Peatland streams have repeatedly been shown to be highly supersaturated in both CO 2 and CH 4 with respect to the atmosphere, and in combination with dissolved (DOC) and particulate organic carbon (POC) represent a potentially important pathway for catchment greenhouse gas (GHG) and carbon (C) losses. The aim of this study was to create a complete C and GHG (CO 2 , CH 4 , N 2 O) budget for Auchencorth Moss, an ombrotrophic peatland in southern Scotland, by combining flux tower, static chamber and aquatic flux measurements from 2 consecutive years. The sink/source strength of the catchment in terms of both C and GHGs was compared to assess the relative importance of the aquatic pathway. During the study period (2007)(2008) the catchment functioned as a net sink for GHGs (352 g CO 2 -Eq m À2 yr À1 ) and C (69.5 g C m À2 yr À1 ). The greatest flux in both the GHG and C budget was net ecosystem exchange (NEE). Terrestrial emissions of CH 4 and N 2 O combined returned only 4% of CO 2 equivalents captured by NEE to the atmosphere, whereas evasion of GHGs from the stream surface returned 12%. DOC represented a loss of 24% of NEE C uptake, which if processed and evaded downstream, outside of the catchment, may lead to a significant underestimation of the actual catchment-derived GHG losses. The budgets clearly show the importance of aquatic fluxes at Auchencorth Moss and highlight the need to consider both the C and GHG budgets simultaneously.

Understanding of the processes that control CO 2 concentrations in the aquatic environment has been hampered by the absence of a direct method to make continuous measurements over both short-and long-term time intervals. We describe an in situ method in which a non-dispersive infrared (NDIR) sensor is enclosed in a water impermeable, gas permeable polytetrafluoroethylene (PTFE) membrane and deployed in a freshwater environment. This allows measurements of CO 2 concentration to be made directly at a specific depth in the water column without the need for pumps or reagents. We demonstrate the potential of the method using examples from different aquatic environments characterized by a range of CO 2 concentrations (0Ð5-8Ð0 mg CO 2 -C l 1 , equivalent to ca 40-650 µmol CO 2 l 1 ). These comprise streams and ponds from tropical, temperate and boreal regions. Data derived from the sensor was compared with direct measurements of CO 2 concentrations using headspace analysis. Sensor performance following long-term (>6 months) field deployment conformed to manufacturers' specifications, with no drift detected. We conclude that the sensor-based method is a robust, accurate and responsive method, with a wide range of potential applications, particularly when combined with other in situ sensor-based measurements of related variables.

Biology, 19 (7). 2133-2148. 10.1111/gcb.12209 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Accepted ArticleThis article is protected by copyright. All rights reserved.Corresponding Author: Kerry J Dinsmore, 0131 445 8583 kjdi@ceh.ac.uk AbstractThe aquatic pathway is increasingly being recognised as an important component of catchment carbon and greenhouse gas (GHG) budgets, particularly in peatland systems due to their large carbon store and strong hydrological connectivity. In this study we present a complete 5-year dataset of all aquatic carbon and GHG species (POC, DOC, DIC, CO 2 , CH 4 , N 2 O) from an ombrotrophic Scottish peatland. We show that short term variability in concentrations exists across all species and this is strongly linked to discharge. Seasonal cyclicity was only evident in DOC, CO 2 and CH 4 concentration; however temperature correlated with monthly means in all species except DIC. Whilst the temperature correlation with monthly DOC and POC concentrations appeared to be related to biological productivity in the terrestrial system, we suggest the temperature correlation with CO 2 and CH 4 was primarily due to in-stream temperature-dependent solubility. Interannual variability in total aquatic carbon concentration was strongly correlated with catchment GPP indicating a strong potential terrestrial aquatic linkage. DOC represented the largest aquatic carbon flux term (19.3 ± 4.59 g C m -2 yr -1 ), followed by CO 2 evasion (10.0 g C m -2 yr -1 ). Despite an estimated contribution to the total aquatic carbon flux of between 8 -48%, evasion estimates have the greatest uncertainty. Interannual variability in total aquatic carbon export was low in comparison with variability in terrestrial biosphere-atmosphere exchange, and could be explained primarily by temperature and precipitation. Our results therefore suggest that climatic change is likely to have a significant impact on annual carbon losses through the aquatic pathway, and as such aquatic exports are fundamental to the understanding of whole catchment responses to climate change.

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This version available at http://nora.nerc.ac.uk/5459/ NERC has developed NORA to enable users to access research outputs wholly or partially funded by NERC. Copyright and other rights for material on this site are retained by the authors and/or other rights owners. Users should read the terms and conditions of use of this material at http://nora.nerc.ac.uk/policies.html#access This document is the author's final manuscript version of the journal article, incorporating any revisions agreed during the peer review process. Some differences between this and the publisher's version remain. You are advised to consult the publisher's version if you wish to cite from this article. Peatland landscapes typically exhibit large variations in greenhouse gas (GHG) emissions due to microtopographic and vegetation heterogeneity. As many peatland budgets are extrapolated from small-scale chamber measurements it is important to both quantify and understand the processes underlying this spatial variability. Here we carried out a mesocosm study which allowed a comparison to be made between different microtopographic features and vegetation communities, in response to conditions of both static and changing water table. Three mesocosm types (hummocks + Juncus effusus, hummocks + Eriophorum vaginatum, and hollows dominated by moss) were subjected to 2 water table treatments (0-5 cm and 30-35 cm depth). Measurements were made of soil-atmosphere GHG exchange, GHG concentration within the peat profile and soil water solute concentrations. After 14 weeks the high water table group was drained and the low water table group flooded. Measurement intensity was then increased to examine the immediate response to change in water table position.Mean CO 2 , CH 4 and N 2 O exchange across all chambers was 39.8 µg m -2 s -1 , 54.7 µg m -2 h -1 and -2.9 µg m -2 h -1 , respectively. Hence the GHG budget was dominated in this case by CO 2 exchange. CO 2 and N 2 O emissions were highest in the low water table treatment group; CH 4 emissions were highest in the saturated mesocosms. We observed a strong interaction between mesocosm type and water table for CH 4 emissions. In contrast to many previous studies, we found that the presence of aerenchyma-containing vegetation reduced CH 4 emissions. A significant pulse in both CH 4 and N 2 O emissions occurred within 1-2 days of switching the water table treatments. This pulsing could potentially lead to significant underestimation of landscape annual GHG budgets when widely spaced chamber measurements are upscaled. Introduction:Northern peatlands are estimated to contain 455 Gt of carbon (Gorham 1991), representing approximately a third of the estimated total global soil carbon pool.They are considered to be net sinks of CO 2 and net sources of CH 4 (Bartlett and Harriss 1993;Gorham 1991;Huttunen et al. 2003) , though annual and inter-annual variation can be extremely high. Peatlands also represent an important source of dissolved organic carbon to drainage waters (Urban et al. 1989;Billett et al. 2004;Daw...