Lake CO2 Research Articles

The Relative Contribution of Winter Under-Ice and Summer Hypolimnetic CO2 Accumulation to the Annual CO2 Emissions from Northern Lakes

We explored the whole-lake accumulation of CO2 during winter ice cover and in the hypolimnion during summer stratification, in 15 temperate and boreal lakes, and how these processes vary with lake trophic state and morphometry. We further estimated an annual CO2 budget for each lake that incorporates the fluxes resulting from winter ice-cover and summer hypolimnetic CO2 accumulation to assess their relative importance. The volumetric rates of CO2 accumulation during the winter ice cover ranged from 4.1 to 42.1 mg C m−3 d−1, and from 9.3 to 54.5 mg C m−3 d−1 in the summer hypolimnion, and both varied mainly as a function of water temperature. The total CO2 accumulation at the end of winter/summer was most strongly related to lake mean depth and was significantly greater under ice than in the hypolimnion. This greater CO2 accumulation resulted in relatively high and unvarying spring outflux, averaging 404 mg C m−2 d−1 (C.V. 27%). Significant hypolimnetic CO2 accumulation also resulted in relatively high fluxes (average 228 mg C m−2 d−1) during autumnal mixing, which were more variable (C.V. 43%) than spring fluxes. Average fluxes were lowest and most variable in summer (198 mg C m−2 d−1, C.V. 46%). The net annual CO2 flux ranged from 14 to 68 g C m−2 y−1, and was positively related to DOC concentration. The winter ice-cover CO2 accumulation accounted from 3 to 80% (average 17%) of the annual CO2 flux, whereas summer hypolimnetic accumulation accounted for a smaller but still significant proportion (from 1.4 to 46%). These winter and summer hypolimnetic CO2 accumulations are released to the atmosphere through short but intense emission bursts that are rarely captured in regular lake sampling schemes, yet they have the potential to profoundly influence the annual lake CO2 budgets in northern landscapes.

Upscaling carbon dioxide emissions from lakes

AbstractQuantifying CO2 fluxes from lakes to the atmosphere is important for balancing regional and global‐scale carbon budgets. CO2 emissions are estimated through statistical upscaling procedures that aggregate data from a large number of lakes. However, aggregation can bias flux estimates if the physical and chemical factors determining CO2 exchange between water and the atmosphere are not independent. We evaluated the magnitude of aggregation biases with moment expansions and pCO2 data from 5140 Swedish lakes. The direction of the aggregation bias depends on lake size; mean flux was overestimated by 4% for small lakes (0.01–0.1 km2) but underestimated by 13% for large lakes (100–1000 km2). Simple covariance‐based correction factors were generated to adjust for upscaling biases. These correction factors represent an easily interpretable and implemented approach to improving the accuracy of regional and global estimates of lake CO2 emissions.

Large-scale patterns in summer diffusive CH4 fluxes across boreal lakes, and contribution to diffusive C emissions.

Lakes are a major component of boreal landscapes, and whereas lake CO2 emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake CH4 fluxes. Here, we present a large-scale study of the magnitude and regulation of boreal lake summer diffusive CH4 fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH4 and CO2 in 224 lakes across a wide range of lake type and environmental gradients in Québec. The diffusive CH4 flux was highly variable (mean 11.6±26.4 SD mgm(-2) d(-1) ), and it was positively correlated with temperature and lake nutrient status, and negatively correlated with lake area and colored dissolved organic matter (CDOM). The relationship between CH4 and CO2 concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH4 flux but does not significantly affect CO2 flux, whereas increasing CDOM concentration leads to higher CO2 flux but lower CH4 flux. CH4 contributed to 8±23% to the total lake C emissions (CH4 +CO2 ), but 18±25% to the total flux in terms of atmospheric warming potential, expressed as CO2 -equivalents. The incorporation of ebullition and plant-mediated CH4 fluxes would further increase the importance of lake CH4 . The average Q10 of CH4 flux was 3.7, once other covarying factors were accounted for, but this apparent Q10 varied with lake morphometry and was higher for shallow lakes. We conclude that global climate change and the resulting shifts in temperature will strongly influence lake CH4 fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning.

Rising CO2 levels will intensify phytoplankton blooms in eutrophic and hypertrophic lakes.

Harmful algal blooms threaten the water quality of many eutrophic and hypertrophic lakes and cause severe ecological and economic damage worldwide. Dense blooms often deplete the dissolved CO2 concentration and raise pH. Yet, quantitative prediction of the feedbacks between phytoplankton growth, CO2 drawdown and the inorganic carbon chemistry of aquatic ecosystems has received surprisingly little attention. Here, we develop a mathematical model to predict dynamic changes in dissolved inorganic carbon (DIC), pH and alkalinity during phytoplankton bloom development. We tested the model in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa at different CO2 levels. The experiments showed that dense blooms sequestered large amounts of atmospheric CO2, not only by their own biomass production but also by inducing a high pH and alkalinity that enhanced the capacity for DIC storage in the system. We used the model to explore how phytoplankton blooms of eutrophic waters will respond to rising CO2 levels. The model predicts that (1) dense phytoplankton blooms in low- and moderately alkaline waters can deplete the dissolved CO2 concentration to limiting levels and raise the pH over a relatively wide range of atmospheric CO2 conditions, (2) rising atmospheric CO2 levels will enhance phytoplankton blooms in low- and moderately alkaline waters with high nutrient loads, and (3) above some threshold, rising atmospheric CO2 will alleviate phytoplankton blooms from carbon limitation, resulting in less intense CO2 depletion and a lesser increase in pH. Sensitivity analysis indicated that the model predictions were qualitatively robust. Quantitatively, the predictions were sensitive to variation in lake depth, DIC input and CO2 gas transfer across the air-water interface, but relatively robust to variation in the carbon uptake mechanisms of phytoplankton. In total, these findings warn that rising CO2 levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters.

Whole-Lake CO2 Dynamics in Response to Storm Events in Two Morphologically Different Lakes

In lentic systems, hydrology can be dramatically altered after storm events, potentially modifying the carbon budget. In particular, rapid increases in the surface water carbon dioxide partial pressure (pCO2) have been observed following such events. Several processes may explain these shifts in lake CO2 dynamics, including vertical mixing, increases in metabolism, and increases in external loading. To evaluate the relative importance of these various processes, we reconstructed the whole-lake daily CO2 budget using concurrent estimates of lake metabolism and daily CO2 mass balance budgets in two lakes with distinct morphometries located in Quebec, Canada. We found that storm events caused variable, but significant, changes in whole-lake CO2 mass. Such events influenced CO2 dynamics indirectly by inducing shifts in lake metabolism, and directly by importing CO2 by the inflowing storm waters. Storm intensity (in terms of total amount of precipitation) influences the balance between these two processes, but the final outcome depends on lake morphometry. Our results suggest that when storms are intense enough to drive lake water renewal rate beyond 1% day−1, external CO2 loadings became the dominant process, overwhelming internal CO2 production. Lakes with slower hydrological turnover, however, are more susceptible to internal regulation and may simply re-allocate CO2 from the hypolimnion to the epilimnion following a storm event. Our results thus suggest that this tightening of the watershed-lake-atmosphere linkage by climatic events is strongly modulated by lake morphometry. These features should be considered when predicting the impact of future climate change on regional C budgets and emissions.

Methane and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH4 ) and carbon dioxide (CO2 ) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well-defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH4 and CO2 in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH4 and a major fraction also emitted CO2 . The total CH4 flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1mmolm(-2) d(-1) , with a mean of 7.8±12.7 (mean±1 SD) mmolm(-2) d(-1) . The mean surface water CH4 concentration was 3.8±14.5μm (range 0.03-92.1μm). The CO2 fluxes ranged from -28.2 to 262.4mmolm(-2) d(-1) and the mean flux was 51.9±71.1mmolm(-2) d(-1) . The mean partial pressure of CO2 was 2927±3269μatm (range: 400-11467μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH4 yr(-1) and 22.0 Tg CO2 yr(-1) from India's inland waters. When expressed as CO2 equivalents, this amounts to 75 Tg CO2 equivalentsyr(-1) (53-98 Tg CO2 equivalentsyr(-1) ; ±1 SD), with CH4 contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30-55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments.

High emission of carbon dioxide and methane during ice thaw in high latitude lakes

The winter period is seldom included in the estimates of aquatic‐atmospheric carbon exchange. In this study we quantified the flux of carbon dioxide (CO2) and methane (CH4) over 3 years from 12 small subarctic lakes. The lakes accumulated consistent and high amounts of CO2 and CH4 (56–97% as CO2) over the winter, resulting in a high flux during ice thaw. The CO2 flux during ice thaw increased with increasing mean depth of the lakes, while the CH4 flux was high in lakes surrounded by mires. The ice thaw period was quantitatively important to the annual gas balances of the lakes. For nine of the lakes, 11 to 55% of the annual flux occurred during thaw. For three of the lakes with an apparent net annual CO2 uptake, including the thaw period reversed the balance from sink to source. Our results suggest that the ice thaw period is critically important for the emissions of CO2 and CH4 in small lakes.

Catchment productivity controls CO2 emissions from lakes

Most lakes are net sources of CO2; conventionally the CO2 in lake waters is attributed to in-lake oxidation of terrestrially-produced dissolved organic carbon. Now research indicates that CO2 in lakes may be delivered directly via inflowing streams. These findings suggest that future CO2 emissions from lakes will be strongly related to productivity in the lake catchment. Most lakes are oversaturated with CO2 and are net CO2 sources to the atmosphere, yet their contribution to the global carbon cycle is poorly constrained1,2,3,4. Their CO2 excess is widely attributed to in-lake oxidation of terrestrially produced dissolved organic carbon5. Here we use data collected over 26 years to show that the CO2 in 20 lakes is primarily delivered directly through inflowing streams rather than being produced in situ by degradation of terrestrial carbon. This implies that high CO2 concentrations and atmospheric emissions are not necessarily symptoms of heterotrophic lake ecosystems. Instead, the annual mean CO2 concentration increased with lake productivity and was proportional to the estimated net primary productivity of the catchment. Overall, about 1.6% of net primary productivity (range 1.2–2.2%) was lost to the atmosphere. Extrapolating globally, this is equivalent to CO2 losses of ∼0.9 Pg C yr−1 (range 0.7–1.3), consistent with existing estimates. These data and our catchment productivity hypothesis re-enforce the high connectivity found between lakes, their catchment and the global C cycle6. They indicate that future concentrations of CO2 in lakes, and losses to the atmosphere, will be highly sensitive to altered catchment management and concomitant effects of climate change that modify catchment productivity.

Climate, catchment runoff and limnological drivers of carbon and oxygen isotope composition of diatom frustules from the central Andean Altiplano during the Lateglacial and Early Holocene

Diatom-based carbon and oxygen isotope analyses (δ13Cdiatom and δ18Odiatom) were performed on diatom-rich laminated sediments of Lake Chungará (Andean Altiplano, northern Chile) deposited during the Lateglacial and Early Holocene (12,400–8300 cal years BP) to reconstruct climate change and environmental response across this major climate transition. The δ13Cdiatom data show both centennial-to-millennial scale changes related to fluctuations in lake productivity and CO2 concentration in the lake water, and variations in carbon sources to the lake through time. The δ18Odiatom data reflect changes in lake hydrology and climate. The combination of δ13Cdiatom and δ18Odiatom data reveals interactions between the internal lake processes and its catchment runoff. During wet periods (low δ18Odiatom values) δ13Cdiatom indicates an enhanced contribution of allochthonous carbon, whereas during dry periods (high δ18Odiatom) δ13Cdiatom values suggest more autochthonous carbon production and recycling. These decadal-to-centennial oscillations are not recognized after 10,000 cal years BP, possibly as a result of ENSO-like phenomenon weakening. Humid conditions during the Lateglacial–Early Holocene transition (12,400–10,100 cal years BP) were possibly due to the establishment of La Niña-like conditions in the tropical South Pacific. Whereas, dry conditions in the Early Holocene (10,100–9100 cal years BP) may be caused by the northward migration of the ITCZ due to both ENSO-like weakening and an insolation minimum. Finally, a return to humid conditions at the end of the Early Holocene (9100–8300 cal years BP) is coincident with an increase in summer insolation.

Carbon dioxide supersaturation promotes primary production in lakes

A majority of the world's lakes are supersaturated with respect to carbon dioxide (CO(2) ). By experimental manipulation of the CO(2) concentration in supersaturated boreal lakes, we demonstrate that phytoplankton primary production was up to 10 times higher in supersaturated lake water in comparison with water with CO(2) at equilibrium concentrations and that CO(2) , together with nutrients, explained most of the variation in pelagic primary production and phytoplankton biomass over a wide variety of unproductive lakes. These results suggest that phytoplankton can be co-limited by CO(2) and nutrients in unproductive lakes. As import of terrestrial organic carbon and its subsequent microbial mineralisation in lakes is a driving force of CO(2) -supersaturation our results suggest that lake productivity and carbon cycling may respond to variations in terrestrial organic carbon export, (e.g. caused by land use or climate change) in ways not described before.

Changes in benthic macroinvertebrate abundance and lake isotope (C, N) signals following biomanipulation: an 18-year study in shallow Lake Vaeng, Denmark

Change in the abundance of benthic macroinvertebrates and the stable isotope composition (C, N) of benthic invertebrates and zooplankton in Lake Vaeng, Denmark, was investigated over an 18-year period following biomanipulation (removal of cyprinids). During the first nine years after biomanipulation, the lake was clear and submerged macrophytes were abundant; after this period, a shift occurred to low plant abundance and high turbidity. Two years after the biomanipulation, total density of benthic macroinvertebrates reached a maximum of 17042 (±2335 SE) individuals m−2 and the density was overall higher when the lake was in a clear state. Redundancy analysis (RDA) suggested macrophyte abundance and total nitrogen (TN) concentration were the dominant structuring forces on the benthic macroinvertebrate assemblage. Stable isotope analysis revealed that δ13C of macroinvertebrates and zooplankton was markedly higher in years with high submerged macrophyte abundance than in years without macrophytes, most likely reflecting elevated δ13C of phytoplankton and periphyton mediated by a macrophyte-induced lowering of lake water CO2 concentrations. We conclude that the strong relationship between macrophyte coverage and δ13C of macroinvertebrates and cladocerans may be useful in paleoecological studies of past changes in the dynamics of shallow lakes, as change in macrophyte abundance may be tracked by the δ13C of invertebrate remains in the sediment.

Seasonal Dynamics of CO2 Flux Across the Surface of Shallow Temperate Lakes

We used data collected from 1989 to 2009 from 151 shallow (mean depth < 3 m) temperate lakes in Denmark to explore the influence of lake trophic status, surface area and catchment size on the seasonal dynamics of the air–water flux of CO2. Monthly CO2 fluxes were derived from measurements of acid neutralizing capacity (ANC), pH, ionic strength, temperature, and wind speed. CO2 fluxes exhibited large seasonal variability, in particular in oligo-mesotrophic lakes. Most of the lakes emitted CO2 during winter (median rates ranging 300–1,900 mg C m−2 day−1), and less CO2 during summer or, in the case of some of the highly eutrophic lakes, retained CO2 during summer. We found that seasonal CO2 fluxes were strongly negatively correlated with pH (r = −0.65, P < 0.01), which in turn was correlated with chlorophyll a concentrations (r = 0.48, P < 0.01). Our analysis suggests that lake trophic status (a proxy for pelagic production) interacts with the lake ANC to drive the seasonal dynamics of CO2 fluxes, largely by changing pH and thereby the equilibrium of the free CO2 and bicarbonate relation. Long-term observations from four lakes, which have all undergone a period of oligotrophication during the past two decades, provide further evidence that CO2 efflux generally increases as trophic status decreases, as a consequence of decreased pH. Across these four lakes, the annual average CO2 emission has increased by 32% during the past two decades, thus, demonstrating the strong link between lake trophic status and CO2 flux.

Carbon dioxide concentrations in eutrophic lakes: undersaturation implies atmospheric uptake

Understanding concentrations and contributions of carbon dioxide (CO2) in aquatic ecosystems is an important part of a comprehensive global carbon budget. Current dogma suggests that world lakes are important emitters of CO2 to the atmosphere. We estimated the partial pressure of carbon dioxide (ρCO2) in 131 agriculturally eutrophic lakes over a 7 year sampling period. Values of ρCO2 in these lakes ranged from 0.1 to 40 392 μatm with a median of 322 μatm (n = 3049). In contrast to previous analyses of CO2 in lakes, 60% of the eutrophic lake samples were undersaturated with CO2. Correlation analysis implied that nutrient-driven primary production, reflected by high oxygen concentrations, drives CO2 concentrations below atmospheric equilibrium. Multiple regression analysis showed several limnological and catchment characteristics that explained a statistically significant amount of variability in ρCO2 (R2 = 0.32). Important variables included chlorophyll a concentration and the ratio of total nitrogen to total phosphorus. Our estimated ρCO2 values were significantly (p < 0.0001) lower than a previously published dataset of world lake ρCO2 values derived primarily from oligotrophic–mesotrophic lakes. High-nutrient lakes, especially those that are small and rich in oxygen from primary production, could act as net atmospheric CO2 uptake sites.

Linking forest fires to lake metabolism and carbon dioxide emissions in the boreal region of Northern Québec

AbstractNatural fires annually decimate up to 1% of the forested area in the boreal region of Québec, and represent a major structuring force in the region, creating a mosaic of watersheds characterized by large variations in vegetation structure and composition. Here, we investigate the possible connections between this fire‐induced watershed heterogeneity and lake metabolism and CO2 dynamics. Plankton respiration, and water–air CO2 fluxes were measured in the epilimnia of 50 lakes, selected to lie within distinct watershed types in terms of postfire terrestrial succession in the boreal region of Northern Québec. Plankton respiration varied widely among lakes (from 21 to 211 μg C L−1 day−1), was negatively related to lake area, and positively related to dissolved organic carbon (DOC). All lakes were supersaturated in CO2 and the resulting carbon (C) flux to the atmosphere (150 to over 3000 mg C m2 day−1) was negatively related to lake area and positively to DOC concentration. CO2 fluxes were positively related to integrated water column respiration, suggesting a biological component in this flux. Both respiration and CO2 fluxes were strongly negatively related to years after the last fire in the basin, such that lakes in recently burnt basins had significantly higher C emissions, even after the influence of lake size was removed. No significant differences were found in nutrients, chlorophyll, and DOC between lakes in different basin types, suggesting that the fire‐induced watershed features influence other, more subtle aspects, such as the quality of the organic C reaching lakes. The fire‐induced enhancement of lake organic C mineralization and C emissions represents a long‐term impact that increases the overall C loss from the landscape as the result of fire, but which has never been included in current regional C budgets and future projections. The need to account for this additional fire‐induced C loss becomes critical in the face of predictions of increasing incidence of fire in the circumboreal landscape.

Climate and CO2 saturation in an alpine lake throughout the Holocene

This study shows that diatom sediment records can be used to investigate the long‐term inorganic carbon dynamics in oligotrophic and poorly acid‐buffered lakes. Using a training set of 115 high‐mountain lakes in the Pyrenees, we found that both alkalinity and potential hydrogen (pH) independently explained some of the variability in diatom assemblages. Transfer functions for both variables were developed and applied to a Holocene record from Lake Redon and CO2 changes calculated. CO2 saturation broadly followed alkalinity, which in turn was related to summer and autumn air‐temperature fluctuations. In general, warmer climate during the ice‐free period led to higher supersaturation, due to increased alkalinity, which facilitated retention of CO2 from respiration, and decreased primary production (assessed by diatom fluxes). Only during the early Holocene, there were periods of extreme undersaturation, corresponding to cold periods of low alkalinity (&lt;20 microequivalents per liter [µeq L−1]), and suggesting carbon limitation of primary production. The winter and spring climate, which determines the ice cover duration, appears to be relevant for CO2 saturation only during periods when the organic‐matter content of the sediments was low (&lt;22%). Longer periods of ice cover led to lower lake CO2 saturation, suggesting that the ice cover influence on internal nutrient loading may regulate lake productivity fluctuations under low allocthonous nutrient and organic‐matter inputs. Alkalinity ~20 µeq L−1 and sediment organic matter ~22% appear as critical thresholds in the way lake CO2 levels respond to climate fluctuations.

Controls of organic and inorganic carbon in randomly selected Boreal lakes in varied catchments

Organic and inorganic carbon concentrations in lakes and the links to catchment and water quality were studied in variable landscapes using the Finnish Lake Survey data base including 874 randomly selected lakes sampled during autumn overturn. The median total organic carbon (TOC) in these boreal lakes was 7.8 mg l−1, the median total inorganic carbon (TIC) 1.6 mg l−1 and the median partial pressure of CO2 (pCO2) 900 μatm. When the data was divided into subgroups according to land use in the catchment, the proportion of TIC of the total carbon (TC) in lakes was highest (31%) in agricultural areas and lowest (10%) in peatland areas. Elevated TIC concentrations were associated with agricultural land in the catchment, whereas elevated TOC concentrations were observed in lakes with high peatland proportion in the catchment. Two contrasting important sources of CO2 in lakes were identified on the basis of statistical analysis of the data; weathering processes in the catchments and decomposition of organic matter. CO2 was also strongly associated with total nutrients TN and TP, implying the importance of quality of organic matter and availability of nutrients for the decomposition processes.

Interannual variation and climatic regulation of the CO2 emission from large boreal lakes

AbstractWe studied the interannual variation of surface water partial pressure of CO2 (pCO2) and the CO2 emissions from the 37 large Finnish lakes linking them to the water quality, catchment and climate attributes in 1996–2001. The lake water CO2 was measured three times a year in the study lakes in 1998 and 1999 and for the rest of the years the CO2 was modeled by measured alkalinity. The median annual CO2 emission to the atmosphere ranged between 1.49 and 2.29 mol m−2 a−1. The annual CO2 emission followed closely the annual precipitation pattern with the highest emission during the years when the precipitation was highest (r2=0.81–0.97, P&lt;0.05). There was a strong negative correlation (r2=0.50–0.82, P&lt;0.001) between O2 and CO2 saturation in the lake water during stratification suggesting effective decomposition of organic matter in the lakes. Furthermore, total phosphorus and the proportion of agricultural land in the catchment had significant positive correlations with CO2 saturation.

Temperature independence of carbon dioxide supersaturation in global lakes

A growing body of evidence suggests that most of the world's lakes are supersaturated with CO2 and export significant amounts of CO2 to the atmosphere. Still, the temperature dependence of the partial pressure of CO2 (pCO2) in lakes, which is the main driver of carbon flux across the air‐water interface, has not yet been assessed. Analyzing a global‐scale database of 4902 lakes, we show that temperature is not an important regulator of pCO2 in lakes. Instead, the concentration of dissolved organic carbon (DOC), a substrate for microbial respiration, explains significant variation in lake pCO2. Contrary to what may be expected from the physiological constraints of temperature, effects of climate change on the carbon balance of lakes may not be due to rising temperature per se, but rather to climatically induced changes in the export of DOC from terrestrial soils to aquatic habitats.

Production of methane from alasses in eastern Siberia: Implications from its 14C and stable isotopic compositions

The radiocarbon (14C) and stable isotopic (13C and D) compositions of methane and carbon dioxide from alasses (typical landforms in permafrost terrain, consisting of lakes and wetlands) were measured around Yakutsk, eastern Siberia, where few isotope studies have been done. The carbon isotopic compositions of methane and carbon dioxide were used to study the pathways of methane formation in alasses. The mean value of methane and of carbon dioxide in each alass ranged from −63.9 to −58.2‰ and from −16.7 to −0.6‰, respectively. In small alasses, where the supply of fresh organic matter from surrounding wetland ecosystems is large, methane was produced almost equally from acetate fermentation and CO2 reduction pathways. In larger alasses the CO2 reduction pathway slightly dominates over acetate fermentation, owing to a smaller supply of labile organic matter. The 13C enrichment in CO2 in large lakes indicates depletion of methane precursors. Gas‐bubble methane is depleted in deuterium (the mean δD value from lakes is −360 ± 14‰ and from shores is −380 ± 20‰), which reflects the deuterium‐depleted environmental water (from −136 to −117‰) of alasses. Lake methane relatively enriched in D is interpreted to be the result of depletion in the hydrogen pool in lake sediments. Methane in shallow lakes is somewhat enriched in 14C relative to modern biogenic methane and is produced from fresh, recently fixed organic matter. The 14C content of methane from deeper lakes is 10–20% less than that of shallow lake methane, indicating a greater contribution from older methane derived from deeper parts of the sediment. The mean 13C, D, and 14C of methane from the alasses in general are estimated to be −61.1 ± 4.4‰, −363 ± 20‰, and 104 ± 6 pMC, respectively. This corresponds to the reported mean isotopic composition of methane from wetlands for δ13C, but shows lower value for δD and 14C content.

Avoid Bumpy Road to Removing Lake's CO2; Just Pay the Piper

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