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1. Diel variation in CO₂ flux is substantial in many lakes

   https://doi.org/10.1002/lol2.70066  

   de_Eyto, Elvira; Smyth, Robyn L; Pilla, Rachel M; Laas, Alo; Shahabinia, Amir Reza; Baldocchi, Angela; Desai, Ankur R; Lupon, Anna; Lohila, Annalea; Obrador, Biel; et al (December 2025, Limnology and Oceanography Letters)

   Abstract Lakes play a significant role in the global carbon cycle, acting as sources and sinks of carbon dioxide (CO₂). In situ measurements of CO₂flux (FCO₂) from lakes have generally been collected during daylight, despite indications of significant diel variability. This introduces bias when scaling up to whole‐lake annual aquatic carbon budgets. We conducted an international sampling program to ascertain the extent of diel variation in FCO₂across lakes. We sampled 21 lakes over 41 campaigns and measured FCO₂at 4‐h intervals over a full diel cycle. Rates of FCO₂ranged from −3.16 to 4.39 mmol m⁻² h⁻¹. Integrated over a day, FCO₂ranged from −381.68 to 878.49 mg C m⁻²d⁻¹(mean = 76.54) across campaigns. We identified three characteristic diel patterns in FCO₂related to trophic status and show that for half of the campaigns, daily flux estimates were biased by > 50% if based on a single (daytime) measurement. 

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2. Lotic‐SIPCO2 : Adaptation of an open‐source CO₂ sensor system and examination of associated emission uncertainties across a range of stream sizes and land uses

   https://doi.org/10.1002/lom3.10600  

   Robison, Andrew L.; Koenig, Lauren E.; Potter, Jody D.; Snyder, Lisle E.; Hunt, Christopher W.; McDowell, William H.; Wollheim, Wilfred M. (April 2024, Limnology and Oceanography: Methods)

   Abstract River networks play a crucial role in the global carbon cycle, as relevant sources of carbon dioxide (CO₂) to the atmosphere. Advancements in high‐frequency monitoring in aquatic environments have enabled measurement of dissolved CO₂concentration at temporal resolutions essential for studying carbon variability and evasion from these dynamic ecosystems. Here, we describe the adaptation, deployment, and validation of an open‐source and relatively low‐cost in situpCO₂sensor system for lotic ecosystems, the lotic‐SIPCO2. We tested the lotic‐SIPCO2 in 10 streams that spanned a range of land cover and basin size. Key system adaptations for lotic environments included prevention of biofouling, configuration for variable stage height, and reduction of headspace equilibration time. We then examined which input parameters contribute the most to uncertainty in estimating CO₂emission rates and found scaling factors related to the gas exchange velocity were the most influential when CO₂concentration was significantly above saturation. Near saturation, sensor measurement ofpCO₂contributed most to uncertainty in estimating CO₂emissions. We also found high‐frequency measurements ofpCO₂were not necessary to accurately estimate median emission rates given the CO₂regimes of our streams, but daily to weekly sampling was sufficient. High‐frequency measurements ofpCO₂remain valuable for exploring in‐stream metabolic variability, source partitioning, and storm event dynamics. Our adaptations to the SIPCO2 offer a relatively affordable and robust means of monitoring dissolved CO₂in lotic ecosystems. Our findings demonstrate priorities and related considerations in the design of monitoring projects of dissolved CO₂and CO₂evasion dynamics more broadly. 

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3. Variability and Controls of p CO ₂ and Air‐Water CO ₂ Fluxes in a Temperate River

   https://doi.org/10.1029/2024JG008434  

   Young, Fischer L; Colman, Benjamin P; Carter, Alice M; Fiejó_de_Lima, Rafael; Shangguan, Qipei; Payn, Robert A; DeGrandpre, Michael D (February 2025, Journal of Geophysical Research: Biogeosciences)

   Abstract Measurements of riverine dissolved inorganic carbon, total alkalinity (A_T), pH, and the partial pressure of carbon dioxide (pCO₂) can provide insights into the biogeochemical function of rivers, including the processes that control biological production, chemical speciation, and air‐water CO₂fluxes. The complexity created by these combined processes dictates that studies of inorganic carbon be made over broad spatial and temporal scales. Time‐series data like these are relatively rare, however, because sampling and measurements are labor intensive and, for some variables, good measurement quality is difficult to achieve (e.g., pH). In this study, spectrophotometric pH and A_Twere quantified with high precision and accuracy at biweekly to monthly intervals over a four‐year period (2018–2021) along 216 km of the Upper Clark Fork River (UCFR) in the northern Rocky Mountains, USA. We use these and other time‐series data to provide insights into the processes that control river inorganic carbon, with a focus onpCO₂and air‐water CO₂fluxes. We found that seasonal snowmelt runoff increasedpCO₂and that expected increase and decrease ofpCO₂due to seasonal heating and cooling were likely offset by an increase and loss of algal biomass, respectively. Overall, the UCFR was a small net source (0.08 ± 0.14 mol m⁻² d⁻¹) of CO₂to the atmosphere over the four‐year study period with highly variable annual averages (0.0–0.10 mol m⁻² d⁻¹). The seasonally correlated, offsetting mechanisms highlight the challenges in predictingpCO₂and air‐water CO₂fluxes in rivers. 

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4. Interannual, summer, and diel variability of CH ₄ and CO ₂ effluxes from Toolik Lake, Alaska, during the ice-free periods 2010–2015

   https://doi.org/10.1039/D0EM00125B  

   Eugster, Werner; DelSontro, Tonya; Shaver, Gaius R.; Kling, George W. (January 2020, Environmental Science: Processes & Impacts)

   Accelerated warming in the Arctic has led to concern regarding the amount of carbon emission potential from Arctic water bodies. Yet, aquatic carbon dioxide (CO 2 ) and methane (CH 4 ) flux measurements remain scarce, particularly at high resolution and over long periods of time. Effluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) from Toolik Lake, a deep glacial lake in northern Alaska, were measured for the first time with the direct eddy covariance (EC) flux technique during six ice-free lake periods (2010–2015). CO 2 flux estimates from the lake (daily average efflux of 16.7 ± 5.3 mmol m −2 d −1 ) were in good agreement with earlier estimates from 1975–1989 using different methods. CH 4 effluxes in 2010–2015 (averaging 0.13 ± 0.06 mmol m −2 d −1 ) showed an interannual variation that was 4.1 times greater than median diel variations, but mean fluxes were almost one order of magnitude lower than earlier estimates obtained from single water samples in 1990 and 2011–2012. The overall global warming potential (GWP) of Toolik Lake is thus governed mostly by CO 2 effluxes, contributing 86–93% of the ice-free period GWP of 26–90 g CO 2,eq m −2 . Diel variation in fluxes was also important, with up to a 2-fold (CH 4 ) to 4-fold (CO 2 ) difference between the highest nighttime and lowest daytime effluxes. Within the summer ice-free period, on average, CH 4 fluxes increased 2-fold during the first half of the summer, then remained almost constant, whereas CO 2 effluxes remained almost constant over the entire summer, ending with a linear increase during the last 1–2 weeks of measurements. Due to the cold bottom temperatures of this 26 m deep lake, and the absence of ebullition and episodic flux events, Toolik Lake and other deep glacial lakes are likely not hot spots for greenhouse gas emissions, but they still contribute to the overall GWP of the Arctic. 

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5. Carbon dioxide fluxes of Arctic coastal ecosystems controlled by seasonal patterns of land‐to‐ocean connectivity

   https://doi.org/10.1002/lno.70101  

   Spera, Alina C; Lougheed, Vanessa L (June 2025, Limnology and Oceanography)

   Abstract The strength of coastal Arctic Ocean CO₂uptake is vulnerable to landscape‐scale changes such as hydrological intensification, changing climate, and alterations to terrestrial and aquatic biogeochemistry. Across a period of 4 yr (2019–2023) and three distinct sampling periods, we visited five coastal ecosystems of the Beaufort Sea with varying barrier island coverage to understand drivers of Arctic coastal CO₂flux. The ice cover sampling period was characterized by the highest pCO₂saturation and dissolved O₂undersaturation. We observed a > 100μatm difference in pCO₂over shallow depths (up to 2 m) at 73% of ice‐cover site visits. Notably, the geomorphology of barrier islands and channels controlled the flushing of colder Beaufort Sea waters across the systems and influenced the strength and appearance of both vertical thermohaline and pCO₂stratification. The ice breakup period reflected spring freshet and was a net CO₂sink during sampling, likely related to freshwater riverine dilution, CaCO₃dissolution from sea ice melt, and water column algal activity. During the open water sampling period, the interaction of marine and terrestrial contributions predicted the strength of the CO₂efflux, with freshwater inputs introducing higher temperatures and organic material, which increased remineralization. The capacity of these systems to act as CO₂sources or sinks varies throughout the year and is largely driven by geomorphic conditions. Any spatially integrative studies of CO₂flux or coastal productivity should consider the physical and biogeochemical heterogeneity of Arctic coastal ecosystems. 

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