Methane-Derived Carbon in the Benthic Food Web in Stream Impoundments
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Methane-Derived Carbon in the Benthic Food Web in Stream Impoundments John Gichimu Mbaka*, Celia Somlai, Denis Ko¨ pfer, Andreas Maeck, Andreas Lorke, Ralf B. Scha¨fer Institute for Environmental Sciences, University of Koblenz-Landau, Landau, Rhineland-Palatinate State, Germany Abstract Methane gas (CH4) has been identified as an important alternative source of carbon and energy in some freshwater food webs. CH4 is oxidized by methane oxidizing bacteria (MOB), and subsequently utilized by chironomid larvae, which may exhibit low d13C values. This has been shown for chironomid larvae collected from lakes, streams and backwater pools. 13 However, the relationship between CH4 concentrations and d C values of chironomid larvae for in-stream impoundments is unknown. CH4 concentrations were measured in eleven in-stream impoundments located in the Queich River catchment area, South-western Germany. Furthermore, the d13C values of two subfamilies of chironomid larvae (i.e. Chironomini and 13 Tanypodinae) were determined and correlated with CH4 concentrations. Chironomini larvae had lower mean d C values (2 29.2 to 225.5 %), than Tanypodinae larvae (226.9 to 225.3 %). No significant relationships were established between CH4 concentrations and d13C values of chironomids (p.0.05). Mean d13C values of chironomid larvae (mean: 226.8%, range: 2 29.2% to 225.3%) were similar to those of sedimentary organic matter (SOM) (mean: 228.4%, range: 229.3% to 227.1%) and tree leaf litter (mean: 229.8 %, range: 230.5% to 229.1%). We suggest that CH4 concentration has limited influence on the benthic food web in stream impoundments. Citation: Mbaka JG, Somlai C, Ko¨pfer D, Maeck A, Lorke A, et al. (2014) Methane-Derived Carbon in the Benthic Food Web in Stream Impoundments. PLoS ONE 9(10): e111392. doi:10.1371/journal.pone.0111392 Editor: David William Pond, Scottish Association for Marine Science, United Kingdom Received July 22, 2014; Accepted September 23, 2014; Published October 31, 2014 Copyright: ß 2014 Mbaka et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: JGM was funded by the German Academic Exchange service (DAAD) (Grant number: A/12/91652). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: [email protected] Introduction MOB have lower d13C values (typically ,240%; [14]), in comparison to organisms that feed on plant organic matter (232 Allochthonous and autochthonous plant organic matter are to 221 %; [15]). Bunn & Boon [16] determined the d13C values of major sources of carbon and energy for freshwater ecosystems [1]. invertebrates in backwater pools and found Chironominae larvae Recent studies have revealed that also methane, which can be to have d13C values (,235%) that were lower than for particulate produced by microbial degradation of organic matter under organic matter (229 to 225 %). Kiyashko et al. [18] and Jones & anoxic conditions, can significantly contribute to the carbon Grey [19] also reported lower (264 to 255 %) d13C values for budget of freshwater ecosystems [2]. Part of this gas is released to some chironomid larvae than for particulate organic matter in the atmosphere, where it contributes to the pool of green house lakes. The observed differences between d13C values of chirono- gases [3], or is oxidized by methane oxidizing bacteria (MOB) [4]. mid larvae and potential food resources led to the conclusion that The biogenic methane in MOB can contribute to the biomass of the chironomid larvae might have fed on MOB, which has very chironomid larvae [5]. Chironomids are one of the most dominant low d13C values. The carbon isotope composition of consumers invertebrate groups in the soft sediments in freshwater ecosystems (e.g. insects) is determined by their diet and usually potrays an and their larvae feed mainly on algae or allochthonous organic enrichment by about 1 %, even though the d13C can deviate from material and associated microroganisms [6]. CH derived organic 4 23 % to +3 % [20]. Given that methane is isotopically very carbon may constitute a crucial source of carbon and energy for distinct, stable carbon isotopes are particularly useful for tracing chironomids, in comparison to other aquatic invertebrates, methane derived carbon [17]. because their burrowing habit creates and exposes them to Although most existing studies on the importance of CH oxyclines at the sediment-water interface, where MOB density and 4 derived carbon in freshwater food webs mainly focused on lakes CH oxidation rates are usually high [7–10]. For example, 4 [2], a wide array of anoxic habitats with high potential for CH chironomid larvae collected from some lakes were sustained (up to 4 70%) by CH derived carbon [11]. production also exist in rivers and streams [21–23]. Particularly, 4 impoundments increases the residence time of water, promotes These quantitative estimates are based on the stable carbon 13 accumulation of organic matter and sediment, and have been isotope signature (d C) of CH4 which is highly depleted due to carbon isotopic fractionation related to methanogenesis [12]. identified as hot spots of CH4 emissions [24]. Maeck et al. [24] measured CH4 concentrations in riverine and impoundment Additionally, MOB that oxidize CH4 are usually characterized by further depletion in d13C [13]. Therefore, organisms that consume reaches and found sediment accumulation in dams to be the main PLOS ONE | www.plosone.org 1 October 2014 | Volume 9 | Issue 10 | e111392 d13C of Chironomids and Methane Gas Concentrations in Streams source of CH4 emissions. Gue´rin et al. [25] reported an increase in CH4 emissions at the downstream sides of impoundments as a result of release of water enriched with CH4. V T~ ðiÞ In shallow aquatic systems such as rice paddies and small lakes, Q CH4 has been shown to be an important source of energy in the benthic food webs [26–28]. In spite of the high abundance of small where: T is the water residence time, V is the volume of water in-stream ponds in smaller streams [29], the relationship between stored in the impoundment, and Q is the water discharge [32]. 13 CH4 concentrations and stable carbon isotope ratios (d C) of Nitrate and phosphate concentrations in stream water were chironomid larvae in such systems has not been examined. determined in the laboratory using Macherey-Nagel viscolor kits Globally, there exist millions of small impoundments (height , (Macherey-Nagel, Du¨ren, Germany). 15 m; [30]). Within this study, we assessed (i) CH4 concentrations in stream and pore-water and (ii) the relationship between CH4 CH4 concentrations concentrations and d13C values of chironomid larvae in impound- Concentrations of dissolved CH4 in stream and pore-water were ments located in the Queich River catchment area, South-western measured at the impoundments. Water samples were collected Germany. We hypothesized that CH4 would have a significant from each study impoundment using 20 mL serum bottles. The d13 influence on the C of chironomid larvae. samples for stream water CH4 analysis were collected by filling water to the sample bottles from the bottom to top, and Methods overflowing the sample bottles several times over. Three bottles were completely filled with water at each sampling site and several Ethics statement drops (250 mL) of mercuric chloride were added to each bottle as This study was conducted in the Queich River catchment area preservative [33]. The bottles were capped and sealed and (see coordinates below) and was not conducted in an area transported to the laboratory. A headspace was prepared by requiring research permit (e.g. national park) or private land. This replacing 10% of the bottle (i.e. 2 mL) with nitrogen gas. To study did not involve endangered or protected species. generate the headspace, each sample bottle was held upside down and a 20 gauge needle was inserted through the septum. Then Study area and sites 2 mL nitrogen gas was added to each bottle using a syringe, while The study was conducted in the Queich River catchment area, the replaced water sample escaped through the needle. The Rhineland-Palatinate State, South-western Germany. The Queich samples were manually shaken, for 1 minute, to equilibrate the gas River (length: 52 km) originates from the Palatinate forest between the headspace and the water [34]. The samples were o o o o 3 21 (49 1096 N750948 E) and flows (mean discharge: 1.31 m s ; analyzed using a CH4 analyser (Los Gatos Research Inc., www.geoportal-wasser.rlp.de) through the upper Rhine Valley to Mountain view, CA, U.S.A.). A closed loop was created between its confluence with the Rhine River in Germersheim (49o13939oN the gas inlet and outlet of the analyser. A gas tight syringe was then o o 2 8 2394 E). The catchment (area: 271 km ) is primarily covered by used to inject 0.5 mL gas sample into the closed loop. The CH4 sandstone and is between 100 m and 673 m above sea level. The concentrations were averaged over 30 seconds before and after gas Rhineland-Palatinate region has dry climate conditions in injection. Concentration of the injected gas was computed as: summer. Typical for most stream networks in central Europe, 67 small DcLosGatos(VLosGatoszVInjection) in-stream impoundments (www.geoportal-wasser.rlp.de) have been csample~ ðiiÞ VInjection constructed on the main stem and tributaries of the Queich River, South-western Germany, for various purposes such as hydropower where: csample is the mol fraction of the sampled gas in parts per generation and flood control.