Boreal Lake Sediments Research Articles

Redox conditions influence the chemical composition of iron-associated organic carbon in boreal lake sediments: A synchrotron-based NEXAFS study

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Québec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Seasonal variation observed in microplastic deposition rates in boreal lake sediments

PurposeThe sediment trap method allows measurements of vertical microplastic flux rate into sediments and provides comparable information of the spatial microplastic deposition rates. Such data are essential for comparison of the microplastic pollution rates in different sedimentary systems and for future risk assessments.Materials and methodsWe monitored microplastic fluxes using sediment traps in a boreal lake seasonally during 1 year. The sites represent different level of exposure to anthropogenic activities, from construction work to the open water site. Microplastic fluxes were compared to sediment characteristics (organic content) and sediment accumulation rates.Results and discussionThe highest annual microplastic deposition rate (2300 items m−2 year−1) was recorded at snow disposal site, a location where the snow collected from the city streets during winter is transported. The lowest rate was observed at the control site (660 items m−2 year−1) upstream from the city. Our results reveal the seasonal variation in microplastic deposition rates. In general, the highest microplastic flux rates were measured during growing season, accompanied with higher sedimentation rate. The low microplastic deposition rate during winter is likely explained by ice cover, frozen soil, and snow cover in the catchment. In contrast, microplastic concentration was higher in winter samples due to ceased sediment transport from catchment to lake. The sediment accumulation rate did not predict microplastic accumulation rate.ConclusionOur data suggest seasonal variation in microplastic deposition rates. The microplastic flux rates compared to their concentrations indicates that sites with high sedimentation rates can lead to underestimation of microplastic deposition and hence hamper recognition of hot spots.Graphical

The role of organic matter and microbial community controlling nitrate reduction under elevated ferrous iron concentrations in boreal lake sediments

The nitrogen availability, that affects the greenhouse gas emission and the trophic level of lakes, is controlled mainly by microbial processes. We measured in a boreal nitrate and iron rich lake how the rates of potential denitrification and dissimilatory nitrate reduction to ammonia (DNRA) are affected by degradability of organic matter and availability of aqueous ferrous iron. We also investigated the microbial community by using 16S rRNA gene and shotgun metagenomic sequencing approach, which allows taxonomic analyses and detection of metagenome-assembled genomes (MAGs) containing genes for both nitrate reduction and iron oxidation. The results show that truncated denitrification, leading to release of nitrous oxide, is favored over dinitrogen production in conditions where the degradability of the organic matter is low. DNRA rates were always minor compared to denitrification and appeared to be independent of the degradability of organic carbon. Reduced iron stimulated nitrate reducing processes, although consistently only DNRA. However, the proportion of MAGs containing DNRA genes was low suggesting chemistry driven stimulation by reduced iron. Nevertheless, the metagenomic analyses revealed unique taxa genetically capable of oxidizing iron and reducing nitrate simultaneously. Overall, the results highlight the spatial variability in microbial community and nitrous oxide emissions in boreal lake sediments.

Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modelled using reaction-transport equation vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Furthermore, we modelled solute porewater profiles reported in the literature for four other seasonally anoxic lake basins. A total of 17 independent porewater datasets are analyzed. CH4 and DIC production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting OM has a mean (± SD) carbon oxidation state (COS) value of -1.4±0.3. This value is much lower than the value of zero expected from carbohydrate fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM in hypolimnetic sediments and propose to include the COS in the formulation of OM fermentation in models applied to lake sediments to better quantify sediment CH4 outflux. This study highlights the potential of mass balancing the products of OM mineralization to characterize labile substrates undergoing fermentation in sediments.

The reconstruction of burned area and fire severity using charcoal from boreal lake sediments

Although lacustrine sedimentary charcoal has long been used to infer paleofires, their quantitative reconstructions require improvements of the calibration of their links with fire regimes (i.e. oc...

The reconstruction of burned area and fire severity using charcoal from boreal lake sediments

Although lacustrine sedimentary charcoal has long been used to infer paleofires, their quantitative reconstructions require improvements of the calibration of their links with fire regimes (i.e. occurrence, area, and severity) and the taphonomic processes that affect charcoal particles between the production and the deposition in lake sediments. Charcoal particles >150 µm were monitored yearly from 2011 to 2016 using traps submerged in seven head lakes situated in flat-to-rolling boreal forest landscapes in eastern Canada. The burned area was measured, and the above-ground fire severity was assessed using the differentiated normalized burn ratio (dNBR) index, derived from LANDSAT images, and measurements taken within zones radiating 3, 15, and 30 km from the lakes. In order to evaluate potential lag effects in the charcoal record, fire metrics were assessed for the year of recorded charcoal recording (lag 0) and up to 5 years before charcoal deposition (lag 5). A total of 92 variables were generated and sorted using a Random Forest-based methodology. The most explanatory variables for annual charcoal particle presence, expressed as the median surface area, were selected. Results show that, temporally, sedimentary charcoal accurately recorded fire events without a temporal lag; spatially, fires were recorded up to 30 km from the lakes. Selected variables highlighted the importance of burned area and fire severity in explaining lacustrine charcoal. The charcoal influx was thus driven by fire area and severity during the production process. The dispersion process of particles resulted mostly of wind transportation within the regional (<30 km) source area. Overall, charcoal median surface area represents a reliable proxy for reconstructing past burned areas and fire severities.

Identifying novel treeline biomarkers in lake sediments using an untargeted screening approach

Paleolimnology uses sedimentary biomarkers as proxies to reconstruct long-term changes in environmental conditions from lake sediment cores. This work describes an untargeted metabolomics-based approach and uniquely applies it to the field of paleolimnology to identify novel sediment biomarkers to track long-term patterns in treeline dynamics. We identified new potential biomarkers across the Canadian northern Arctic, non-alpine, treeline using high-resolution accurate mass spectrometry, and pattern recognition analysis. This method was applied to 120 sediment core extracts from 14 boreal, 25 forest-tundra, and 21 tundra lakes to assess long-term fluctuations in treeline position. High resolution accurate mass spectrometry resolved many compounds from complex mixtures with low mass accuracy errors. This generated a large dataset that required metabolomics styled statistical analyses to identify potential biomarkers. In total, 29 potential biomarkers discriminated between boreal and tundra lakes. Tetrapyrrole-type phorbides and squalene derivatives dominated in boreal regions, while biohopane-type lipids were in the tundra regions. Tetrapyrroles were in both surface and subsurface sediments of boreal lakes indicating these compounds can survive long-term burial in sediments. At the ecozone level, tetrapyrroles were more abundant in boreal Taiga Shield, and Taiga Plains. Boreal plant extracts belonging to Pinaceae and Ericaceae also contained tetrapyrroles. Squalene derivatives demonstrated long-term preservation, but wider distribution than tetrapyrroles. Hopanoids were present in tundra and forest-tundra lake regions, specifically the Low Arctic and Taiga Shield, and were absent in all boreal lake sediments. Herein, we describe a method that can systematically identify new paleolimnological biomarkers. Novel biomarkers would facilitate multi-proxy paleolimnological studies and potentially lead to more accurate paleoenvironmental reconstructions.

Ecotoxicity assessment of boreal lake sediments affected by metal mining: Sediment quality triad approach complemented with metal bioavailability and body residue studies

There are several methods for studying metal-contaminated freshwater sediments, but more information is needed on which methods to include in ecological risk assessment. In this study, we compliment the traditional Sediment Quality Triad (SQT) approach – including information on chemistry, toxicity and ecological status – with studies on metal bioavailability and metal body residues in local organisms.We studied four mining-affected boreal lakes in Finland by conducting chemical analyses of sediment and water, toxicity tests (L. variegatus, V. fischeri, C. riparius, L. stagnalis), and analysis of benthic organism community structure. In addition, we studied the relationships between metal loading, toxicity, metal bioavailability, and metal body residues in the field-collected biota.Chemistry and benthic organism community structures show adverse effects in those lakes, where the metal concentrations are the highest. However, toxicity was connected to low sediment pH during the experiment, rather than to high metal concentrations. Toxicity was observed in 4 out of 6 toxicity tests including growth test with L. variegatus, bulk sediment test with V. fischeri, and the L. stagnalis toxicity test. The C. riparius test did not show toxicity. Metal body residues in biota were not high enough to induce adverse effects (0.1–4.1 mg Cu/kg fw, 0.01–0.3 mg Ni/kg fw, 2.9–26.7 mg Zn/kg fw and 0.01–0.7 mg As/kg fw).Chemical analyses, metal bioavailability assessment and benthic community structures survey revealed adverse effects in the sediments, where metal concentrations are highest (Lake SJ and Lake KS). Standard toxicity tests were not suitable for studying acid, sulfide-rich sediments and, therefore, benthic structure study and chemical analyses are believed to give more reliable results of the ecological status of these sediments.

Methanogens and Iron-Reducing Bacteria: the Overlooked Members of Mercury-Methylating Microbial Communities in Boreal Lakes.

Methylmercury is a potent human neurotoxin which biomagnifies in aquatic food webs. Although anaerobic microorganisms containing the hgcA gene potentially mediate the formation of methylmercury in natural environments, the diversity of these mercury-methylating microbial communities remains largely unexplored. Previous studies have implicated sulfate-reducing bacteria as the main mercury methylators in aquatic ecosystems. In the present study, we characterized the diversity of mercury-methylating microbial communities of boreal lake sediments using high-throughput sequencing of 16S rRNA and hgcA genes. Our results show that in the lake sediments, Methanomicrobiales and Geobacteraceae also represent abundant members of the mercury-methylating communities. In fact, incubation experiments with a mercury isotopic tracer and molybdate revealed that only between 38% and 45% of mercury methylation was attributed to sulfate reduction. These results suggest that methanogens and iron-reducing bacteria may contribute to more than half of the mercury methylation in boreal lakes.IMPORTANCE Despite the global awareness that mercury, and methylmercury in particular, is a neurotoxin to which millions of people continue to be exposed, there are sizable gaps in the understanding of the processes and organisms involved in methylmercury formation in aquatic ecosystems. In the present study, we shed light on the diversity of the microorganisms responsible for methylmercury formation in boreal lake sediments. All the microorganisms identified are associated with the processing of organic matter in aquatic systems. Moreover, our results show that the well-known mercury-methylating sulfate-reducing bacteria constituted only a minor portion of the potential mercury methylators. In contrast, methanogens and iron-reducing bacteria were important contributors to methylmercury formation, highlighting their role in mercury cycling in the environment.

Resistant ammonia‐oxidizing archaea endure, but adapting ammonia‐oxidizing bacteria thrive in boreal lake sediments receiving nutrient‐rich effluents

SummaryClimate change along with anthropogenic activities changes biogeochemical conditions in lake ecosystems, modifying the sediment microbial communities. Wastewater effluents introduce nutrients and organic material but also novel microbes to lake ecosystems, simulating forthcoming increases in catchment loadings. In this work, we first used 16s rRNA gene sequencing to study how the overall sediment microbial community responds to wastewater in six boreal lakes. To examine forthcoming changes in the lake biogeochemistry, we focused on the ammonia‐oxidizing archaea (AOA) and bacteria (AOB), and examined their functional and compositional community response to wastewater. Although we found the least diverse and least resistant prokaryotic communities from the most wastewater‐influenced sediments, the community changed fast toward the natural composition with the diminishing influence of wastewater. Each lake hosted a unique resistant AOA community, while AOB communities were adapting, responding to environmental conditions as well as receiving new members from WWTPs. In general, AOB dominated in numbers in wastewater‐influenced sediments, while the ratio between AOA and AOB increased when moving toward pristine conditions. Our results suggest that although future climate‐change‐driven increases in nutrient loading and microbial migration might significantly disrupt lake sediment microbiomes, they can promote nitrification through adapting and abundant AOB communities.

High variability in iron-bound organic carbon among five boreal lake sediments

Being both stable carbon sinks and greenhouse gas sources, boreal lake sediments represent significant players in carbon (C) cycling. The release of dissolved organic carbon (DOC) into anoxic water is a widespread phenomenon in boreal lakes with impact on sediment C budgets. The association of OC with iron (Fe) is assumed to play an important role for this anoxic OC release via the dissimilatory reduction of Fe, but also to influence the stabilization of OC in sediments. To investigate the role of Fe–OC association for OC dynamics in different boreal lake sediments, we compared the content of Fe-bound OC [Fe–OC, defined as citrate bicarbonate dithionite (CBD) extractable OC] and the extent of reductive dissolution of solid-phase Fe and OC at anoxia. We found high among-lake variability in Fe–OC content, and while the amount of Fe–OC was high in three of the lakes (980–1920 µmol g−1), the overall contribution of Fe–OC to the sediment OC pool in all study lakes was not higher than 11%. No linkages between the amount of the Fe–OC pool and lake or sediment characteristics (e.g., pH, DOC concentration, sediment OC content, C:N ratio) could be identified. The observed release of OC from anoxic sediment may be derived from dissolution of Fe–OC in the lake sediments with high Fe–OC, but in other lake sediments, OC release during anoxia exceeded the sediment Fe–OC pool, indicating low contribution of reductive Fe dissolution to OC release from these lake sediments. The range of the investigated boreal lakes reflects the high variability in the size of the sediment Fe–OC pool (0–1920 µmol g−1) and CBD-extractable Fe (123–4050 µmol g−1), which was not mirrored in the extent of reductive dissolution of Fe (18.9–84.6 µmol g−1) and OC (1080–1700 µmol g−1) during anoxia, suggesting that Fe-bound OC may play a minor role for sediment OC release in boreal lakes. However, studies of redox-related OC cycling in boreal lake sediments should consider that the amount of Fe–OC can be high in some lakes.

Modeling the carbon isotope signatures of methane and dissolved inorganic carbon to unravel mineralization pathways in boreal lake sediments

Vertical profiles of the concentration and isotopic composition (δ13C) of methane (CH4) and dissolved inorganic carbon (DIC), as well as of ancillary parameters, were obtained in the top 25 cm of a sediment column in a seasonally anoxic basin from an oligotrophic boreal lake. Modeling the profiles of CH4 and DIC concentrations and those of their δ13C signatures with reaction-transport equations allowed us to determine the organic matter (OM) degradation rates according to various reactions and to constrain the in situ isotopic fractionation factors and diffusivity coefficients of CH4 and DIC. This exercise reveals inter alia that (i) CH4 production occurs below a depth of 5 cm, with the highest production rate between 5 and 7.5 cm depth, (ii) all CH4 is produced through hydrogenotrophy, and (iii) methanogenesis yields a production rate of CH4 about three times greater than that of DIC. This latter observation indicates either that fermentation of OM is not the exclusive source of H2 sustaining hydrogenotrophy, or that the commonly assumed model molecule CH2O does not adequately represent the fermenting OM, since its fermentation yields identical rates of CH4 and DIC production. The porewater profiles of Fe and SO42- suggest that some H2 may be produced during the reoxidation of reduced sulfur by Fe(III), but the rate of H2 production via this process, if active, would be insignificant in comparison to that required to sustain the estimated rate of hydrogenotrophy. We deduce that the imbalance between CH4 and DIC production rates is rather due to the fermentation of organic substrates that are more reduced than CH2O, i.e., having a negative average carbon oxidation state (COS). From the constraints on reaction rates and on fermentation pathways imposed by the δ13C data, we infer that the organic substrate fermenting between depths of 5 and 7.5 cm should have a COS of −1.87. We thus submit that CH4 is produced in the sediments of the seasonally anoxic basin of our boreal lake through hydrogenotrophy coupled to the fermentation of reduced organic substrates that can be represented by a mixture of fatty acids (e.g. C16H32O2; COS of −1.75) and fatty alcohols (e.g., C16H34O; COS of −2.00). This study emphasizes the importance of characterizing the sedimentary OM undergoing mineralization in order to improve diagenetic model predictions of CH4 cycling in boreal lakes and of its significance in climate change.

Microbial Community Response on Wastewater Discharge in Boreal Lake Sediments.

Despite high performance, municipal wastewater treatment plants (WWTPs) still discharge significant amounts of organic material and nitrogen and even microbes into the receiving water bodies, altering physico-chemical conditions and microbial functions. In this study, we examined how nitrified wastewater affects the microbiology of boreal lake sediments. Microbial community compositions were assessed with next generation sequencing of the 16S rRNA gene, and a more detailed view on nitrogen transformation processes was gained with qPCR targeting on functional genes (nirS, nirK, nosZI, nosZII, amoAarchaea, and amoAbacteria). In both of the two studied lake sites, the microbial community composition differed significantly between control point and wastewater discharge point, and a gradual shift toward natural community composition was seen downstream following the wastewater gradient. SourceTracker analysis predicted that ∼2% of sediment microbes were of WWTP-origin on the study site where wastewater was freely mixed with the lake water, while when wastewater was specially discharged to the sediment surface, ∼6% of microbes originated from WWTP, but the wastewater-influenced area was more limited. In nitrogen transformation processes, the ratio between nitrifying archaea (AOA) and bacteria (AOB) was affected by wastewater effluent, as the AOA abundance decreased from the control point (AOA:AOB 28:1 in Keuruu, 11:1 in Petäjävesi) to the wastewater-influenced sampling points, where AOB dominated (AOA:AOB 1:2–1:15 in Keuruu, 1:3–1:19 in Petäjävesi). The study showed that wastewater can affect sediment microbial community through importing nutrients and organic material and altering habitat characteristics, but also through bringing wastewater-originated microbes to the sediment, and may thus have significant impact on the freshwater biogeochemistry, especially in the nutrient-poor boreal ecosystems.

Widespread release of dissolved organic carbon from anoxic boreal lake sediments

Sediments in boreal lakes are important components of the carbon cycle because they receive and store large amounts of organic carbon (OC) and at the same time are a source of greenhouse gases. Under anoxic conditions, sediment OC can be lost through dissolution from the solid phase and subsequent diffusion to the water column. Although this process may have implications for sediment OC budgets, it has yet to be studied systematically. We combined laboratory sediment incubation experiments from 4 boreal lakes in central Sweden that differed widely in their biogeochemical conditions with data from the Swedish monitoring program covering >100 lakes to analyze the frequency of occurrence of sediment DOC loss in boreal lakes and identify lake characteristics and the conditions related to high DOC fluxes from anoxic sediments. We found DOC diffusion from anoxic sediments in all the anoxic sediment incubations but at different mean rates (0.7–3.7 mmol m−2 d−1). Similarly, 16 of 17 of the monitoring lakes that developed anoxic bottom water exhibited an increase in bottom-water DOC concentration, corresponding to a mean (and standard deviation) DOC diffusion flux from anoxic sediment of 11.1 (35.4) mmol m−2 d−1. The observed variability between lakes was related to particularly large DOC fluxes in humic-rich lakes, which we attribute to their low pH and high share of terrestrial OC favoring the formation of OC-iron aggregates. Accordingly, increasing pH might facilitate the dissolution of sediment OC because high dissolved OC fluxes from anoxic sediments were accompanied by high microbial iron and sulfate reduction, resulting in concomitant pH increase.

Preferential sequestration of terrestrial organic matter in boreal lake sediments

AbstractThe molecular composition and origin has recently been demonstrated to play a critical role in the persistence of organic matter in lake water, but it is unclear to what degree chemical attributes and sources may also control settling and burial of organic matter in lake sediments. Here we compared the annual contribution of allochthonous and autochthonous sources to the organic matter settling in the water column and present in the sediments of 12 boreal lakes. We used the fluorescence properties and elemental composition of the organic matter to trace its origin and found a consistent pattern of increasing contribution of terrestrial compounds in the sediments as compared to the settling matter, with an annual average allochthony of ~87% and ~57%, respectively. Seasonal data revealed a predominance of in‐lake‐produced compounds sinking in the water column in summer. Yet only a slight concurrent decrease in the contribution of terrestrial C to lake sediments was observed during the same period, and sediment allochthony increased again to high levels in autumn. Our results reveal a preferential preservation of allochthonous matter in the sediments and highlight the role of lakes as sequesters of organic carbon primarily originating from the surrounding landscape.

Molecular composition of organic matter controls methylmercury formation in boreal lakes

A detailed understanding of the formation of the potent neurotoxic methylmercury is needed to explain the large observed variability in methylmercury levels in aquatic systems. While it is known that organic matter interacts strongly with mercury, the role of organic matter composition in the formation of methylmercury in aquatic systems remains poorly understood. Here we show that phytoplankton-derived organic compounds enhance mercury methylation rates in boreal lake sediments through an overall increase of bacterial activity. Accordingly, in situ mercury methylation defines methylmercury levels in lake sediments strongly influenced by planktonic blooms. In contrast, sediments dominated by terrigenous organic matter inputs have far lower methylation rates but higher concentrations of methylmercury, suggesting that methylmercury was formed in the catchment and imported into lakes. Our findings demonstrate that the origin and molecular composition of organic matter are critical parameters to understand and predict methylmercury formation and accumulation in boreal lake sediments.

Rates and pathways of sedimentary organic matter mineralization in two basins of a boreal lake: Emphasis on methanogenesis and methanotrophy

AbstractSediment porewater was analyzed at several sampling dates in two adjacent basins of an oligotrophic boreal lake, one basin perennially oxygenated (Basin A) and the other occasionally anoxic (Basin B). Depth concentration profiles of methane (CH4), dissolved inorganic carbon (DIC), and electron acceptors were modeled with a one‐dimensional transport‐reaction equation to constrain the depth intervals (zones) where solutes are produced/consumed in the top 10 cm of the sediment column, and to obtain the net reaction rates in each zone. This multicomponent geochemical modeling reveals that CH4 was produced below 4–7 cm depth at lower rates in Basin A (250–800 fmol cm−2 s−1) than in Basin B (1900–6500 fmol cm−2 s−1) and that methanogenesis accounted for 30–64% and 84–100% of the sediment organic matter (OM) mineralization in Basins A and B, respectively. We show that methanogenesis did not always yield equimolar amount of CH4 and DIC, as would be expected from the fermentation of the model molecule CH2O. While ∼50% of the CH4 produced in Basin A is oxidized in the sediment column, this proportion decreases to ∼20% in Basin B. Dioxygen is by far the main electron acceptor for CH4 and OM oxidations in both basins. Methanotrophy in the sediment, however, is not limited to the ∼4‐mm thick surface layer in which O2 diffuses from bottom water but occurs down to 4–7 cm depth where O2 is transported through bioirrigation. Thermodynamic calculations suggest that, in addition to O2, Fe oxyhydroxides, and sulfate may serve as oxidants for methanotrophy in that zone. We predict that Basin B sediments release more CH4 than DIC whereas Basin A sediments mainly export DIC. This study highlights that small changes in hypolimnetic O2 levels may significantly alter the magnitude of OM mineralization pathways and the fate of CH4 in boreal lake sediments.

The role of sediments in the carbon budget of a small boreal lake

Inland waters are active sites of carbon (C) processing and emitters of carbon dioxide (CO2) and methane (CH4) to the atmosphere. In the boreal zone, where surface waters receive large quantities of organic carbon (OC) from surrounding forests and wetlands, lakes and streams act as strong sources of these greenhouse gases. Lake sediments provide the only long-term sink of C in boreal inland waters, through burial of OC. However, mineralization of OC counteracts the efficiency of lake sediments in removing C from the short-term C cycle. In this context, this thesis provides a better insight into the dual role of boreal lake sediments as C source and C sink.The presented work is based on empirical assessments of OC burial and OC mineralization rates in boreal lakes. The temporal variability of OC burial and the stability of the buried OC was assessed on both centennial and millennial timescales. The quantitative importance of sediment OC burial and mineralization in comparison both to other C fluxes within the lake, and to C fluxes within the tributary stream network, was quantified. By simulating the effect of climate change on water temperature, we also gauged the potential future efficiency of lake sediments in storing C.The results demonstrate that OC mineralization in sediments dominates three-fold over OC burial when observed at a whole-basin and annual scale. The contribution of sediment OC mineralization to annual C emission from the assessed study lake was, however, found to be small (16%), when compared to OC mineralization in the water column (37%) and catchment import of C (47%). Furthermore, C emission from headwater streams was found to dominate greatly over the lake C emission, mainly triggered by the higher gas transfer velocity of streams compared to lakes.On a long-term (Holocene) scale, the continuous OC burial flux results in a large amount of C stored in sediments. The temporal variability of this OC accumulation was found to vary across lakes, with, however, time-dependent patterns: On a millennial scale, smaller lakes exhibited a higher variability than larger lakes of the study area. For the last century, similar variability and a trend to increased OC accumulation was found for most study lakes, irrespective of their size. Analysis of lignin phenols in the accumulated OC did not indicated post-depositional degradation, independent of the age of the sediment OC, implying that sediments are a very stable sink for land-derived OC in boreal lakes.Simulation of warming water temperatures in boreal lakes resulted in declines of the OC burial efficiency BE (OCBE; OC burial/OCdeposition) up to 16%, depending, however, on basin morphometry. Predicted declines in OCBE were higher for the more shallow lake compared to the deeper lake.In conclusion, this thesis illustrates that sediments play, despite a small quantitative impact on aquatic C cycling, an important role as a very stable C sink in boreal lakes. However, the efficiency of this C sink is likely to be reduced in the future.

The effects of temperature and resource availability on denitrification and relative N2O production in boreal lake sediments

Anthropogenic environmental stressors (like atmospheric deposition, land use change, and climate warming) are predicted to increase inorganic nitrogen and organic carbon loading to northern boreal lakes, with potential consequences for denitrification in lakes. However, our ability to predict effects of these changes is currently limited as northern boreal lakes have been largely neglected in denitrification studies. The aim of this study was therefore to assess how maximum potential denitrification and N2O production rates, and the relationship between the two (relative N2O production), is controlled by availability of nitrate (NO3−), carbon (C), phosphorus (P), and temperature. Experiments were performed using the acetylene inhibition technique on sediments from a small, nutrient poor boreal lake in northern Sweden in 2014. Maximum potential denitrification and N2O production rates at 4°C were reached already at NO3− additions of 106–120μg NO3−–N/L, and remained unchanged with higher NO3 amendments. Higher incubation temperatures increased maximum potential denitrification and N2O production rates, and Q10 was somewhat higher for N2O production (1.77) than for denitrification (1.69). The relative N2O production ranged between 13% and 64%, and was not related to NO3− concentration, but the ratio increased when incubations were amended with C and P (from a median of 16% to 27%). Combined, our results suggests that unproductive northern boreal lakes currently have low potential for denitrification but are susceptible to small changes in NO3 loading especially if these are accompanied by enhanced C and P availability, likely promoting higher N2O production relative to N2.

Uncoupled organic matter burial and quality in boreal lake sediments over the Holocene

AbstractBoreal lake sediments are important sites of organic carbon (OC) storage, which have accumulated substantial amounts of OC over the Holocene epoch; the temporal evolution and the strength of this Holocene carbon (C) sink is, however, not well constrained. In this study we investigated the temporal record of carbon mass accumulation rates (CMARs) and assessed qualitative changes of terrestrially derived OC in the sediment profiles of seven Swedish boreal lakes, in order to evaluate the variability of boreal lake sediments as a C sink over time. CMARs were resolved on a short‐term (centennial) and long‐term (i.e., over millennia of the Holocene) timescale, using radioactive lead (210Pb) and carbon (14C) isotope dating. Sources and degradation state of terrestrially derived OC were identified and characterized by molecular analyses of lignin phenols. We found that CMARs varied substantially on both short‐term and long‐term scales and that the variability was mostly attributed to sedimentation rates and uncoupled from the OC content in the sediment profiles. The lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant OC source to the sediments over the Holocene. Furthermore, lignin‐based degradation indices, such as acid‐to‐aldehyde ratios, indicated that postdepositional degradation in the sediments was very limited on longer timescales, implying that terrestrial OC is stabilized in the sediments on a permanent basis.