Future directions for river carbon biogeochemistry observations

Rivers carry large quantities of carbon and form an important link between terrestrial, marine and atmospheric biogeochemical cycles, yet our observations of river carbon are severely limited. Here we provide a blueprint to build a global River Observation System that would improve our ability to observe and predict changes in this crucial piece of the global carbon cycle

The potential hidden age of dissolved organic carbon exported by peatland streams

Radiocarbon (14C) is a key tracer for detecting the mobilization of previously stored terrestrial organic carbon (C) into aquatic systems. Old C (>1,000 years BP) may be “masked” by postbomb C (fixed from the atmosphere post‐1950 CE), potentially rendering bulk aquatic dissolved organic C (DOC) 14C measurements insensitive to old C. We collected DOC with a modern 14C signature from a temperate Scottish peatland stream and decomposed it to produce CO2 under simulated natural conditions over 140 days. We measured the 14C of both DOC and CO2 at seven time points and found that while DOC remained close to modern in age, the resultant CO2 progressively increased in age up to 2,356 ± 767 years BP. The results of this experiment demonstrate that the bulk DO14C pool can hide the presence of old C within peatland stream DOC export, demonstrating that bulk DO14C measurements can be an insensitive indicator of peatland disturbance. Our experiment also demonstrates that this old C component is biologically and photochemically available for conversion to the greenhouse gas CO2, and as such, bulk DO14C measurements do not reflect the 14C signature of the labile organic C pool exported by inland water systems more broadly. Moreover, our experiment suggests that old C may be an important component of CO2 emissions to the atmosphere from peatland aquatic systems, with implications for tracing and modeling interactions between the hydrological and terrestrial C cycles

Ancient dissolved methane in inland waters revealed by a new collection method at low field concentrations for radiocarbon (14C) analysis

Methane (CH4) is a powerful greenhouse gas that plays a prominent role in the terrestrial carbon (C) cycle, and is released to the atmosphere from freshwater systems in numerous biomes globally. Radiocarbon (14C) analysis can indicate both the age and source of CH4 in natural environments. In contrast to CH4 present in bubbles released from aquatic sediments (ebullition), dissolved CH4 in lakes and streams can be present in low concentrations compared to carbon dioxide (CO2), and therefore obtaining sufficient aquatic CH4 for radiocarbon (14C) analysis remains a major technical challenge. Previous studies have shown that freshwater CH4, in both dissolved and ebullitive form, can be significantly older than other forms of aquatic C, and it is therefore important to characterise this part of the terrestrial C balance. This study presents a novel method to capture sufficient amounts of dissolved CH4 for 14C analysis in freshwater environments by circulating water across a hydrophobic, gas-permeable membrane and collecting the CH4 in a large headspace volume. The results of laboratory and field tests show that reliable dissolved δ13CH4 and 14CH4 samples can be readily collected over short time periods (∼4–24h), at relatively low cost and from a variety of surface water types. The initial results further support previous findings that dissolved CH4 may be significantly older than other forms of aquatic C, and is currently unaccounted for in many terrestrial C balances and models. This method is suitable for use in remote locations, and could potentially be used to detect the leakage of unique 14CH4 signatures from point sources into waterways, e.g. coal seam gas and landfill gas

Wastewater discharges and urban land cover dominate urban hydrology signals across England and Wales

Urbanisation is an important driver of changes in streamflow. These changes are not uniform across catchments due to the diverse nature of water sources, storage, and pathways in urban river systems. While land cover data are typically used in urban hydrology analyses, other characteristics of urban systems (such as water management practices) are poorly quantified which means that urbanisation impacts on streamflow are often difficult to detect and quantify. Here, we assess urban impacts on streamflow dynamics for 711 catchments across England and Wales. We use the CAMELS-GB dataset, which is a large-sample hydrology dataset containing hydro-meteorological timeseries and catchment attributes characterising climate, geology, water management practices and land cover. We quantify urban impacts on a wide range of streamflow dynamics (flow magnitudes, variability, frequency, and duration) using random forest models. We demonstrate that wastewater discharges from sewage treatment plants and urban land cover dominate urban hydrology signals across England and Wales. Wastewater discharges increase low flows and reduce flashiness in urban catchments. In contrast, urban land cover increases flashiness and frequency of medium and high flow events. We highlight the need to move beyond land cover metrics and include other features of urban river systems in hydrological analyses to quantify current and future drivers of urban streamflow

Methanotrophic potential of Dutch canal wall biofilms is driven by Methylomonadaceae

Global urbanization of waterways over the past millennium has influenced microbial communities in these aquatic ecosystems. Increased nutrient inputs have turned most urban waters into net sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4). Here, canal walls of five Dutch cities were studied for their biofilm CH4 oxidation potential, alongside field observations of water chemistry, and CO2 and CH4 emissions. Three cities showed canal wall biofilms with relatively high biological CH4 oxidation potential up to 0.48 mmol gDW-1 d-1, whereas the other two cities showed no oxidation potential. Salinity was identified as the main driver of biofilm bacterial community composition. Crenothrix and Methyloglobulus methanotrophs were observed in CH4-oxidizing biofilms. We show that microbial oxidation in canal biofilms is widespread and is likely driven by the same taxa found across cities with distinctly different canal water chemistry. The oxidation potential of the biofilms was not correlated with the amount of CH4 emitted but was related to the presence or absence of methanotrophs in the biofilms. This was controlled by whether there was enough CH4 present to sustain a methanotrophic community. These results demonstrate that canal wall biofilms can directly contribute to the mitigation of greenhouse gases from urban canals

Filtration artefacts in bacterial community composition can affect the outcome of dissolved organic matter biolability assays

Inland waters are large contributors to global carbon dioxide (CO2) emissions, in part due to the vulnerability of dissolved organic matter (DOM) to microbial decomposition and respiration to CO2 during transport through aquatic systems. To assess the degree of this vulnerability, aquatic DOM is often incubated in standardized biolability assays. These assays isolate the dissolved fraction of aquatic OM by size filtration prior to incubation. We test whether this size selection has an impact on the bacterial community composition and the consequent dynamics of DOM degradation using three different filtration strategies: 0.2 μm (filtered and inoculated), 0.7 μm (generally the most common DOM filter size) and 106 μm (unfiltered). We found that bacterial community composition, based on 16S rRNA amplicon sequencing, was significantly affected by the different filter sizes. At the same time, the filtration strategy also affected the DOM degradation dynamics, including the δ13C signature. However, the dynamics of these two responses were decoupled, suggesting that filtration primarily influences biolability assays through bacterial abundance and the presence of their associated predators. By the end of the 41-day incubations all treatments tended to converge on a common total DOM biolability level, with the 0.7 μm filtered incubations reaching this point the quickest. These results suggest that assays used to assess the total biolability of aquatic DOM should last long enough to remove filtration artefacts in the microbial population. Filtration strategy should also be taken into account when comparing results across biolability assays

Effect of Eucalyptus plantations, geology, and precipitation variability on water resources in upland intermittent catchments

Land-use change and climate variability have the potential to alter river flow and groundwater resources dramatically, especially by modifying actual evapotranspiration. Seven catchments with intermittent flow dominated by either winter-active perennial pastures (4 catchments) or Eucalyptus globulus plantations (3 catchments), located in 3 geologic settings of southeastern Australia, were studied for over 6 years to determine the primary controls on water resources. Groundwater levels in the pasture sites were stable through the 2011–2016 study period, while levels in the plantations declined in the same period. Streamflow occurred mainly during winter. Annual streamflow showed no difference clearly attributable to pasture versus plantation land use. The presence of grass buffers along streams enhances groundwater recharge and saturation-dependent overland flow, reducing the impacts of the plantations on streamflow. Site water balances indicated that the average annual actual evapotranspiration was 87–93% of precipitation for pasture catchments and 102–108% of precipitation for plantation catchments. Actual evapotranspiration greater than precipitation at the plantations was attributed to uptake of groundwater by the root system in parts of the catchments. Thus, change to groundwater storage is a critical component in the water balance. Actual evapotranspiration from pasture catchments was higher than previously estimated from global pasture and cropping data, instead matching global precipitation versus actual evapotranspiration curves for treed catchments

Amsterdam urban canals contain novel niches for methane-cycling microorganisms

Urbanised environments have been identified as hotspots of anthropogenic methane emissions. Especially urban aquatic ecosystems are increasingly recognised as important sources of methane. However, the microbiology behind these emissions remains unexplored. Here, we applied microcosm incubations and molecular analyses to investigate the methane‐cycling community of the Amsterdam canal system in the Netherlands. The sediment methanogenic communities were dominated by Methanoregulaceae and Methanosaetaceae, with co‐occurring methanotrophic Methanoperedenaceae and Methylomirabilaceae indicating the potential for anaerobic methane oxidation. Methane was readily produced after substrate amendment, suggesting an active but substrate‐limited methanogenic community. Bacterial 16S rRNA gene amplicon sequencing of the sediment revealed a high relative abundance of Thermodesulfovibrionia. Canal wall biofilms showed the highest initial methanotrophic potential under oxic conditions compared to the sediment. During prolonged incubations the maximum methanotrophic rate increased to 8.08 mmol g(DW) (−1) d(−1) that was concomitant with an enrichment of Methylomonadaceae bacteria. Metagenomic analysis of the canal wall biofilm lead to the recovery of a single methanotroph metagenome‐assembled genome. Taxonomic analysis showed that this methanotroph belongs to the genus Methyloglobulus. Our results underline the importance of previously unidentified and specialised environmental niches at the nexus of the natural and human‐impacted carbon cycle