Seasonal variability of sediment controls of carbon cycling in an agricultural stream

Whether interstitial space features were the main factors affecting sediment microbial community structures in Chaohu Lake

Sediments cover a majority of Earth's surface and are essential for global biogeochemical cycles. The effects of sediment physiochemical features on microbial community structures have attracted attention in recent years. However, the question of whether the interstitial space has significant effects on microbial community structures in submerged sediments remains unclear. In this study, based on identified OTUs (operational taxonomic units), correlation analysis, RDA analysis, and Permanova analysis were applied into investigating the effects of interstitial space volume, interstitial gas space, volumetric water content, sediment particle features (average size and evenness), and sediment depth on microbial community structures in different sedimentation areas of Chaohu Lake (Anhui Province, China). Our results indicated that sediment depth was the closest one to the main environmental gradient. The destruction effects of gas space on sediment structures can physically affect the similarity of the whole microbial community in all layers in river dominated sedimentation area (where methane emits actively). However, including gas space, none of the five interstitial space parameters were significant with accounting for the microbial community structures in a sediment layer. Thus, except for the happening of active physical destruction on sediment structures (for example, methane ebullition), sediment interstitial space parameters were ineffective for affecting microbial community structures in all sedimentation areas.

Methane concentrations and fluxes in agricultural and preserved tropical headwater streams

Tropical streams have been intensively impacted by agricultural activities. Among the most important agricultural activities in Brazil, sugarcane production represents a large impact for economic development and for environmental conditions. Permeating sugarcane fields, several headwater streams can be affected by sugarcane cultivation, in special, aquatic biogeochemical cycles because of the deforestation, fertilization, crop residues and higher temperatures in the tropics. In this study, we analyzed the effects of sugarcane cultivation on methane fluxes and concentrations, assuming that carbon cycles are influenced by agricultural activities in headwater streams. Our study aimed to (1) measure methane fluxes and concentrations in tropical streams located in Southeastern Brazil, (2) Analyze whether seasonal cycles influence methane fluxes and concentrations, (3) Evaluate the influence of sugarcane cultivation on methane fluxes and (4) Analyze the association between water chemistry in the methane concentrations in tropical streams. We found mean fluxes of CH₄ of 0.280 mmol m^-2 d^-1, with higher fluxes during the summer and in streams draining preserved catchments. The average CH₄ concentrations were 0.695 μmol L^-1, with higher values during the summer and in streams draining preserved catchments. Methane concentrations in the studied streams was influenced by dissolved oxygen (negatively), dissolved organic carbon (negatively), water velocity (positively) and conductivity (negatively). Methane concentrations were significantly higher than concentrations found in Temperate Grasslands, Savannas & Shrublands and similar to concentrations found in other tropical biomes (excluding Tropical & Subtropical Moist Broadleaf Forests which receives large amounts of organic inputs). We conclude that sugarcane influence methane concentrations and fluxes in tropical streams by reducing the organic matter availability provided by the native vegetation in soil and water.

Removal of Fecal Indicator Bacteria by River Networks

Fecal contamination is a significant source of water quality impairment globally. Aquatic ecosystems can provide an important ecosystem service of fecal contamination removal. Understanding the processes that regulate the removal of fecal contamination among river networks across flow conditions is critical. We applied a river network model, the Framework for Aquatic Modeling in the Earth System (FrAMES-Ecoli), to quantify removal of fecal indicator bacteria by river networks across flow conditions during summers in a series of New England watersheds of different characteristics. FrAMES-Ecoli simulates sources, transport, and riverine removal of Escherichia coli (E. coli). Aquatic E. coli removal was simulated in both the water column and the hyporheic zone, and is a function of hydraulic conditions, flow exchange rates with the hyporheic zone, and die-off in each compartment. We found that, at the river network scale during summers, removal by river networks can be high (19–99%) with variability controlled by hydrologic conditions, watershed size, and distribution of sources in the watershed. Hydrology controls much of the variability, with 68–99% of network scale inputs removed under base flow conditions and 19–85% removed during storm events. Removal by the water column alone could not explain the observed pattern in E. coli, suggesting that processes such as hyporheic removal must be considered. These results suggest that river network removal of fecal indicator bacteria should be taken into consideration in managing fecal contamination at critical downstream receiving waters.

Hot spot of CH4 production and diffusive flux in rivers with high urbanization

Rivers and streams play a central role in global carbon budget, but our knowledge is limited on the magnitude and extent of urbanization influence on riverine methane (CH₄) dynamics. In this study, we investigated dissolved CH₄ (dCH₄) concentration and CH₄ diffusive fluxes in 27 river segments of two 4^th-order and three 3^rd-order tributary rivers to the Yangtze River in China, which drained land areas with varied urbanization intensities. We found that urban development was the key factor responsible for high fluvial dCH₄ concentration and diffusive flux, exceeding the influence of agricultural farming, and these headwater rivers were over-saturated in CH₄ with respect to atmospheric equilibrium. dCH₄ concentration (3546 ± 6770 nmol L^-1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L^-1 and 764 ± 708 nmol L^-1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). River segments draining higher urban area also acted as important sources of CH₄ to the atmosphere (8.93 ± 14.29 mmol m^-2 d^-1). Total nitrogen (TN) concentration in river water showed the best prediction capacity when compared to other water parameters. Based on urban land use grouping, nutrient elements could predict dCH₄ well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (TN, ammonia nitrogen, and total phosphorus). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH₄ concentration. These findings highlight that combining distal geomorphic and hydrologic drivers can be effective in determining the relationship between riverine CH₄ and the proximal controls (e.g., nutrients, dissolved oxygen, dissolved organic carbon), as well as in identifying their key drivers. Being rapid urbanization a common feature of catchments worldwide, our results suggest riverine CH₄ emissions will increase into the future.

Ebullition Controls on CH4 Emissions in an Urban, Eutrophic River: A Potential Time-Scale Bias in Determining the Aquatic CH4 Flux

Rivers and streams contribute significant quantities of methane (CH₄) to the atmosphere. However, there is a lack of CH₄ flux and ebullitive (bubble) emission data from urban rivers, which might lead to large underestimations of global aquatic CH₄ emissions. Here, we conducted high-frequency surveys using the boundary layer model (BLM) supplemented with floating chambers (FCs) and bubble traps to investigate the seasonal and diurnal variability in CH₄ emissions in a eutrophic urban river and to evaluate whether the contribution of bubbles is important. We found that ebullition contributed nearly 99% of CH₄ emissions and varied on hourly to seasonal time scales, ranging from 0.83 to 230 mmol m^-2 d^-1, although diffusive emissions and CH₄ concentrations in bubbles did not exhibit temporal variability. Ebullitive CH₄ emissions presented high temperature sensitivity (r = 0.6 and p < 0.01) in this urban river, and eutrophication might have triggered this high temperature sensitivity. The ebullitive CH₄ flux is more likely to be underestimated at low temperatures because capturing the bubble flux is more difficult, given the low frequency of ebullition events. This study suggests that future ebullition measurements on longer time scales are needed to accurately quantify the CH₄ budgets of eutrophic urban rivers.

River temperature research and practice: Recent challenges and emerging opportunities for managing thermal habitat conditions in stream ecosystems

There is growing evidence that river temperatures are increasing under climate change, which is expected to be exacerbated by increased abstractions to satisfy human water demands. Water temperature research has experienced crucial advances, both in terms of developing new monitoring and modelling tools, as well as understanding the mechanisms of temperature feedbacks with biogeochemical and ecological processes. However, water practitioners and regulators are challenged with translating the widespread and complex technological, modelling and conceptual advances made in river temperature research into improvements in management practice. This critical review provides a comprehensive overview of recent advances in the state-of-the-art monitoring and modelling tools available to inform ecological research and practice. In so doing, we identify pressing research gaps and suggest paths forward to address practical research and management challenges. The proposed research directions aim to provide new insights into spatio-temporal stream temperature dynamics and unravel drivers and controls of thermal river regimes, including the impacts of changing temperature on metabolism and aquatic biogeochemistry, as well as aquatic organisms. The findings of this review inform future research into ecosystem resilience in the face of thermal degradation and support the development of new management strategies cutting across spatial and temporal scales.

Methane and nitrous oxide porewater concentrations and surface fluxes of a regulated river

Greenhouse gas (GHG) emissions from rivers are a critical missing component of current global GHG models. Their exclusion is mainly due to a lack of in-situ measurements and a poor understanding of the spatiotemporal dynamics of GHG production and emissions, which prevents optimal model parametrization. We combined simultaneous observations of porewater concentrations along different beach positions and depths, and surface fluxes of methane and nitrous oxide at a plot scale in a large regulated river during three water stages: rising, falling, and low. Our goal was to gain insights into the interactions between hydrological exchanges and GHG emissions and elucidate possible hypotheses that could guide future research on the mechanisms of GHG production, consumption, and transport in the hyporheic zone (HZ). Results indicate that the site functioned as a net source of methane. Surface fluxes of methane during river water stages at three beach positions (shallow, intermediate and deep) correlated with porewater concentrations of methane. However, fluxes were significantly higher in the intermediate position during the low water stage, suggesting that low residence time increased methane emissions. Vertical profiles of methane peaked at different depths, indicating an influence of the magnitude and direction of the hyporheic mixing during the different river water stages on methane production and consumption. The site acted as either a sink or a source of nitrous oxide depending on the elevation of the water column. Nitrous oxide porewater concentrations peaked at the upper layers of the sediment throughout the different water stages. River hydrological stages significantly influenced porewater concentrations and fluxes of GHG, probably by influencing heterotrophic respiration (production and consumption processes) and transport to and from the HZ. Our results highlight the importance of including dynamic hydrological exchanges when studying and modeling GHG production and consumption in the HZ of large rivers.