, Spatio-temporal variability of lake CH4 fluxes and its influence on annual whole lake emission estimates, 2015, Limnology and Oceanography.http://dx

Abstract. Fluxes of CO2 are important for our understanding of the global carbon cycle and greenhouse gas balances. Several significant CO2 fluxes in nature may still be unknown as illustrated by recent findings of high CO2 emissions from aquatic environments, previously not recognized in global carbon balances. Therefore, it is important to develop convenient and affordable ways to measure CO2 in many types of environments. At present, direct measurements of CO2 fluxes from soil or water, or CO2 concentrations in surface water, are typically labor intensive or require costly equipment. We here present an approach with measurement units based on small inexpensive CO2 loggers, originally made for indoor air quality monitoring, that were tested and adapted for field use. Measurements of soil–atmosphere and lake–atmosphere fluxes, as well as of spatiotemporal dynamics of water CO2 concentrations (expressed as the equivalent partial pressure, pCO2aq) in lakes and a stream network are provided as examples. Results from all these examples indicate that this approach can provide a cost- and labor-efficient alternative for direct measurements and monitoring of CO2 flux and pCO2aq in terrestrial and aquatic environments.

Global stream and river greenhouse gas emissions seem to be as large as the oceanic C uptake. However, stream and river emissions are uncertain until both spatial and temporal variability have been quantified. Here we investigated in detail the stream CH4 and CO2 emissions within a hemiboreal catchment in Southwest Sweden primarily covered by coniferous forest. Gas transfer velocities (k600), CH4 and CO2 concentrations were measured with multiple methods. Our data supported modelling approaches accounting for various stream slopes, water velocities and discharge. The results revealed large but partially predictable spatio-temporal variabilities in k600, dissolved gas concentrations, and emissions. The variability in CO2 emission was best explained by the variability in k, while dissolved CH4 concentrations explained most of the variability in CH4 emission, having implications for future measurements. There were disproportionately large emissions from high slope stream reaches including waterfalls, and from high discharge events. In the catchment, stream reaches with low slope and time periods of moderate discharge dominated (90% of area and 69% of time). Measurements in these stream areas and time periods only accounted for <36% of the total estimated emissions. Hence, not accounting for local or episodic high emissions can lead to substantially underestimated emissions.

Abstract. Fluxes of CO2 are important for our understanding of the global carbon cycle and greenhouse gas balances. Several significant CO2 fluxes in nature may still be neglected as illustrated by recent findings of high CO2 emissions from aquatic environments, previously not recognized in global carbon balances. Therefore it is important to develop convenient and affordable ways to measure CO2 in many types of environments. At present, direct measurements of CO2 fluxes from soils or waters, or CO2 concentrations in surface water, are typically labour intensive or require costly equipment. We here present an approach with measurement units based on small inexpensive CO2 loggers, originally made for indoor air quality monitoring, that were tested and adapted for field use. Measurements of soil–atmosphere and lake–atmosphere fluxes, as well as of spatio-temporal dynamics of water CO2 concentrations (expressed as the equivalent partial pressure, pCO2aq) in lakes and a stream network are provided as examples. Results from all these examples indicate that this approach can provide a cost- and labor efficient alternative for direct measurements and monitoring of CO2 flux and pCO2aq in terrestrial and aquatic environments.

Globally, lakes are frequently supersaturated with carbon dioxide (CO2) and are major emitters of carbon to the atmosphere. Recent studies have generated awareness of the high variability in pCO2aq (the partial pressure corresponding to the concentration in water) and CO2 fluxes to the atmosphere and the need for better accounting for this variability. However, studies simultaneously accounting for both spatial and temporal variability of pCO2aq and CO2 fluxes in lakes are rare. We measured pCO2aq (by both manual sampling and mini loggers) and CO2 fluxes, covering spatial variability in open water areas of three lakes of different character in a Swedish catchment for 2 years. Spatial pCO2aq variability within lakes was linked to distance from shore, proximity to stream inlets, and deepwater upwelling events. Temporally, pCO2aq variability was linked with variability in dissolved organic carbon, total nitrogen, and dissolved oxygen. While previous studies over short time periods (1 to 6 h) observed gas transfer velocity (k) to be more variable than pCO2aq, our work shows that over longer time (days to weeks) pCO2aq variability was greater and affected CO2 fluxes much more than k. We demonstrate that ≥8 measurement days distributed over multiple seasons in combination with sufficient spatial coverage (≥8 locations during stratification periods and 5 or less in spring and autumn) are a key for representative yearly whole lake flux estimates. This study illustrates the importance of considering spatiotemporal variability in pCO2aq and CO2 fluxes to generate representative whole lake estimates.

Low-order streams are suggested to dominate the atmospheric CO 2 source of all inland waters. Yet, many large-scale stream estimates suffer from methods not designed for gas emission determination and rarely include other greenhouse gases such as CH 4 . Here, we present a compilation of directly measured CO 2 and CH 4 concentration data from Swedish low-order streams (> 1600 observations across > 500 streams) covering large climatological and land-use gradients. These data were combined with an empirically derived gas transfer model and the characteristics of a ca. 400,000 km stream network covering the entire country. The total *Correspondence: marcus.wallin@geo.uu.se Author Contribution Statement: MBW brought the idea and compiled the concentration data. MBW and TG designed the study, analyzed the data and conducted the modelling component. AC, JA, DB, KB, JK, HJ, EL, SL, SN, SB, CT and GAW provided data, ideas and catchment/region specific information. MBW wrote the manuscript with great support from all co-authors.Data Availability Statement: Data are available in the Uppsala University data repository at http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-332472.Additional Supporting Information may be found in the online version of this article.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. This article is part of the Special Issue: Carbon cycling in inland waters Edited by: Emily Stanley and Paul del Giorgio Scientific Significance StatementStreams have been identified as disproportional emitters of CO 2 to the atmosphere across all inland waters. Despite their suggested importance, reliable large-scale stream C emission data are often lacking which makes current estimates uncertain. Here, we show that Swedish low-order streams emit much higher amounts of C to the atmosphere than previously reported, corresponding to 21% of the estimated terrestrial C sequestration. We also show that local scale spatiotemporal variability in stream gas concentrations often exceeds variability across regions, and that stream surface area matters. Without such fundamental information, large-scale stream C emission estimates will always be associated with a large degree of uncertainty.1

Freshwaters are important sources of the greenhouse gases methane (CH4) and carbon dioxide (CO2) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH4 and CO2 flux from a small shallow pond during a period of 4 months. Mean CH4 flux and surface water CH4 concentration were 8.0 [3.3-15.1] +/- A 3.1 mmol m(-2) day(-1) (mean [range] +/- A 1 SD) and 1.3 [0.3-3.5] +/- A 0.9 A mu M respectively. Mean CO2 flux was 1.1 [-9.8 to 16.0] +/- A 6.9 mmol m(-2) day(-1). Substantial diel changes in CO2 flux and surface water CH4 concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH4 emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO2 flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO2 consumption by plants was higher relative to CO2 production by respiration during warm sunny days. Interestingly, CH4 fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH4 and CO2 summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH4 emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons

7Inland waters were recently recognized to be important sources of methane (CH4) and carbon 8 dioxide (CO2) to the atmosphere, and including inland water emissions in large scale greenhouse 9 gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the 10 lack of GHG flux measurements and well-defined inland water areas for extrapolation, make the 11 magnitude of the potential offset unclear. This study presents coordinated flux measurements of 12 CH4 and CO2 in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. 13All these inland water types, representative of common aquatic ecosystems in India, emitted 14 substantial amounts of CH4 and a major fraction also emitted CO2. The total CH4 flux (including 15 ebullition and diffusion) from all the 45 systems ranged from from 0.01 to 52.1 mmol m with CH4 contributing 71%. Hence, average inland water GHG emissions, which were not 24 previously considered, correspond to 42% (30 -55%) of the estimated land carbon sink of India. 25