Sediment methane dynamics along the Elbe River

Authors

Adam Bednařík, Martin Blaser, Anna Matoušů, Michal Tušer, Prem Prashant Chaudhary, Karel Šimek, Martin Rulík

Methane (CH₄) is an important atmospheric trace gas mostly released from wet anoxic soils and sediments. While many studies have focused on relatively homogenous environments like rice fields and lake sediments, the changing contribution of heterogeneous sediments e.g. along the longitudinal profile of a rivers has not been covered very frequently. Here we investigated sediment samples from 11 locations of the Elbe River. Sediments were incubated to measure methanogenic/methanotrophic potentials and contribution of individual methanogenic pathways using isotope analysis of δ¹³C. Additionally, we determined the diversity of the methanogenic communities (analysis of T-RFLP targeting the mcr-A gene in the sediment samples), while abundances of archaea, methanogens and methanotrophs were determined by qPCR. The CH₄ production was detected in six samples (out of 11 examined) and ranged from 0.12 to 644.72 nmol gDW^-1 d^-1. Methanotrophy was found in all examined sediment samples and ranged from 654 to 10,875 nmol gDW^-1 d^-1. Abundance of methanogens and methanotrophs (Mcr-A and pmo-A gene copy numbers) was not significantly different and quite stable around 10⁶ to 10⁷ copies gDW^-1. The group specific qPCR showed high fluctuations, while the highest counts were reported for Methanomicrobiales and Methanosarcinales (10⁵ to 10⁸ copies per gram dry sediment), followed by Methanobacteriales (10³ to 10⁵ copies per gram dry sediment). A significant proportion of unidentified methanogens was found in almost every locality. Isotope analysis of δ¹³C showed that (CH₄) is produced mainly by hydrogenotrophic methanogens. We see no trend in the studied parameters along the Elbe River. The molecular data showed no spatial characteristics, while we found hotspots of the measured CH₄ processes (CH₄ production and oxidation) due to other local driving factors (e.g. carbon content). Thus, out results indicate that the observed variability of the CH₄ production and oxidation rates is only indirectly linked to the presence or quantities of different microbial guilds.

On the calculation of lake metabolic rates: Diel O2 and 18/16O technique

Authors

Frank Peeters, Hilmar Hofmann, Jorge Encinas Fernández

Metabolic transformations have a major impact on the development of primary producers in aquatic systems and thus affect the dynamics of the entire aquatic food web. Furthermore, metabolic transformations contribute to the carbon budget and thereby influence CO₂ emissions from aquatic systems. Several techniques have been developed that aim at an easy assessment of metabolic rates over long time periods or in many systems. The ^18/16O technique, which utilizes the isotopic fractionation between ¹⁸O and ¹⁶O isotopes due to metabolic transformations, is receiving increasing popularity in studies comparing the metabolism in many different lakes and served as basis for the conclusions that production increases with increasing atmospheric CO₂ and that surprisingly little terrestrial carbon is recycled in lakes of the arid circumpolar landscape. However, we demonstrate here that the steady state assumptions underlying the ^18/16O technique cause large uncertainties in the estimated metabolic rates. This conclusion is based on a sensitivity analysis using a numerical model of dissolved oxygen, DO, and of dissolved ¹⁸O, ¹⁸O_DO, but is also confirmed by published metabolic rates estimated from the ^18/16O and the diel O₂ techniques. Metabolic rates obtained from the ^18/16O technique appear unsuited for correlation analyses between lakes but may provide reasonable estimates in systems with low and long-term stable production. In addition we illustrate that the combination of few ¹⁸O measurements with the diel O₂ technique and an inverse fitting procedure can improve estimates of metabolic rates and in particular of respiration rates.

Urea is both a carbon and nitrogen source for Microcystis aeruginosa: tracking 13C incorporation at bloom pH conditions

Authors

Lauren E. Krausfeldt, Abigail T. Farmer, Hector F. Castro Gonzalez, Brittany N. Zepernick, Shawn R. Campagna and Steven W. Wilhelm

The use of urea as a nitrogenous fertilizer has increased over the past two decades, with urea itself being readily detected at high concentrations in many lakes. Urea has been linked to cyanobacterial blooms as it is a readily assimilated nitrogen (N) – source for cyanobacteria that possess the enzyme urease. We tested the hypothesis that urea may also act as a carbon (C) source to supplemental growth requirements during the alkaline conditions created by dense cyanobacterial blooms, when concentrations of dissolved CO₂ are vanishingly low. High rates of photosynthesis markedly reduce dissolved CO₂ concentrations and drive up pH. This was observed in Lake Erie during the largest bloom on record (2015) over long periods (months) and short periods (days) of time, suggesting blooms experience periods of CO₂-limitation on a seasonal and daily basis. We used ¹³C-urea to demonstrate that axenic cultures of the model toxic cyanobacterium, Microcystis aeruginosa NIES843, assimilated C at varying environmentally relevant pH conditions directly into a spectrum of metabolic pools during urea hydrolysis. Primarily, ¹³C from urea was assimilated into central C metabolism and amino acid biosynthesis pathways, including those important for the production of the hepatotoxin, microcystin, and incorporation into these pathways was at a higher percentage during growth at higher pH. This corresponded to increased growth rates on urea as the sole N source with increasing pH. We propose this ability to incorporate C from urea represents yet another competitive advantage for this cyanobacterium during dense algal blooms.

Urea dynamics during Lake Taihu cyanobacterial blooms in China

Authors

Kaijun Lu, Zhanfei Liu, Ruihua Dai, Wayne S. Gardner

Lake Taihu, the third largest freshwater lake in China, suffers from harmful cyanobacteria blooms caused by Microcystis spp., which do not fix nitrogen (N). Reduced N (i.e., NH₄⁺, urea and other labile organic N compounds) is an important factor affecting the growth of Microcystis. As the world use of urea as fertilizer has escalated during the past decades, an understanding of how urea cycling relates to blooms of Microcystis is critical to predicting, controlling and alleviating the problem. In this study, the cycling rates of urea-N in Lake Taihu ranged from non-detectable to 1.37 μmol N L⁻¹ h⁻¹ for regeneration, and from 0.042 μmol N L⁻¹ h⁻¹ to 2.27 μmol N L⁻¹ h⁻¹ for potential urea-N removal. The fate of urea-N differed between light and dark incubations. Increased ¹⁵NH₄⁺ accumulated and higher quantities of the removed urea-¹⁵N remained in the ¹⁵NH₄⁺ form were detected in the dark than in the light. A follow-up incubation experiment with ¹⁵N-urea confirmed that Microcystis can grow on urea but its effects on urea dynamics were minor, indicating that Microcystis was not the major factor causing the observed fates of urea under different light conditions in Lake Taihu. Bacterial community composition and predicted functional gene data suggested that heterotrophic bacteria metabolized urea, even though Microcystis spp. was the dominant bloom organism.

Effects of catchment area and nutrient deposition regime on phytoplankton functionality in alpine lakes

Authors

Coralie Jacquemin, Céline Bertrand, Evelyne Franquet, Stéphane Mounier, Benjamin Misson, Benjamin Oursel, Laurent Cavalli

High mountain lakes are a network of sentinels, sensitive to any events occurring within their waterbodies, their surrounding catchment and their airshed. In this paper, we investigate how catchments impact the taxonomic and functional composition of phytoplankton communities in high mountain lakes, and how this impact varies according to the atmospheric nutrient deposition regime. For two years, we sampled the post snow-melt and the late summer phytoplankton, with a set of biotic and abiotic parameters, in six French alpine lakes with differing catchments (size and vegetation cover) and contrasting nitrogen (N) and phosphorus (P) deposition regimes. Whatever the nutrient deposition regime, we found that the lakes with the smallest rocky catchments showed the lowest functional richness of phytoplankton communities. The lakes with larger vegetated catchments were characterized by the coexistence of phytoplankton taxa with more diverse strategies in the acquisition and utilization of nutrient resources. The nutrient deposition regime appeared to interact with catchment characteristics in determining which functional groups ultimately developed in lakes. Photoautotroph taxa dominated the phytoplankton assemblages under high NP deposition regime while mixotroph taxa were even more favored in lakes with large vegetated catchments under low NP deposition regime. Phytoplankton functional changes were likely related to the leaching of terrestrial organic matter from catchments evidenced by analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios in seston and zooplankton. Plankton δ¹⁵N values indicated greater water–soil interaction in lakes with larger vegetated catchments, while δ¹³C values indicated the effective mineralization of the organic matter in lakes. The role played by catchments should be considered when seeking to determine the vulnerability of high altitude lakes to future changes, as catchments’ own properties will vary under changes related to climate and airborne contaminants.

Understanding transport and transformation of dissolved inorganic carbon (DIC) in the reservoir system using δ13CDIC and water chemistry

Authors

Wanfa Wang, Si-Liang Li, Jun Zhong, Cai Li, Yuanbi Yi, Sainan Chen, Yimeng Ren

In order to advance our understanding of the driving factors controlling carbon dynamics and evolution of water quality in river-reservoir systems for the generation of hydropower, we conducted a study in a karst deep-water reservoir (Wujiangdu Reservoir), southwest China. Water samples were collected from the inflow/outflow of the Wujiangdu Reservoir, four vertical columns along the reservoir, and from three tributaries to the reservoir in January, April, July, and October 2017. The dissolved inorganic carbon (DIC) concentrations and carbon isotope composition (δ¹³C_DIC) varied greatly (1.99–3.45 mmol/L and −10.7‰ to −6.0‰, respectively) and were controlled by multiple processes including CO₂outgassing, primary production, and organic matter degradation. In the four vertical profiles, the difference between the values of samples with those at 15 m of Δ[DIC] and Δ[δ¹³C_DIC], Δ[DIC] and ΔCa²⁺ in the same water column had positive correlations, and the variation in dissolved O₂, partial pressure of CO₂, and flux of CO₂ suggested that primary production dominated above the epilimnion and degradation of OM dominated below the epilimnion during the warm season. Continuous CO₂ outgassing was found from riverine water to surface water of the reservoir before the dam based on carbon isotopic compositions and water chemistry. These processes would cause isotopic fractionation between the residual DIC and CO₂ (aqueous), the degree of which varied among different seasons (July > April > October > January). Carbon transport and biogeochemical processes were highly controlled by the hydraulic retention time (HRT) in the river-reservoir system. It was estimated that approximately 71.5% of the annual DIC flux was produced in the water column below 15 m depth during the study year, suggesting that the processes of DIC generation and consumption occurred at the same time. The results highlight that carbon behavior in the impounded rivers is influenced by multiple processes. The carbon transformation processes should be taken into account for improving the estimation accuracy of the carbon budget calculations and the management of water quality for river-reservoir systems.

Methane Cycling Contributes to Distinct Patterns in Carbon Stable Isotopes of Wetland Detritus

Authors

Julia A. Hart, Carmella Vizza, William E. West, Dominic T. Chaloner, Stuart E. Jones, Gary A. Lamberti

Increasing global temperatures are changing the balance between carbon sequestration and its microbial processing in wetlands, making the tracking of these processes important. We used detrital carbon stable isotopes (δ¹³C) to trace aerobic decomposition and CH₄ production in two experiments conducted in Alaskan wetlands. In laboratory bottle incubations, larger decreases in detritus δ¹³C corresponded to higher net CH₄ and CO₂ production rates. Because net CH₄production was the stronger predictor and its effect was negative, we hypothesize that decreases in δ¹³C trace concurrent CH₄ production and oxidation. In a field experiment, decreases in detritus δ¹³C were not correlated with aerobic decomposition rates, but were positively correlated with CH₄ production potentials as estimated from bottle incubations. We hypothesize that the positive relationship reflects only CH₄ production, rather than concurrent production and oxidation. Although CH₄ production rates were correlated with changes in detrital δ¹³C in both experiments, the direction of this relationship differed between laboratory and field with important consequences for the scale of ecological experiments. Our study demonstrates that CH₄ cycling can create distinct patterns in δ¹³C of wetland detritus. Future studies should conduct explicit mass balance experiments to clarify mechanisms and determine the importance of scale in shaping isotopic patterns.

Seasonality of nitrate sources and isotopic composition in the Upper Illinois River

Authors

Jiajia Lin, John K. Böhlke, Sheng Huang, Miquel Gonzalez-Meler, Neil C. Sturchio

To improve understanding of spatial, seasonal, and inter-annual variations in nitrate sources and in-stream processes in the Illinois River system, nitrate concentrations and isotopic compositions were measured in 445 water samples collected over a four-year period (2004–2008) from the Upper Illinois River Basin (UIRB). Samples included surface water in the river and major tributaries, effluent samples from Chicago’s largest wastewater treatment plant (WTP), and representative groundwater from shallow wells in agricultural land. Two principal nitrate endmember sources within the UIRB had distinctive isotopic compositions: WTP effluent with δ¹⁵N = 8.6 ± 1.7‰ and δ¹⁸O = 0.8 ± 1.4‰ and agricultural groundwater with δ¹⁵N-NO₃ = 3.4 ± 0.6‰ and δ¹⁸O = 3.7 ± 0.5‰ (when minimally affected by nitrate reduction). Isotopic data indicated that the large pulse of nitrate exported from the river basin during the spring was mostly derived from agricultural land drainage, while nitrate from large WTP effluent point sources was predominant in the upper reaches of the river near Chicago. During low base-flow conditions in late-summer and fall, the agricultural nitrate source was greatly diminished and the headwater WTP source was predominant in the river basin export. Our results indicated biogeochemical nitrate reduction and isotopic fractionation occurred within the river network, affecting both agricultural and urban sources during surface-water transport. In addition, diminished agricultural nitrate export was attributable to preferential discharge of biogeochemically reduced groundwater during low base flow. Isotopic indicators of spatial and seasonal variations in the relative importance of different nitrate sources, and their relative susceptibility to natural attenuation, might be useful for guiding monitoring and management practices to reduce nitrate export from complex watersheds with mixed land uses.

Cumulative effects of cascade dams on river water cycle: Evidence from hydrogen and oxygen isotopes

Authors

Baoli Wang, Haitao Zhang, Xia Liang, Xiaodong Li, Fushun Wang

Cascade dams are known to influence the river water cycle, but their cumulative effects (CEs) are still not well understood. Water hydrogen (H) and oxygen (O) isotopes are hypothesized to be used to characterize the CEs. To test this hypothesis, we investigated water δD, δ¹⁸O, and related environmental factors in cascade reservoirs on the Wujiang River, Southwest China. The δD and δ¹⁸O ranged from −64.2‰ to −45.4‰ and from −9.7‰ to −6.8‰, respectively, and showed obvious temporal and spatial variations. Water temperature is an important factor influencing these variations. After damming, an increase of water retention time caused the enrichment of heavy H-O isotopes in reservoir surface water, and thermal stratification induced a decrease of δD and δ¹⁸O with depth. Due to bottom discharge, released water showed more negative δD and δ¹⁸O than reservoir surface water, and these δD and δ¹⁸O differences were controlled by water retention time and mean water depth of the reservoir. Overall, the CEs of cascade dams caused δD and δ¹⁸O to display a jagged increase from upstream to downstream in the impounded Wujiang River. Therefore H-O isotopes can be used to estimate the CEs of cascade dams. As cascade dams can modify H-O isotope signatures, caution should be exercised when using H and O isotopes to trace the source of the impounded river water.

How the Distribution of Anthropogenic Nitrogen Has Changed in Narragansett Bay (RI, USA) Following Major Reductions in Nutrient Loads

Authors

Autumn Oczkowski, Courtney Schmidt, Emily Santos, Kenneth Miller, Alana Hanson, Donald Cobb, Jason Krumholz, Adam Pimenta, Leanna Heffner, Sandra Robinson, Joaquín Chaves, Rick McKinney

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here, we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ¹⁵N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ¹³C values measured in particulate matter and hard clams have declined by about the same amount. The δ¹⁵N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ¹³C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.