Nitrous oxide (N 2 O) can be both produced and reduced to dinitrogen (N 2 ) during microbial denitrification, with the balance between these steps controlling the net flux of this potent climate gas. Here, we first used a meta‐analysis of published studies to predict how warming may regulate N 2 O and N 2 production in soils and sediments. However, as most of these former studies used nitrate at far higher than ambient concent...

The physical effects of climate warming have been well documented, but the biological responses are far less well known, especially at the ecosystem level and at large (intercontinental) scales. Global warming over the next century is generally predicted to reduce food web complexity, but this is rarely tested empirically due to the dearth of studies isolating the effects of temperature on complex natural food webs. To overcom...

Summary Methane is a powerful greenhouse gas but the microbial diversity mediating methylotrophic methanogenesis is not well‐characterized. One overlooked route to methane is via the degradation of dimethylsulfide (DMS), an abundant organosulfur compound in the environment. Methanogens and sulfate‐reducing bacteria (SRB) can degrade DMS in anoxic sediments depending on sulfate availability. However, we know little about the un...

Summary Anaerobic ammonium oxidation (anammox) and nitrite‐dependent anaerobic methane oxidation (n‐damo) play important roles in nitrogen and carbon cycling in fresh waters but we do not know how these two processes compete for their common electron acceptor, nitrite. Here, we investigated the spatial distribution of anammox and n‐damo across a range of permeable riverbed sediments. Anammox activity and gene abundance were de...

Functional traits are increasingly being used to predict extinction risks and range shifts under long‐term climate change scenarios, but have rarely been used to study vulnerability to extreme climatic events, such as supraseasonal droughts. In streams, drought intensification can cross thresholds of habitat loss, where marginal changes in environmental conditions trigger disproportionate biotic responses. However, these thres...

We investigated the seasonal dynamics of in‐stream metabolism at the reach scale (∼ 150 m) of headwaters across contrasting geological sub‐catchments: clay, Greensand, and Chalk of the upper River Avon (UK). Benthic metabolic activity was quantified by aquatic eddy co‐variance while water column activity was assessed by bottle incubations. Seasonal dynamics across reaches were specific for the three types of geologies. During...

Methane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production. We assessed methanotrophy and photosynthetic production under differing conditions of light and methane concentration. We measured methane oxidation and photosynthesis in gravel sediments from adjacent shaded and unshaded stretches of 15 chalk rivers in southern England, and also i...

Aim The average carbon‐to‐nitrogen‐to‐phosphorus ratio (C:N:P) of marine algae is known to be tightly coupled to that of the inorganic pools of C , N and P in the ocean interior (i.e. the Redfield ratio), and therefore plays a key role in regulating the C and N cycles in the ocean. The C:N:P ratio of algae also varies substantially, both within and among taxa, in response to variation in the abiotic environment, raising the po...

Biodiversity loss is occurring rapidly worldwide, yet it is uncertain whether few or many species are required to sustain ecosystem functioning in the face of environmental change. The importance of biodiversity might be enhanced when multiple ecosystem processes (termed multifunctionality) and environmental contexts are considered, yet no studies have quantified this explicitly to date. We measured five key processes and thei...

Ecosystem respiration is a primary component of the carbon cycle and understanding the mechanisms that determine its temperature dependence will be important for predicting how rates of carbon efflux might respond to global warming. We used a rare model system, comprising a network of geothermally heated streams ranging in temperature from 5 °C to 25 °C, to explore the nature of the relationship between respiration and tempera...

We measured denitrification, anaerobic ammonium oxidation (anammox), oxygen uptake, nutrient exchange, and pore‐water profiles of oxygen in intact sediments at three sites in the southern North Sea, which we experimentally exposed to different oxygen saturations (ambient and ∼ 33% of air‐saturation for oxygen [i.e., our hypoxic treatment]) over 14 months. Denitrification ranged from 1 μmol N m −2 h −1 to 21 μmol N m −2 h −1 ,...

Methane (CH 4 ) and carbon dioxide (CO 2 ) are the dominant gaseous end products of the remineralization of organic carbon and also the two largest contributors to the anthropogenic greenhouse effect. We investigated whether warming altered the balance of CH 4 efflux relative to gross primary production (GPP) and ecosystem respiration (ER) in a freshwater mesocosm experiment. Whole ecosystem CH 4 efflux was strongly related to...

Due to a combination of local methanogenesis and high background concentrations in the groundwater, water in the River Lambourn is 51 times supersaturated with methane (162 nmol CH 4 L −1 ). Pore‐water concentrations of methane in the gravels of the riverbed were much lower throughout the year (71 nmol CH 4 L −1 ), suggesting significant methane oxidation. To investigate the potential for methane oxidation as a novel chemosynt...

We measured the δ 13 C values of dominant primary consumers and their potential food sources in a groundwater‐fed lowland river. The δ 13 C of most consumers, such as Gammarus and Simulium , reflected that of the dominant forms of photosynthetic production, whereas the cased larvae of two caddis flies ( Agapetus and Silo ) were consistently 13 C depleted (mean δ 13 C: ‐41.2% and ‐40.4%, respectively) throughout the year. The r...

We measured the production of N 2 gas from anammox and denitrification simultaneously in intact sediment cores at six sites along a transect of the continental shelf (50 m) and deeper slope (2000 m) in the North Atlantic. Maximum rates of total N 2 production were measured on the shelf and were largely due to denitrification, with anammox contributing, on average, 33% of this production. On the continental slope, the productio...

The oxygen minimum zone (OMZ) of the Arabian Sea is a significant source of nitrous oxide (N 2 O), yet the metabolism responsible for N 2 O production is unclear. High‐resolution profiles identified peaks and troughs of N 2 O and NO 2 ‐ in the top 500 m of the water column. The first peak in N 2 O was not in the oxycline, but deeper at the oxic‐suboxic interface. Peaks and troughs were targeted with a suite of 15 N incubations...