluxes of DIC from the hypolimnion to the epilimnion were low and consequently insignificant for the carbon budget. In comparison to previous studies, the carbon budget and magnitude of individual fluxes followed our expectation for a highly productive lake. Calculated DIC production was higher than in oligotrophic lakes Mirror and Dart in the Adirondacks, USA, and experimentally eutrophied and acidified lakes in the Experimental Lakes Area, but lower than in eutrophic Lake Mendota, Wisconsin, and Lake Onondaga (Table 3). Carbon sedimentation from the epilimnion differed in a similar way between lake systems, and the particulate organic carbon sedimentation into the hypolimnion of Meerfelder Maar fell well into the reported range of 5.3 to 49 mmol m–2 d–1. The main difference to previous budgets was the finding that the estimated DIC production in the hypolimnion exceeded the particulate organic carbon input from the epilimnion. The reason for this phenomenon is not clear. Apart from photosynthetic carbon fixation in the meta- and upper hypolimnion, it seems also possible that resuspension of sediment, or sediment focussing, contributed to the recorded organic carbon sedimentation flux out of the hypolimnion.

Table 2. Comparison of hypolimnetic electron budgets from several lakes. Data include carbon sedimentation rate and DIC production rate during summer stratification.


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esses began near the sediment-water interface, were not clearly spatially and temporally separated, and only later occurred in the upper hypolimnion. Differences in Gibbs free energies among the processes and hydrogenotrophic and acetoclastic pathways clearly occurred. This pattern was likely due to a greater abundance of organic electron donors near the sediment-water interface. Methanogenesis became the predominant process only towards the end of the stratification period and then proceeded at rates that were higher than initial consumption of oxygen and of alternative electron acceptors, possibly triggered by high organic carbon deposition during this period. Acetogenesis, which has been suggested earlier as an important source of DIC, did not play a significant role for DIC production. Thermodynamic controls seem only of limited effectiveness and the availability of electron donors a more important control on the occurrence of redox processes in the hypolimnion of eutrophic waters. The results suggest that depletion of oxygen in shallow eutrophic lakes, such as Meerfelder Maar, will result in accumulation of methane in hypolimnetic waters, even if sulphate and ferric iron are still present as electron acceptors, and thus contribute to atmospheric methane fluxes during fall turnover.

Acknowledgments We gratefully acknowledge the laboratory support of Gerhard Dawen from the Biological Research Station Mosenberg in Bettenfeld, Germany. Discussions with Julia Beer, Beate Fulda and Marieke Oosterwoud also contributed to this work.

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