Abstract
Globally, rivers and streams are important sources of carbon dioxide and methane, with small rivers contributing disproportionately relative to their size. Previous research on greenhouse ...gas (GHG) emissions from surface water lacks mechanistic understanding of contributions from streambed sediments. We hypothesise that streambeds, as known biogeochemical hotspots, significantly contribute to the production of GHGs. With global climate change, there is a pressing need to understand how increasing streambed temperatures will affect current and future GHG production. Current global estimates assume linear relationships between temperature and GHG emissions from surface water. Here we show non-linearity and threshold responses of streambed GHG production to warming. We reveal that temperature sensitivity varies with substrate (of variable grain size), organic matter (OM) content and geological origin. Our results confirm that streambeds, with their non-linear response to projected warming, are integral to estimating freshwater ecosystem contributions to current and future global GHG emissions.
The increasing frequency and severity of UK wildfires, attributed in part to the effects of climate change, highlights the critical role of fuel moisture content (FMC) of live and dead vegetation in ...shaping wildfire behaviour. However, current models used to assess wildfire danger do not perform well in shrub-type fuels such as Calluna vulgaris, requiring in part an improved understanding of fuel moisture dynamics on diurnal and seasonal scales. To this end, 554 samples of upper live Calluna canopy, live Calluna stems, upper dead Calluna canopy, dead Calluna stems, moss, litter and organic layer (top 5 cm of organic material above mineral soil) were sampled hourly between 10:00 and 18:00 on seven days from March-August. Using a novel statistical method for investigating diurnal patterns, we found distinctive diurnal and seasonal trends in FMC for all fuel layers. Notably, significant diurnal patterns were evident in dead Calluna across nearly all sampled months, while diurnal trends in live Calluna canopy were pronounced in March, June, and August, coinciding with the peak occurrence of UK wildfires. In addition, the moisture content of moss and litter was found to fluctuate above and below their relative ignition thresholds throughout the day on some sampling days. These findings underscore the impact of diurnal FMC variations on wildfire danger during early spring and late summer in Calluna dominated peatlands and the need to consider such fluctuations in management and fire suppression strategies.
Agricultural streams receive large inputs of nutrients, such as nitrate (NO₃⁻) and ammonium (NH₄⁺), which impact water quality and stream health. Streambed sediments are hotspots of biogeochemical ...reactivity, characterised by high rates of nutrient attenuation and denitrification. High concentrations of nitrous oxide (N₂O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N₂O. We investigated the effect of sediment type and seasonal variation on denitrification and N₂O production in the streambed of an agricultural UK stream. Denitrification was strongly controlled by sediment type, with sand-dominated sediments exhibiting potential rates of denitrification almost 10 times higher than those observed in gravel-dominated sediments (0.026 ± 0.004 N₂O–N lg g⁻¹ h⁻¹ for sand-dominated and 0.003 ± 0.003 N₂O–N µg g⁻¹ h⁻¹ for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO₃⁻, nitrite (NO₂⁻) and N₂O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from-winter to autumn. Our results indicate highest NO₃⁻ reduction occurred in sand-dominated sediments whilst highest N₂O concentrations occurred in gravel-dominated sediments. This suggests that finer-grained streambeds could play an important role in removing excess nitrogen from agricultural catchments without producing excess N₂O.
Eutrophication resulting from nutrient loading to freshwater habitats is a severe problem, leading to degradation of ecosystems, including deterioration of water quality, water clarity and loss of ...biodiversity. Measures enacted to restore degraded freshwater ecosystems often involve the reintroduction of submerged plants and aquatic animals with beneficial ecological functions. In a mesocosm experiment, three treatments (planting with Vallisneria natans, introduction of the snail Bellamya aeruginosa and a combined treatment with both plants and snails) were compared with controls to evaluate their effects on trophic state. The total nitrogen (TN), total phosphorus (TP) and chlorophyll a (Chl a) concentrations of planktonic and benthic algal samples were determined every two weeks, along with light intensity at the sediment surface. The plant-only treatment significantly reduced the TN levels and planktonic and benthic algal biomass and increased the light intensity at the sediment surface. The snail-only treatment reduced the concentrations of TN and reduced planktonic and benthic algal biomass. The combined treatment decreased the concentrations of TN and TP, reduced planktonic algal biomass and increased the light intensity on the sediment surface. The results indicate that while submerged plants and snails can both improve water quality, the most pronounced effect in aquatic ecosystems is achieved by their presence in combination. A combined reintroduction approach may provide enhanced benefits in restoring the eutrophic ecosystems, following the reduction of external nutrient loading. L'eutrophisation résultant de la charge nutritive dans les habitats d'eau douce est un problème grave, entraînant une dégradation des écosystèmes, y compris la détérioration de la qualité de l'eau, de la clarté de l'eau et la perte de biodiversité. Les mesures adoptées pour restaurer les écosystèmes d'eau douce dégradés impliquent souvent la réintroduction de plantes submergées et d'animaux aquatiques ayant des fonctions écologiques bénéfiques. Dans une expérience en mésocosme, trois traitements (plantation avec Vallisneria natans, introduction de l'escargot Bellamya aeruginosa et traitement combiné avec les plantes et les escargots) ont été comparés aux témoins pour évaluer leurs effets sur l'état trophique. Les concentrations d'azote total (TN), de phosphore total (TP) et de chlorophylle a (Chl a), des échantillons d'algues planctoniques et benthiques ont été déterminées toutes les deux semaines, ainsi que l'intensité de la lumière à la surface des sédiments. Le traitement aux plantes seules a réduit significativement les niveaux de TN et la biomasse d'algues planctoniques et benthiques et a augmenté l'intensité lumineuse à la surface des sédiments. Le traitement avec l'escargot seul a réduit les concentrations de TN et réduit la biomasse d'algues planctoniques et benthiques. Le traitement combiné a diminué les concentrations de TN et TP, réduit la biomasse d'algues planctoniques et augmenté l'intensité de la lumière sur la surface des sédiments. Les résultats indiquent que si les plantes submergées et les escargots peuvent améliorer la qualité de l'eau, l'effet le plus prononcé dans les écosystèmes aquatiques est obtenu par leur présence en combinaison. Une approche de réintroduction combinée peut apporter des avantages accrus dans la restauration des écosystèmes eutrophes, après réduction de la charge nutritive externe.
The surface pattern of vegetation influences the composition and humification of peat laid down during the development of a bog, producing a subsurface hydrological structure that is expected to ...affect both the rate and pattern of water flow. Subsurface peat structures are routinely derived from the inspection of peat cores. However, logistical limits on the number of cores that can be collected means that the horizontal extent of these structures must be inferred. We consider whether subsurface patterns in peat physical properties can be mapped in detail over large areas with ground‐penetrating radar (GPR) and complex conductivity by comparing geophysical measurements with peat core data along a 36 m transect through different microhabitats at Caribou Bog, Maine. The geophysical methods show promise. Peat horizons produced radar reflections because of changes in the volumetric moisture content. Although these reflections could not be directly correlated with the peat core data, they were related to the depth‐averaged peat properties which varied markedly between the microhabitats. Well‐decomposed peat below a hollow was characterized by a discontinuous sequence of chaotic wavy reflections, while distinct layering of the peat below an area of hummocks coincided with a pattern of parallel planar reflections. The complex conductivity survey showed spatial variation in the real and imaginary conductivities which resulted from changes in the pore water conductivity; peat structures may also have influenced the spatial pattern in the complex conductivity. The GPR and complex conductivity surveys enabled the developmental history of the different microhabitats along the studied transect to be inferred.
The structural arrangement of peat constituents controls the hydrological and thermal properties of peat. However, the importance of these structural characteristics on other physical processes ...within a peatland has not been fully assessed. Here, we evaluate the importance of peat structure on its ability to entrain biogenic gas bubbles and control ebullition, an important transport mechanism for methane. X‐ray computed tomography (CT) was applied to characterize the structure of a range of peats at varying levels of decomposition. The structural properties of the peat were quantified from a vector representation of the CT images, and the potential of each sample to entrain biogenic gas bubbles was quantified using a rule‐based Monte Carlo model that calculates the tortuosity of bubbles pathways through the peat. Sixty‐six percent of the variability in the trapping potential of the peat results from porosity variations and 34% from structural variations between samples. A metric that represents this structural control was not identified for all peat types because of difficulties adequately representing some peats as a vector network. However, for S. magellanicum peat we were able to establish that the influence of peat structure on the entrainment of gas bubbles is characterized by v, the average vector length of the stems and branches. Peat characterized by longer structural components (larger v) enhances the entrainment of gas bubbles. Our findings demonstrate the need to incorporate some representation of the peat structure in numerical models of biogenic gas transport in peat.
PDF
