We performed a series of gas exchange measurements in 12 diverse aquatic systems to develop the direct relationship between near-surface turbulence and gas transfer velocity. The relationship was ...log-linear, explained 78% of the variation in instantaneous gas transfer velocities, and was valid over a range of turbulent energy dissipation rates spanning about two orders of magnitude. Unlike wind-based relationships, our model is applicable to systems ranging in size from less than 1 km² to over 600 km². Gas fluxes measured with our specific model of floating chambers can be grossly overestimated (by up to 1000%), particularly in low-turbulence conditions. In high-turbulence regimes, flux overestimation decreases to within 50%. Direct measurements of turbulent energy dissipation rate provide reliable estimation of the associated gas transfer velocity even at short temporal and spatial scales.
This essay describes the evolution of our understanding of the carbon cycle of inland waters. Research has evolved from studies of individual lakes with limited attention to the surrounding ...landscapes, to a focus on how lakes are affected by external factors such as import of organic matter from the watershed, thereafter increasingly addressing how inland waters impact the carbon cycle beyond their own limits, for example by emission of gases to the atmosphere. Major steps are described toward the now widely applied concept of the aquatic “active pipe,” and the development of global quantification of inland water carbon cycling. Despite the great progress in understanding of the carbon cycle during the last decades, we argue that there is still a need for better integration of inland waters with other habitats in studies of carbon biogeochemistry.
Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH4) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of ...isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH4 emission and isotopic signature. Here, we examine the magnitude of CH4 ebullition (bubbling) and stable carbon isotopic signature (δ13C-CH4) of bubble CH4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH4 flux and bubble δ13C-CH4 varied from 0.01 to 37.0 mmol m−2 d−1 and between −71.0‰ and −50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ13C-CH4 signature. Subsequently, in addition to the temperature dependence (1.4 ± 0.1 eV), the rates of ebullition strongly correlated with the variability of δ13C-CH4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ13C-CH4 values (−64.6 ± 0.6‰ to −60.1 ± 3.2‰), yet the signature of the total CH4 emission (ebullition + diffusion) was relatively enriched (−60.7‰ to −52.6‰) due to high methanotrophic activity in the water column. We show that δ13C-CH4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH4 source signature to northern lakes.
Display omitted
•Stable carbon isotopic signature of CH4 (δ13C-CH4) varies greatly within and across four northern temperate lakes.•Ebullition rates and δ13C-CH4 signature are strongly coupled.•Variability of δ13C-CH4 signature is linked to the shift in methanogenic pathways.
Reservoirs are important sources of greenhouse gases (GHGs) to the
atmosphere, and their number is rapidly increasing, especially in tropical
regions. Accurately predicting their current and future ...emissions is
essential but hindered by fragmented data on the subject, which often fail
to include all emission pathways (surface diffusion, ebullition, degassing,
and downstream emissions) and the high spatial and temporal flux
variability. Here we conducted a comprehensive sampling of Batang Ai
reservoir (Malaysia), and compared field-based versus modelled estimates of
its annual carbon footprint for each emission pathway. Carbon dioxide
(CO2) and methane (CH4) surface diffusion were higher in upstream
reaches. Reducing spatial and temporal sampling resolution resulted in up to a 64 % and 33 % change in the flux estimate, respectively. Most GHGs present in
discharged water were degassed at the turbines, and the remainder were
gradually emitted along the outflow river, leaving time for CH4 to be
partly oxidized to CO2. Overall, the reservoir emitted 2475 gCO2eqm-2yr-1, with 89 % occurring downstream of the dam, mostly in
the form of CH4. These emissions, largely underestimated by
predictions, are mitigated by CH4 oxidation upstream and downstream of
the dam but could have been drastically reduced by slightly raising the
water intake elevation depth. CO2 surface diffusion and CH4
ebullition were lower than predicted, whereas modelled CH4 surface
diffusion was accurate. Investigating latter discrepancies, we conclude that
exploring morphometry, soil type, and stratification patterns as predictors
can improve modelling of reservoir GHG emissions at local and global scales.
Dissolved organic carbon (DOC) is a key parameter in lakes that can affect numerous features, including microbial metabolism, light climate, acidity, and primary production. In an attempt to ...understand the factors that regulate DOC in lakes, we assembled a large database (7,514 lakes from 6 continents) of DOC concentrations and other parameters that characterize the conditions in the lakes, the catchment, the soil, and the climate. DOC concentrations were in the range $0.1-332 mg L^{-1}$, and the median was $5.71 mg L^{-1}$. A partial least squares regression explained 48% of the variability in lake DOC and showed that altitude, mean annual runoff, and precipitation were negatively correlated with lake DOC, while conductivity, soil carbon density, and soil C:N ratio were positively related with lake DOC. A multiple linear regression using altitude, mean annual runoff, and soil carbon density as predictors explained 40% of the variability in lake DOC. While lake area and drainage ratio (catchment:lake area) were not correlated to lake DOC in the global data set, these two factors explained significant variation of the residuals of the multiple linear regression model in several regional subsets of data. These results suggest a hierarchical regulation of DOC in lakes, where climatic and topographic characteristics set the possible range of DOC concentrations of a certain region, and catchment and lake properties then regulate the DOC concentration in each individual lake.
Here, we explore the interaction between hydrology and the reactivity of allochthonous dissolved organic carbon (DOCalloch) in determining the potential of DOCalloch to generate CO 2 through ...biological and photo-chemical mineralization in boreal lakes. We developed a mechanistic model that integrates the reactivity continuum (RC) concept to reconstruct in-lake mineralization of DOCalloch under variable hydrologic conditions using empirical measurements of DOCalloch concentrations and reactivity as model inputs. The model predicts lake DOCalloch concentration (L-DOCalloch) and its average overall reactivity ðKalloch Þ, which integrates the distribution of DOC alloch ages within the lake as a function of the DOC loading (DOCin), the initial reactivity of this DOCin (k₀), and the lake water residence time (WRT). The modeled DOCalloch mineralization rates and concentrations were in agreement with expectations based on observed and published values of ambient lake DOC concentrations and reactivity. Results from this modeling exercise reveal that the interaction between WRT and k₀ is a key determinant of the ambient concentration and reactivity of lake DOCalloch, which represents the bulk of DOC in most of these lakes. The steadystate (K̅alloch) also represents the proportion of CO₂ that can be extracted from DOCalloch during its transit through lakes, and partly explains the patterns in surface water pCO₂ oversaturation that have been observed across gradients of lake size and volume. We estimate that in-lake DOCalloch mineralization could potentially contribute on average 30–40% of the observed surface carbon dioxide partial pressure (pCO₂) across northern lakes. Applying the RC framework to in-lake DOCalloch dynamics improves our understanding of DOCalloch transformation and fate along the aquatic network, and results in a predictable mosaic of DOC reactivity and potential CO₂ emissions across lakes within a landscape.
Freshwaters are important emitters of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less ...information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO2 and CH4 surface fluxes. In this study, we quantified sources and sinks of CO2 and CH4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO2 and CH4 (>90 % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO2 and CH4 are uncoupled and driven by different processes. In the main basin, verticalCO2 inputs and sediment CH4 inputs contributed to on average 60 % and 23 % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO2 and CH4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scaleCO2 and CH4 metabolism, which hinder aquatic GHG flux predictions.
Understanding the drivers of aerobic methane (CH₄) oxidation (MOX) is paramount in assessing the current and potential future CH₄ emissions from freshwater aquatic systems. Regulation of MOX kinetics ...is a complex function of CH₄ and oxygen (O₂) concentrations. While MOX activity is usually proportional to the concentration of CH₄ itself, the effects of O₂ have been more conflicting, with maximum MOX rates often restricted to low O₂ concentrations. Despite the complexity involved, MOX kinetics are often modelled as monotonic positive functions of both CH₄ and O₂ concentrations. We conducted a series of incubation experiments using natural and unamended water samples obtained from multiple depths in northern temperate lakes that vary widely and independently in their CH₄ and O₂ concentrations. Our results showed the expected positive effect of CH₄ concentration and temperature but also demonstrated the strong inhibitory effects of O₂ at high concentration. We then developed a general model describing the kinetics of MOX, simultaneously integrating the effects of CH₄ concentration, temperature as well as the non-linear effect O₂ on MOX activity. The model revealed an overall temperature dependency (activation energy = 0.49 ± 0.06 eV) much lower than reported for methanogenesis and an optimal O₂ level around 15 µmol O₂ L⁻¹ where maximum MOX activity occurs, regardless of CH₄ concentration and temperature. We further show that ignoring the inhibitory effect of O₂ can lead to significant bias in calculating the expected MOX rates in different portions of the water column.
Northern forests are important ecosystems for carbon (C) cycling and lakes within them process and bury large amounts of organic-C. Current burial estimates are poorly constrained and may discount ...other shifts in organic-C burial driven by global change. Here we analyse a suite of northern lakes to determine trends in organic-C burial throughout the Anthropocene. We found burial rates increased significantly over the last century and are up to five times greater than previous estimates. Despite a correlation with temperature, warming alone did not explain the increase in burial, suggesting the importance of other drivers including atmospherically deposited reactive nitrogen. Upscaling mean lake burial rates for each time period to global northern forests yields up to 4.5 Pg C accumulated in the last 100 years--20% of the total burial over the Holocene. Our results indicate that lakes will become increasingly important for C burial under future global change scenarios.
Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely constrained. Although part of this uncertainty can be ...attributed to the difficulties in measuring highly variable fluxes, it is also the result of a lack of a clear accounting methodology, particularly about what constitutes new emissions and potential new sinks. In this paper, we review the main processes involved in the generation of GHG in reservoir systems and propose a simple approach to quantify the reservoir GHG footprint in terms of the net changes in GHG fluxes to the atmosphere induced by damming, that is, ‘what the atmosphere sees.’ The approach takes into account the pre-impoundment GHG balance of the landscape, the temporal evolution of reservoir GHG emission profile as well as the natural emissions that are displaced to or away from the reservoir site resulting from hydrological and other changes. It also clarifies the portion of the reservoir carbon burial that can potentially be considered an offset to GHG emissions.
PDF
