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Methane (CH4) emissions from the boreal and arctic region are globally significant and highly sensitive to climate change. There is currently a wide range in estimates of high-latitude annual CH4 fluxes, where estimates based on land cover inventories and empirical CH4 flux data or process models (bottom-up approaches) generally are greater than atmospheric inversions (top-down approaches). A limitation of bottom-up approaches has been the lack of harmonization between inventories of site-level CH4 flux data and the land cover classes present in high-latitude spatial datasets. Here we present a comprehensive dataset of small-scale, surface CH4 flux data from 540 terrestrial sites (wetland and non-wetland) and 1247 aquatic sites (lakes and ponds), compiled from 189 studies. The Boreal–Arctic Wetland and Lake Methane Dataset (BAWLD-CH4) was constructed in parallel with a compatible land cover dataset, sharing the same land cover classes to enable refined bottom-up assessments. BAWLD-CH4 includes information on site-level CH4 fluxes but also on study design (measurement method, timing, and frequency) and site characteristics (vegetation, climate, hydrology, soil, and sediment types, permafrost conditions, lake size and depth, and our determination of land cover class). The different land cover classes had distinct CH4 fluxes, resulting from definitions that were either based on or co-varied with key environmental controls. Fluxes of CH4 from terrestrial ecosystems were primarily influenced by water table position, soil temperature, and vegetation composition, while CH4 fluxes from aquatic ecosystems were primarily influenced by water temperature, lake size, and lake genesis. Models could explain more of the between-site variability in CH4 fluxes for terrestrial than aquatic ecosystems, likely due to both less precise assessments of lake CH4 fluxes and fewer consistently reported lake site characteristics. Analysis of BAWLD-CH4 identified both land cover classes and regions within the boreal and arctic domain, where future studies should be focused, alongside methodological approaches. Overall, BAWLD-CH4 provides a comprehensive dataset of CH4 emissions from high-latitude ecosystems that are useful for identifying research opportunities, for comparison against new field data, and model parameterization or validation. BAWLD-CH4 can be downloaded from https://doi.org/10.18739/A2DN3ZX1R (Kuhn et al., 2021).

While the sentinel nature of freshwater systems is now well-recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources., Arctic Science, 9 (2), ISSN:2368-7460

Extensive floodplains and numerous lakes in the Amazon basin are well suited to examine the role of floodable lands within the context of the sources and processing of carbon within inland waters. We measured diel, seasonal and inter-annual variations of CO2 concentrations and related environmental variables in open water and flooded vegetation and estimated their habitat area using remote sensing in a representative Amazon floodplain lake, Lake Janauaca. Variability in CO2 concentrations in open water resulted from changes in the extent of inundation and exchange with vegetated habitats. Depth-averaged values of CO2 in the open water of the lake, 157 ± 91 µM (mean ± SD), were less than those in an embayment near aquatic vegetation, 285 ± 116 µM, and were variable over 24-h periods at both sites. Within floating herbaceous plant mats, the mean concentration was 275 ± 77 µM, while in flooded forests it was 217 ± 78 µM. The best statistical model that included CO2 in aquatic plant mats, water clarity, rate of change in water level and chlorophyll-a concentrations explained around 90% of the variability in CO2 concentration. Three-dimensional hydrodynamic modeling demonstrated that diel differences in water temperature between plant mats and open water as well as basin-scale motions caused lateral exchanges of CO2 between vegetated habitats and open water. Our findings extend understanding of CO2 in tropical lakes and floodplains with measurements and models that emphasize the importance of flooded forests and aquatic herbaceous plants fringing floodplain lakes as sources of CO2 to open waters.

Lakes emit CO2 to the atmosphere at magnitudes significant for the global carbon cycle, in the form of diffusive CO2 flux (FCO2). As direct FCO2 measurements are time-consuming, FCO2 is often estimated from the air-water CO2 concentration gradient (ΔpCO2) and the gas transfer velocity (k), representing the two components considered to regulate FCO2. However, extrapolating measurements of ΔpCO2 and k to whole-year estimates require understanding of their variability in time and across different types of lakes, which is often insufficient. As a result, simple linear interpolations are typically used in extrapolations which risk producing bias as spatiotemporal variability is not included. Further insight to the variability of ΔpCO2 and k may contribute to more representative extrapolations and provide guidance for focusing sampling campaigns on capturing times of high variability. We used floating flux chambers and surface water samples to measure FCO2 and ΔpCO2, respectively, both within-weeks and over seasons during the open water period at 12 locations in each of 15 boreal lakes across a latitudinal gradient in Sweden. We combined these measurements to derive spatially resolved values of k in order to identify: i) the contributions of ΔpCO2 and k to FCO2 variability over time; and ii) if differences in the contributions of ΔpCO2 and k to FCO2 variability can be related to lake characteristics. The results presented are relevant for improved modelling of lake CO2 emissions.

Lakes and reservoirs contribute to regional carbon budgets via significant emissions of climate forcing trace gases. Here, for improved modelling, we use 8 years of floating chamber measurements from three small, shallow subarctic lakes (2010–2017, n=1306 ) to separate the contribution of physical and biogeochemical processes to the turbulence-driven, diffusion-limited flux of methane ( CH4 ) on daily to multi-year timescales. Correlative data include surface water concentration measurements (2009–2017, n=606 ), total water column storage (2010–2017, n=237 ), and in situ meteorological observations. We used the last to compute near-surface turbulence based on similarity scaling and then applied the surface renewal model to compute gas transfer velocities. Chamber fluxes averaged 6.9±0.3 mg CH4 m −2 d −1 and gas transfer velocities ( k600 ) averaged 4.0±0.1 cm h −1 . Chamber-derived gas transfer velocities tracked the power-law wind speed relation of the model. Coefficients for the model and dissipation rates depended on shear production of turbulence, atmospheric stability, and exposure to wind. Fluxes increased with wind speed until daily average values exceeded 6.5 m s −1 , at which point emissions were suppressed due to rapid water column degassing reducing the water–air concentration gradient. Arrhenius-type temperature functions of the CH4 flux ( E a ′ = 0.90 ± 0.14 eV) were robust ( R2≥0.93 , p ) and also applied to the surface CH4 concentration ( E a ′ = 0.88 ± 0.09 eV). These results imply that emissions were strongly coupled to production and supply to the water column. Spectral analysis indicated that on timescales shorter than a month, emissions were driven by wind shear whereas on longer timescales variations in water temperature governed the flux. Long-term monitoring efforts are essential to identify distinct functional relations that govern flux variability on timescales of weather and climate change.

Large uncertainties in estimates of methane (CH4) emissions from tropical inland waters reflect the paucity of information at appropriate temporal and spatial scales. CH4 concentrations, diffusive and ebullitive fluxes, and environmental parameters in contrasting aquatic habitats of Lake Janauaca, an Amazon floodplain lake, measured for two years revealed patterns in temporal and spatial variability related to different aquatic habitats and environmental conditions. CH4 concentrations ranged from below detection to 96 µM, CH4 diffusive fluxes from below detection to 2342 µmol m−2 h−1, and CH4 ebullitive fluxes from 0 to 190 mmol m−2 d−1. Vegetated aquatic habitats had higher surface CH4 concentrations than open water habitats, and no significant differences in diffusive CH4 fluxes, likely due to higher k values measured in open water habitats. CH4 emissions were enhanced after a prolonged low water period, when the exposed sediments were colonized by herbaceous plants that decomposed after water levels rose, possibly fueling CH4 production. Statistical models indicated the importance of variables related to CH4 production (temperature, dissolved organic carbon) and consumption (dissolved nitrogen, oxygenated water column), as well as maximum depth, in controlling surface water CH4 concentrations.

Mountain lakes experience extreme interannual climate variation as well as rapidly warming air temperatures, making them ideal systems to understand lake‐climate responses. Snowpack and water temperature are highly correlated in mountain lakes, but we lack a complete understanding of underlying mechanisms. Motivated by predicted declines in snowfall with future temperature increases, we investigated how surface heat fluxes and lake warming responded to variation in snowpack, ice‐off, and summer weather patterns in a high elevation lake in the Sierra Nevada, California. Ice‐off timing determined the phenology of lake exposure to solar radiation, and was the dominant mechanism linking snowpack to lake temperature. The relative importance of heat loss fluxes (longwave radiation, latent and sensible heat exchange) varied among wet and dry years. Declines in snowpack and ice cover in mountain systems will reduce variability in lake thermal responses and increase the responsiveness of lake warming to atmospheric forcing.

To ensure the long-term sustainable use of African Great Lakes (AGL), and to better understand the functioning of these ecosystems, authorities, managers and scientists need regularly collected scientific data and information of key environmental indicators over multi-years to make informed decisions. Monitoring is regularly conducted at some sites across AGL; while at others sites, it is rare or conducted irregularly in response to sporadic funding or short-term projects/studies. Managers and scientists working on the AGL thus often lack critical long-term data to evaluate and gauge ongoing changes. Hence, we propose a multi-lake approach to harmonize data collection modalities for better understanding of regional and global environmental impacts on AGL. Climate variability has had strong impacts on all AGL in the recent past. Although these lakes have specific characteristics, their limnological cycles show many similarities. Because different anthropogenic pressures take place at the different AGL, harmonized multi-lake monitoring will provide comparable data to address the main drivers of concern (climate versus regional anthropogenic impact). To realize harmonized long-term multi-lake monitoring, the approach will need: (1) support of a wide community of researchers and managers; (2) political goodwill towards a common goal for such monitoring; and (3) sufficient capacity (e.g., institutional, financial, human and logistic resources) for its implementation. This paper presents an assessment of the state of monitoring the AGL and possible approaches to realize a long-term, multi-lake harmonized monitoring strategy. Key parameters are proposed. The support of national and regional authorities is necessary as each AGL crosses international boundaries. publishedVersion

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change., Measurement(s) temperature of water • temperature profile Technology Type(s) digital curation Factor Type(s) lake location • temporal interval Sample Characteristic - Environment lake • reservoir Sample Characteristic - Location global Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14619009

Seasonally flooded forests along tropical rivers cover extensive areas, yet the processes driving air-water exchanges of radiatively active gases are uncertain. To quantify the controls on gas transfer velocities, we combined measurements of water-column temperature, meteorology in the forest and adjacent open water, turbulence with an acoustic Doppler velocimeter, gas concentrations, and fluxes with floating chambers. Under cooling, measured turbulence, quantified as the rate of dissipation of turbulent kinetic energy (ε), was similar to buoyancy flux computed from the surface energy budget, indicating convection dominated turbulence production. Under heating, turbulence was suppressed unless winds in the adjacent open water exceeded 1 m/s. Gas transfer velocities obtained from chamber measurements ranged from 1 to 5 cm/hr and were similar to or slightly less than predicted using a turbulence-based surface renewal model computed with measured ε and ε predicted from wind and cooling. ©2019. The Authors.

Northern lakes are important sources of the climate forcing trace gases methane (CH4) and carbon dioxide (CO2). A substantial portion of lakes' annual emissions can take place immediately after ice melt in spring. The drivers of these fluxes are neither well constrained nor fully understood. We present a detailed carbon gas budget for three subarctic lakes, using 6 years of eddy covariance and 9 years of manual flux measurements. We combine measurements of temperature, dissolved oxygen, and CH4 stable isotopologues to quantify functional relationships between carbon gas production and conversion, energy inputs, and the redox regime. Spring emissions were regulated by the availability of oxygen in winter, rather than temperature as during ice‐free conditions. Under‐ice storage increased predictably with ice‐cover duration, and CH4 accumulation rates (25 ± 2 mg CH4‐C·m−2·day−1) exceeded summer emissions (19 ± 1 mg CH4‐C·m−2·day−1). The seasonally ice‐covered lakes emitted 26–59% of the annual CH4 flux and 15–30% of the annual CO2 flux at ice‐off. Reduced spring emissions were associated with winter snowmelt events, which can transport water downstream and oxygenate the water column. Stable isotopes indicate that 64–96% of accumulated CH4 escaped oxidation, implying that a considerable portion of the dissolved gases produced over winter may evade to the atmosphere.

Large, shallow lakes are common in the extensive floodplains throughout the tropics. To determine controls on their mixing dynamics, we instrumented 5 stations in two shallow, connected tropical lakes on the lower Amazon floodplain with meteorological and temperature sensors. A tight relation between changes in thermal structure and LMO/h (the ratio of the Monin-Obukhov length scale to the depth of the actively mixing layer) indicates the sensitivity of thermal structure to wind speed relative to heating and cooling. Four regimes led to variations in mixing: (i) high solar radiation with light winds in the mid-morning to early afternoon resulted in shallow stratification, 0 1; (iii) by late afternoon, buoyancy flux became negative and LMO/h 0.06 m s−1, mixing from wind and cooling co-occurred; and (iv) convection dominated mixing on nights with light winds, −1

Variability in physico-chemical water column structure along a gradient between Nyanza Gulf and Lake Victoria was characterized, and the exchange between the two basins estimated. Specific conductivity (SC) measurements between March 2005 and March 2006 identified three morphologically distinct zones, the inner-gulf, the connecting Rusinga Channel and the main lake basin. The relatively shallow, wide and river-influenced inner-gulf had higher turbidity and SC compared to the channel and the main lake. The deep, narrow and more wind-exposed mid-channel area was significantly cooler than the rest of the study areas. Observational data revealed “density-driven” underflow of the cooler channel water into the main lake and compensatory surface flow into the gulf as well as seasonal patterns of gulf-main lake exchange in relation to variation in tributary input. The wind field was dominated by a diurnal lake-land breeze which resulted in wind-driven diurnal reversal of water flow in and out of the gulf, where the constriction of flow within the channel induced strong currents. The exchange between the gulf and the main lake was estimated using a box model, with SC as a conservative tracer. A net annual residual water flow of 35 m3 s−1 occurred from the gulf into the main lake although during months with low tributary discharge the flow was towards the gulf. The daily flow in and out of the gulf resulted in relatively high exchange flux compared to residual flow across the Rusinga Channel leading to the development of the well-defined physicochemical gradient along the Rusinga Channel.

Methane (CH4) emissions from the Boreal and Arctic region are globally significant and highly sensitive to climate change. There is currently a wide range in estimates of high-latitude annual CH4 fluxes, where estimates based on land cover inventories and empirical CH4 flux data or process models (bottom-up approaches) generally are greater than atmospheric inversions (top-down approaches). A limitation of bottom-up approaches has been the lack of harmonization between inventories of site-level CH4 flux data and the land cover classes present in high-latitude spatial datasets. Here we present a comprehensive dataset of small-scale, surface CH4 flux data from 540 terrestrial sites (wetland and non-wetland) and 1247 aquatic sites (lakes and ponds), compiled from 189 studies. The Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4) was constructed in parallel with a compatible land cover dataset, sharing the same land cover classes to enable refined bottom-up assessments. BAWLD-CH4 includes information on site-level CH4 fluxes, but also on study design (measurement method, timing, and frequency) and site characteristics (vegetation, climate, hydrology, soil, and sediment types, permafrost conditions, lake size and depth, and our determination of land cover class). The different land cover classes had distinct CH4 fluxes, resulting from definitions that were either based on or co-varied with key environmental controls. Fluxes of CH4 from terrestrial ecosystems were primarily influenced by water table position, soil temperature, and vegetation composition, while CH4 fluxes from aquatic ecosystems were primarily influenced by water temperature, lake size, and lake genesis. Models could explain more of the between-site variability in CH4 fluxes for terrestrial than aquatic ecosystems, likely due to both less precise assessments of lake CH4 fluxes and fewer consistently reported lake site characteristics. Analysis of BAWLD-CH4 identified both land cover classes and regions within the Boreal and Arctic domain where future studies should be focused, alongside methodological approaches. Overall, BAWLD-CH4 provides a comprehensive dataset of CH4 emissions from high-latitude ecosystems that are useful for identifying research opportunities, for comparison against new field data, and model parameterization or validation. BAWLD-CH4 can be downloaded from https://doi.org/10.18739/A27H1DN5S.

The ice-covered period in lakes is increasingly recognized for its unique hydrodynamic and biogeochemical phenomena and ecological relevance yet it remains poorly studied compared to the ice-free season. Knowledge gaps exist where research areas – hydrodynamics, biogeochemistry and biology – intersect. For example, density-driven circulation under ice coincides with an expansion of the anoxic zone, but abiotic and biotic controls on oxygen depletion have not been disentangled. While heterotrophic microorganisms and migrating phytoplankton often thrive at the oxycline, the extent to which physical processes induce fluxes of heat and substrates that further support under-ice food webs is uncertain. Similarly, radiatively-driven convection under ice in spring can promote growth of motile phytoplankton or diatoms depending on flow velocity, water clarity and mixing depth, but links between functional trait selection, trophic transfer to zooplankton and fish and the prevalence of microbial versus classical food webs in seasonally ice-covered lakes remain unclear. Under-ice processes cascade into and from the ice-free season, and are relevant to annual cycling of energy and carbon through aquatic food webs. Understanding the coupling between state transitions and the reorganization of trophic hierarchies is essential for predicting complex ecosystem responses to climate change. In this presentation, we briefly review existing knowledge regarding physical processes in lakes in winter and the parallel developments in under-ice biogeochemistry and ecology. We then illustrate interactions between these processes, identify extant knowledge gaps whose solution requires interdisciplinary approaches, and present (novel) methods to address outstanding questions.

Inland waters, such as lakes, reservoirs and rivers, are important sources of climate forcing trace gases. A key parameter that regulates the gas exchange between water and the atmosphere is the gas transfer velocity, which itself is controlled by near-surface turbulence in the water. While in lakes and reservoirs, near-surface turbulence is mainly driven by atmospheric forcing, in shallow rivers and streams it is generated by bottom friction of gravity-forced flow. Large rivers represent a transition between these two cases. Near-surface turbulence has rarely been measured in rivers and the drivers of turbulence have not been quantified. We analyzed continuous measurements of flow velocity and quantified turbulence as the rate of dissipation of turbulent kinetic energy over the ice-free season in a large regulated river in Northern Finland. Measured dissipation rates agreed with predictions from bulk parameters, including mean flow velocity, wind speed, surface heat flux, and with a one-dimensional numerical turbulence model. Values ranged from to . Atmospheric forcing or gravity was the dominant driver of near-surface turbulence for similar fraction of the time. Large variability in near-surface dissipation rate occurred at diel time scales, when the flow velocity was strongly affected by downstream dam operation. By combining scaling relations for boundary-layer turbulence at the river bed and at the air-water interface, we derived a simple model for estimating the relative contributions of wind speed and bottom friction of river flow as a function of depth. Plain Language Summary Inland water bodies such as lakes, reservoirs and rivers are an important source of climate forcing trace gases to the atmosphere. Gas exchange between water and the atmosphere is regulated by the gas transfer velocity and the concentration difference between the water surface and the atmosphere. The gas transfer velocity depends on near-surface turbulence, but robust formulations have not been developed for river systems. Their surface area is sufficiently large for meteorological forcing to cause turbulence, as in lakes and reservoirs, but turbulence generated from bed and internal friction of gravity-driven flows is also expected to contribute. Here we quantify near-surface turbulence using data from continuous air and water side measurements conducted over the ice-free season in a large subarctic regulated river in Finland. We find that turbulence, quantified as the dissipation rate of turbulent kinetic energy, is well described using equations for predicting turbulence from meteorological data for sufficiently high wind speeds whereas the contribution from bottom shear dominated at higher flow velocities. A one-dimensional river model successfully captured these processes. We provide a fundamental model for estimating the relative contributions of atmospheric forcing and bottom friction as a function of depth.

Lakes are generally supersaturated in carbon dioxide (CO2) and emitters of CO2 to the atmosphere. However, estimates of CO2 flux (FCO2) from lakes are seldom based on direct flux measurements and usually do not account for nighttime emissions, yielding risk of biased assessments. Here, we present direct FCO2 measurements from automated floating chambers collected every 2-3 hr and spanning 115 24 hr periods in three boreal lakes during summer stratification and before and after autumn mixing in the most eutrophic lake of these. We observed 40%-67% higher mean FCO2 in daytime during periods of surface water CO2 supersaturation in all lakes. Day-night differences in wind speed were correlated with the day-night FCO2 differences in the two larger lakes, but in the smallest and most wind-sheltered lake peaks of FCO2 coincided with low-winds at night. During stratification in the eutrophic lake, CO2 was near equilibrium and diel variability of FCO2 insignificant, but after autumn mixing FCO2 was high with distinct diel variability making this lake a net CO2 source on an annual basis. We found that extrapolating daytime measurements to 24 hr periods overestimated FCO2 by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated FCO2 by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO2 emission estimates. Funding Agencies: Knut & Alice Wallenberg Foundation [2016.0083]; European Research Council (ERC) [725546]; Swedish Research Council, European Commission [2017-00635, 2016-04829]; FORMAS (the Swedish Research Council for Sustainable Development) [2018-01794]

The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (epsilon) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface epsilon varied from 10(-8) to 10(-6) m(2) s(-3) for the 0-4 m s(-1) winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10(-3) m(2) s(-1) on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L-N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using epsilon agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s(-1), a condition that can lead to elevated near-surface epsilon and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes. Funding Agencies|Swedish Research CouncilSwedish Research Council [210-2012-1461, 2009-1692, 2011-3575, 2012-00048, 2016-04829]; European Research Council (ERC)European Research Council (ERC) [725546]; FormasSwedish Research Council Formas [2018-2022]; U.S. National Science Foundation through DEB grant [1753856, DEB 0919603, ANS 01204267, ANS 1737411]

The goals of our study were to (1) quantify production of CO2 during winter ice‐cover in arctic lakes, (2) develop methodologies which would enable prediction of CO2 production from readily measured variables, and (3) improve understanding of under‐ice circulation as it influences the distribution of dissolved gases under the ice. To that end, we combined in situ measurements with profile data. CO2 production averaged 20 mg C m−2 d−1 in a 3 m deep lake and ∼ 45 mg C m−2 d−1 in four larger lakes, similar to experimental observations at temperatures below 4°C. CO2 production was predicted by the initial rate of loss of oxygen near the sediments at ice‐on and by the full water column loss of oxygen throughout the winter. The time series data also showed the lake‐size and time dependent contribution of sediment respiration to under‐ice circulation and the decreased near‐bottom flows enabling anoxia and CH4 accumulation.

Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO2) and methane (CH4) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO2 partial pressure resulted in an overall reduction of CO2 efflux. Ebullition represented between 40 and 99% of the total CH4 efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO2 equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.

The extent to which snowmelt flowing into ice-covered lakes spreads horizontally and mixes vertically influences retention of solutes derived from the landscape. To quantify these transport processes and retention, we combine time series temperature and specific conductance measurements in Toolik Lake (Alaska) and its major inflow, with measurements of discharge and meteorology, and profiles of specific conductance, temperature, fluorescence, chlorophyll a, and dissolved organic carbon (DOC) in spring of 3 yr. During early snowmelt, the concentration of DOC in the stream was 750 μM, twice that in the lake. During slow melt (discharge (Q) 6 m3 s−1, the incoming water partially flushed the inlet basin and the more dilute water flowed over the original intrusion with a preferential flowpath to the outlet. Penetrative convection was restricted by the increased density gradients from the incoming plume and initially constrained to shallow mixing zones associated with the step changes in density. As ice thickness decreased to less than 1 m, heating caused density instabilities at the base of the intrusions that mixed solutes ∼ 10 m vertically, contributing to retention. Near-surface layers enriched with DOC persisted for ∼ 10 d during a rapid melt and for over 3 weeks when the melt was slow. Retention, of order 10–20%, also depended on the rapidity of melt and magnitude of discharge.

Lakes and reservoirs are important emitters of climate forcing trace gases. Various environmental drivers of the flux, such as temperature and wind speed, have been identified, but their relative importance remains poorly understood. Here we use an extensive field dataset to disentangle physical and biogeochemical controls on the turbulence-driven diffusive flux of methane (CH4) on daily to multi-year timescales. We compare 8 years of floating chamber fluxes from three small, shallow subarctic lakes (2010–2017, n = 1306) with fluxes computed using 9 years of surface water concentration measurements (2009–2017, n = 606) and a small-eddy surface renewal model informed by in situ meteorological observations. Chamber fluxes averaged 6.9 ± 0.3 mg m−2 d−1 and gas transfer velocities (k600) from the chamber-calibrated surface renewal model averaged 4.0 ± 0.1 cm h−1. We find robust (R2 ≥ 0.93, p

Mixing regime and CO2 availability may control cyanobacterial blooms in polymictic lakes, but the underlying mechanisms still remain unclear. We integrated detailed results from a natural experiment comprising an average-wet year (2011) and one with heat waves (2012), a long-term meteorological dataset (1960–2010), historical phosphorus concentrations and sedimentary pigment records, to determine the mechanistic controls of cyanobacterial blooms in a eutrophic polymictic lake. Intense warming in 2012 was associated with: 1) increased stability of the water column with buoyancy frequencies exceeding 40 cph at the surface, 2) high phytoplankton biomass in spring (up to 125 mg WW L−1), 3) reduced downward transport of heat and 4) depleted epilimnetic CO2 concentrations. CO2 depletion was maintained by intense uptake by phytoplankton (influx up to 30 mmol m−2 d−1) in combination with reduced, internal and external, carbon inputs during dry, stratified periods. These synergistic effects triggered bloom of buoyant cyanobacteria (up to 300 mg WW L−1) in the hot year. Complementary evidence from polynomial regression modelling using historical data and pigment record revealed that warming explains 78% of the observed trends in cyanobacterial biomass, whereas historical phosphorus concentration only 10% thereof. Together the results from the natural experiment and the long-term record indicate that effects of hotter and drier climate are likely to increase water column stratification and decrease CO2 availability in eutrophic polymictic lakes. This combination will catalyze blooms of buoyant cyanobacteria.

Non

We review alternative hypotheses and associated mechanisms to explain Lake Victoria’s Nile perch (Lates niloticus) takeover and concurrent reduction in haplochromines through a (re)analysis of long-term climate, limnological, and stock observations in comparison with size-spectrum model predictions of co-existence, extinction, and demographic change. The empirical observations are in agreement with the outcomes of the model containing two interacting species with life histories matching Nile perch and a generalized haplochromine. The dynamic interactions may have depended on size-related differences in early juvenile mortality: mouth-brooding haplochromines escape predation mortality in early life stages, unlike Nile perch, which have miniscule planktonic eggs and larvae. In our model, predation on the latter by planktivorous haplochromine fry acts as a stabilizing factor for co-existence, but external mortality on the haplochromines would disrupt this balance in favor of Nile perch. To explain the observed switch, mortality on haplochromines would need to be much higher than the fishing mortality that can be realistically reconstructed from observations. Abrupt concomitant changes in algal and zooplankton composition, decreased water column transparency, and widespread hypoxia from increased eutrophication most likely caused haplochromine biomass decline. We hypothesize that the shift to Nile perch was a consequence of an externally caused, climate-triggered decrease in haplochromine biomass and associated recruitment failure rather than a direct cause of the introduction.