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1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function. 2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 lm) dominated by the sand-sized fraction (63-1000 lm). 3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH 4 transport through Ranunculus stems and an increase in water column CH 4 concentration, while the sediment CH 4 concentration increased 100-fold between February and April. A marked seasonal enrichment in the d 15 N of N 2 dissolved in the pore water correlated with CH 4 flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin. 4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems. 5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 · 10 )6 Tg CH 4 year )1 ), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.

[1] There is widespread recognition that the groundwater-surface water interface can have significant influence on the pattern and form of the transfer of nutrient-rich groundwater to rivers. Characterizing and quantifying this influence is critical for successful management of water resources in many catchments, particularly those threatened by rising nitrate levels in groundwater. Building on previous experimental investigations in one such catchment: the River Leith, UK, we report on a multimeasurement, multiscale program aimed at developing a conceptualization of groundwater-surface water flow pathways along a 200 m reach. Key to this conceptualization is the quantification of vertical and horizontal water fluxes, which is achieved through a series of Darcian flow estimates coupled with in-stream piezometer tracer dilution tests. These data, enhanced by multilevel measurements of chloride concentration in riverbed pore water and water-borne geophysical surveying, reveal a contrast in the contribution of flow components along the reach. In the upper section of the reach, a localized connectivity to regional groundwater, that appears to suppress the hyporheic zone, is identified. Further downstream, horizontal (lateral and longitudinal) flows appear to contribute more to the total subsurface flow at the groundwater-surface water interface. Although variation in hydraulic conductivity of the riverbed is observed, localized variation that can account for the spatial variability in flow pathways is not evident. The study provides a hydrological conceptualization for the site, which is essential for future studies which address biogeochemical processes, in relation to nitrogen retention/release. Such a conceptualization would not have been possible without a multiexperimental program.Citation: Binley, A., S. Ullah, A. L. Heathwaite, C. Heppell, P. Byrne, K. Lansdown, M. Trimmer, and H. Zhang (2013), Revealing the spatial variability of water fluxes at the groundwater-surface water interface, Water Resour. Res., 49,[3978][3979][3980][3981][3982][3983][3984][3985][3986][3987][3988][3989][3990][3991][3992]

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Rivers are an important global sink for excess bioavailable nitrogen: they convert approximately 4 40% of terrestrial N-runoff per year (~47 Tg) to biologically unavailable N 2 gas and return it to 5 the atmosphere. 1 Currently, riverine N 2 production is conceptualised and modelled as 6 denitrification. [2][3][4] The contribution of anaerobic ammonium oxidation (or anammox), an alternate 7 pathway of N 2 production important in marine environments, is not well understood. 5,6 Here we 8 use in situ and laboratory measurements of anammox activity using 15 N tracers and molecular 9 analyses of microbial communities to evaluate anammox in clay, sand, and chalk-dominated rates. In spite of requiring anoxic conditions, anammox, most likely coupled to partial 14 nitrification, contributed up to 58% of in situ N 2 production in oxic, permeable riverbeds.. In 15 contrast, denitrification dominated in low permeability clay-bed rivers, where anammox 16 contributes roughly 7% to the production of N 2 gas. We conclude that anammox can represent an 17 important nitrogen loss pathway in permeable river sediments. and increases a river's capacity to attenuate nitrogen. 49Much of what is known about anammox in the environment comes from estuaries and 50 coastal seas where anammox varies in response to sediment reactivity. The relative 51 contribution of anammox to marine N 2 production (ra) decreases with proximity to the shore 52 as supply of carbon stimulates denitrification over anammox. 12,13 Extrapolating this trend 53 further inshore suggested anammox activity would be insignificant in estuaries but anammox 54 potential actually increased. 14,15 In both estuaries and coastal seas, however, anammox is 55 important in low permeability sediments (ra <1 to 11 %) 9,16 , where oxygen penetration is 56 restricted 12,15 and it is these muddy sediments that the few studies of riverine anammox have 57 occurred. 5,6 In addition, anammox is widespread in marine sediments but the affiliated 64Using a combination of in situ and laboratory-based 15 N tracer techniques 12,18 and molecular 65 assays we characterised both the anammox community and its activity within rivers from 66 clay, sand and chalk-dominated sub-catchments under summer, base flow conditions (Table 67 S1). For rivers in which in situ measurements were performed we indexed catchment 68 permeability by calculating the base-flow index (BFI , Table S2), the proportion of river flow 69 4 derived from deep groundwater sources. In clay catchments, low soil permeability leads to 70 routing of rainfall overland or through shallow, more permeable soils into the river (low BFI). 71Whilst in chalk or sand catchments, the higher soil permeability allows infiltrated water to 72 percolate deeper into the aquifer and follow much longer flow paths to towards the river 73 (high BFI). 74We began by characterising the anammox hzo functional gene that encodes hydrazine 75 oxidoreductase which catalyses the oxidation of hydrazine to N 2 . The hzo gene was detected 76 in all sediment...

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Flows of water, soil, litter, and anthropogenic materials in and around rivers lead to the mixing of their resident microbial communities and subsequently to a resultant community distinct from its precursors. Consideration of these events through a new conceptual lens, namely, community coalescence, could provide a means of integrating physical, environmental, and ecological mechanisms to predict microbial community assembly patterns better in these habitats. Here, we review field studies of microbial communities in riverine habitats where environmental mixing regularly occurs, interpret some of these studies within the community coalescence framework and posit novel hypotheses and insights that may be gained in riverine microbial ecology through the application of this concept. Particularly in the face of a changing climate and rivers under increasing anthropogenic pressures, knowledge about the factors governing microbial community assembly is essential to forecast and/or respond to changes in ecosystem function. Additionally, there is the potential for microbial ecology studies in rivers to become a driver of theory development: riverine systems are ideal for coalescence studies because regular and predictable environmental mixing occurs. Data appropriate for testing community coalescence theory could be collected with minimal alteration to existing study designs.