Inland waters transport and transform substantial amounts of carbon and account for ∼18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (∼0.23 mmol CH4 m(-2) d(-1) vs ∼19.7 mmol CH4 m(-2) d(-1), respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%.

The impact of ocean acidification on benthic habitats is a major preoccupation of the scientific community. However, the natural variability of pCO2 and pH in those habitats remains understudied, especially in temperate areas. In this study we investigated temporal variations of the carbonate system in nearshore macrophyte meadows of the western Baltic Sea. These are key benthic ecosystems, providing spawning and nursery areas as well as food to numerous commercially important species. In situ pCO2, pH (total scale), salinity and PAR irradiance were measured with a continuous recording sensor package dropped in a shallow macrophyte meadow (Eckernförde bay, western Baltic Sea) during three different weeks in July (pCO2 and PAR only), August and September 2011.The mean (± SD) pCO2 in July was 383±117 µatm. The mean (± SD) pCO2 and pHtot in August were 239±20 µatm and 8.22±0.1, respectively. The mean (± SD) pCO2 and pHtot in September were 1082±711 µatm and 7.83±0.40, respectively. Daily variations of pCO2 due to photosynthesis and respiration (difference between daily maximum and minimum) were of the same order of magnitude: 281±88 µatm, 219±89 μatm and 1488±574 µatm in July, August and September respectively. The observed variations of pCO2 were explained through a statistical model considering wind direction and speed together with PAR irradiance. At a time scale of days to weeks, local upwelling of elevated pCO2 water masses with offshore winds drives the variation. Within days, primary production is responsible. The results demonstrate the high variability of the carbonate system in nearshore macrophyte meadows depending on meteorology and biological activities. We highlight the need to incorporate these variations in future pCO2 scenarios and experimental designs for nearshore habitats.

Ice sheets are currently ignored in global methane budgets 1,2. They have been proposed to cap large reserves of methane that may contribute to a rise in atmospheric methane concentrations if released during periods of rapid ice retreat 3,4 , but no data on the current methane footprint of ice sheets currently exist. Here we find that subglacially-produced methane is rapidly flushed to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland Ice Sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice sheet bed during the melt season. Pulses of high CH4(aq) concentrations coincided with supraglacially-forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season were indicative of subglacial methane reserves in excess of export, with an estimated 6.3 (2.4-11) tonnes of CH4(aq) laterally transported from the ice sheet bed. Stable isotope analyses revealed a microbial origin for methane; most likely derived from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation 5 to about 17% of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive Author contribution J.L.W. and G.L.G. designed the study. B.S.L. supervised stable-isotope analyses. S.A. performed the reaction-transport hydrate model calculations. P.F. assisted in the interpretation and analysis of the CONTROS HydroC ®-CH4 raw results. G.

The final published version of this manuscript will replace the preliminary version at the above DOI once it is available.If you would like to cite this EOR in a separate work, please use the following full citation:Fietzek, P., B. Fiedler, T. Steinhoff, and A. Körtzinger, 2013: In situ quality assessment of a novel underwater pCO2 sensor based on membrane equilibration and NDIR spectrometry.

In recent years, profiling floats, which form the basis of the successful international Argo observatory, are also being considered as platforms for marine biogeochemical research. This study showcases the utility of floats as a novel tool for combined gas measurements of CO 2 partial pressure ( pCO 2 ) and O 2 . These float prototypes were equipped with a small-sized and submersible pCO 2 sensor and an optode O 2 sensor for highresolution measurements in the surface ocean layer. Four consecutive deployments were carried out during November 2010 and June 2011 near the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic. The profiling float performed upcasts every 31 h while measuring pCO 2 , O 2 , salinity, temperature, and hydrostatic pressure in the upper 200 m of the water column. To maintain accuracy, regular pCO 2 sensor zeroings at depth and surface, as well as optode measurements in air, were performed for each profile. Through the application of data processing procedures (e.g., time-lag correction), accuracies of floatborne pCO 2 measurements were greatly improved (10-15 matm for the water column and 5 matm for surface measurements). O 2 measurements yielded an accuracy of 2 mmol kg 21. First results of this pilot study show the possibility of using profiling floats as a platform for detailed and unattended observations of the marine carbon and oxygen cycle dynamics.