Streams and rivers can substantially modify organic carbon (OC) inputs from terrestrial landscapes, and much of this processing is the result of microbial respiration. While carbon dioxide (CO2) is the major end‐product of ecosystem respiration, methane (CH4) is also present in many fluvial environments even though methanogenesis typically requires anoxic conditions that may be scarce in these systems. Given recent recognition of the pervasiveness of this greenhouse gas in streams and rivers, we synthesized existing research and data to identify patterns and drivers of CH4, knowledge gaps, and research opportunities. This included examining the history of lotic CH4 research, creating a database of concentrations and fluxes (MethDB) to generate a global‐scale estimate of fluvial CH4 efflux, and developing a conceptual framework and using this framework to consider how human activities may modify fluvial CH4 dynamics. Current understanding of CH4 in streams and rivers has been strongly influenced by goals of understanding OC processing and quantifying the contribution of CH4 to ecosystem C fluxes. Less effort has been directed towards investigating processes that dictate in situ CH4 production and loss. CH4 makes a meager contribution to watershed or landscape C budgets, but streams and rivers are often significant CH4 sources to the atmosphere across these same spatial extents. Most fluvial systems are supersaturated with CH4 and we estimate an annual global emission of 26.8 Tg CH4, equivalent to ~15‐40% of wetland and lake effluxes, respectively. Less clear is the role of CH4 oxidation, methanogenesis, and total anaerobic respiration to whole ecosystem production and respiration. Controls on CH4 generation and persistence can be viewed in terms of proximate controls that influence methanogenesis (organic matter, temperature, alternative electron acceptors, nutrients) and distal geomorphic and hydrologic drivers. Multiple controls combined with its extreme redox status and low solubility result in high spatial and temporal variance of CH4 in fluvial environments, which presents a substantial challenge for understanding its larger‐scale dynamics. Further understanding of CH4 production and consumption, anaerobic metabolism, and ecosystem energetics in streams and rivers can be achieved through more directed studies and comparison with knowledge from terrestrial, wetland, and aquatic disciplines.

Growing awareness of ongoing and rapid changes in Earth's carbon cycle is motivating a new era of research aimed at improving our understanding of ecosystems as both responders to, and drivers of larger-scale biogeochemical dynamics. In the case of streams and rivers, this has often taken the form of elucidating their role as processors of organic carbon (OC), a capacity that far exceeds their meager size and significantly influences the export of continental OC to marine environments (Cole et al. 2007, Battin et al. 2009, Aufdenkampe et al. 2011). Amplified OC processing has been inferred from observations of smaller export loads relative to inputs, rates of ecosystem respiration that exceed gross primary production, and/or occurrence of supersaturated concentrations of the products of OC decomposition, namely, carbon dioxide (CO 2) and methane (CH 4).

Many physical, chemical, and biological processes in freshwater ecosystems mobilize the nutrient phosphorus (P) from sediments, which in turn may contribute to the formation of harmful algal blooms. Here, we critically reviewed internal P loading in Canadian fresh waters to understand the geographic patterns and environmental drivers of this important process. From 43 publications, we consolidated 618 estimates of internal P loading from Canadian freshwater ponds, lakes, reservoirs, and coastal wetlands (n = 70). Expressed in terms of total P, short-term gross rates in sediment samples (L gross ) ranged from −27 to 54 mg·m −2 ·day −1 (n = 461), while long-term net rates in whole ecosystems (L net ) ranged from −1694 to 10 640 mg·m −2 ·year −1 (n = 157). The main environmental drivers of this variation were oxygen, pH, geology, and trophic state. Internal P loading tended to be higher during the open-water season and most prominent in small prairie lakes. Priorities for future research on internal P loading should include resolving methodological problems, assessing the relative importance of different mechanisms, examining the influence of anthropogenic activities, and quantifying rates in understudied ecosystems. Résumé :De nombreux processus physiques, chimiques et biologiques dans les écosystèmes d'eau douce mobilisent le phosphore (P), un élément nutritif, des sédiments, ce qui peut favoriser la formation de fleurs d'eau néfastes. Nous avons effectué un examen critique de l'apport interne de P dans les plans d'eau douce canadiens afin de comprendre les motifs de répartition géographique et les facteurs environnementaux qui influent sur cet important processus. À la lumière de 43 publications, nous avons colligé 618 estimations de l'apport interne de P d'étangs, de lacs, de réservoirs et de milieux humides littoraux d'eau douce canadiens (n = 70). Exprimés en termes de P total, les taux d'apport de P à court terme pour les échantillons de sédiments (L gross ) vont de −27 à 54 mg·m -2 ·jour -1 (n = 461), alors que les taux nets à long terme à l'échelle de l'écosystème (L net ) vont de −1694 à 10 640 mg·m -2 ·an -1 (n = 157). Les principaux facteurs environnementaux influant sur ces variations sont l'oxygène, le pH, la géologie et l'état trophique. L'apport interne de P tend à être plus élevé durant la période d'eau libre et le plus marqué dans les petits lacs de prairie. La priorité des travaux futurs sur l'apport interne de P devrait porter sur la résolution de problèmes méthodologiques, l'évaluation de l'importance relative de différents mécanismes, l'examen de l'influence des activités humaines et la quantification des taux dans les écosystèmes sous-étudiés. [Traduit par la Rédaction]

Advanced sensor technology is widely used in aquatic monitoring and research. Most applications focus on temporal variability, whereas spatial variability has been challenging to document. We assess the capability of water chemistry sensors embedded in a high-speed water intake system to document spatial variability. This new sensor platform continuously samples surface water at a range of speeds (0 to >45 km h(-1)) resulting in high-density, mesoscale spatial data. These novel observations reveal previously unknown variability in physical, chemical, and biological factors in streams, rivers, and lakes. By combining multiple sensors into one platform, we were able to detect terrestrial-aquatic hydrologic connections in a small dystrophic lake, to infer the role of main-channel vs backwater nutrient processing in a large river and to detect sharp chemical changes across aquatic ecosystem boundaries in a stream/lake complex. Spatial sensor data were verified in our examples by comparing with standard lab-based measurements of selected variables. Spatial fDOM data showed strong correlation with wet chemistry measurements of DOC, and optical NO3 concentrations were highly correlated with lab-based measurements. High-frequency spatial data similar to our examples could be used to further understand aquatic biogeochemical fluxes, ecological patterns, and ecosystem processes, and will both inform and benefit from fixed-site data.

Winter is an understudied but key period for the socioecological systems of northeastern North American forests. A growing awareness of the importance of the winter season to forest ecosystems and surrounding communities has inspired several decades of research, both across the northern forest and at other mid‐ and high‐latitude ecosystems around the globe. Despite these efforts, we lack a synthetic understanding of how winter climate change may impact hydrological and biogeochemical processes and the social and economic activities they support. Here, we take advantage of 100 years of meteorological observations across the northern forest region of the northeastern United States and eastern Canada to develop a suite of indicators that enable a cross‐cutting understanding of (1) how winter temperatures and snow cover have been changing and (2) how these shifts may impact both ecosystems and surrounding human communities. We show that cold and snow covered conditions have generally decreased over the past 100 years. These trends suggest positive outcomes for tree health as related to reduced fine root mortality and nutrient loss associated with winter frost but negative outcomes as related to the northward advancement and proliferation of forest insect pests. In addition to effects on vegetation, reductions in cold temperatures and snow cover are likely to have negative impacts on the ecology of the northern forest through impacts on water, soils, and wildlife. The overall loss of coldness and snow cover may also have negative consequences for logging and forest products, vector‐borne diseases, and human health, recreation, and tourism, and cultural practices, which together represent important social and economic dimensions for the northern forest region. These findings advance our understanding of how our changing winters may transform the socioecological system of a region that has been defined by the contrasting rhythm of the seasons. Our research also identifies a trajectory of change that informs our expectations for the future as the climate continues to warm.