Journal of Geophysical Research: Biogeosciences最新文献

River damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river-reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO₂ among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO₂ exchange. Conversely, the distribution of CH₄ and N₂O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH₄ in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river-reservoir systems.

The quality and quantity of organic matter (OM) in a river system directly affects ecosystem health; thus, managers benefit from an in-depth understanding of the drivers and sources of OM. In the Snake River, a highly altered river-reservoir system in the semi-arid western United States, OM production and loading are key drivers of reservoir anoxia, which leads to several deleterious processes such as mercury methylation. However, sources and quantities of OM to the Snake River, and the effects of impoundment on OM moving through the river-reservoir system, are not well understood. Particulate organic carbon (POC), dissolved organic carbon (DOC), particulate nitrogen (PN), chlorophyll a (chl-a), and δ¹⁵N–PN and δ¹³C–POC isotopic ratios were measured bi-weekly for over 2 years at four locations through the Snake River Hells Canyon Reservoir Complex to determine spatial and temporal patterns of OM quantities and sources. POC concentrations increased through the riverine zone upstream of the reservoirs, likely due to in situ primary production and/or inputs from tributaries and agricultural drains; then decreased through the most upstream reservoir likely due to particle settling. Isotopic ratios and other OM source indicators (δ¹⁵N–PN, δ¹³C–POC, POC:PN, chl-a:POC) show that the dominant source of particulate OM was phytoplankton with seasonal terrestrial/macrophytic inputs. Results highlight the effects of major tributary and agricultural drain inputs, primary production, and impoundment on OM composition and concentration through a large river-reservoir system and may inform water quality management efforts in this and similar systems.

Organic by-products are released into the surrounding soil during the terrestrial decomposition of animal remains. The affected area, known as the Cadaver Decomposition Island (CDI), can undergo biochemical changes that contribute to landscape heterogeneity. Soil bacteria are highly sensitive to labile inputs, but it is unknown how they respond to shifts in dissolved organic matter (DOM) quantity and quality resulting from animal decomposition. We aimed to evaluate the relationship between soil DOM composition and bacterial activity/function in CDIs under a Canadian temperate continental climate. This was studied in soils surrounding adult pig carcasses (n = 3) that were surface deposited within a mixed forested environment (Trois-Rivières, Québec) in June 2019. Using fluorescence spectroscopy and dissolved organic carbon analyses, we detected a pulse of labile protein-like DOM during the summer season (day 55). This was found to be an important driver of heightened soil bacterial respiration, cell abundance and potential carbohydrate metabolism. These bacterial disturbances persisted into the cooler autumn season (day 156) and led to the gradual transformation of labile DOM inputs into microbially sourced humic-like compounds. By the spring (day 324), DOM quantities and bacterial measures almost recovered, but DOM quality remained distinct from surrounding vegetal humic signals. All observed effects were spatially constrained to the topsoil (A-horizon) and within 20 cm laterally from the carcasses. These findings provide valuable insight into CDI organic matter cycling within a cold-climate ecosystem. Repeated CDI studies will however be required to capture the changing dynamics resulting from increasing global temperatures.

This study is the first assessment of a fjord Ecological Quality Status (EcoQS) by comparing both the traditional morphology-based and emerging metabarcoding techniques in benthic foraminifera. For this, we focus on historically polluted Idefjord on the Swedish Norwegian border, which has experienced high effluent load from pulp and paper mill for almost a century. Based on our results, the morphological data was more sensitive to “naturally stressed” conditions, like course sediments and cascading water inflows at fjord sills. Generally, both data sets report congruous responses in the EcoQS and benthic foraminiferal assemblages to environmental stress factors, showing highest diversity at the coastal reference station and the outer fjord, with a diversity decline in proximity of industrial facilities and at the most oxygen depleted sites in the inner fjord. Genetic methods tend to overestimate EcoQS at highly anoxic sites probably due to a presence of dormant propagules or extraorganismal DNA, emphasizing a need for cross-correlation with morphological methods to validate EcoQS assessment in such conditions.

Freeze-thaw cycles (FTC) alter soil function through changes to physical organization of the soil matrix and biogeochemical processes. Understanding how dynamic climate and soil properties influence FTC may enable better prediction of ecosystem response to changing climate patterns. In this study, we quantified FTC occurrence and frequency across 40 National Ecological Observatory Network (NEON) sites. We used site mean annual precipitation (MAP) and mean annual temperature (MAT) to define warm and wet, warm and dry, and cold and dry climate groupings. Site and soil properties, including MAT, MAP, maximum-minimum temperature difference, aridity index, precipitation as snow (PAS), and organic mat thickness, were used to characterize climate groups and investigate relationships between site properties and FTC occurrence and frequency. Ecosystem-specific drivers of FTC provided insight into potential changes to FTC dynamics with climate warming. Warm and dry sites had the most FTC, driven by rapid diurnal FTC close to the soil surface in winter. Cold and dry sites were characterized by fewer, but longer-duration FTC, which mainly occurred in spring and increased in number with higher organic mat thickness (Spearman's ⍴ = 0.97, p < 0.01). The influence of PAS and MAT on the occurrence of FTC depended on climate group (binomial model interaction p (χ²) < 0.05), highlighting the role of a persistent snowpack in buffering soil temperature fluctuations. Integrating ecosystem type and season-specific FTC patterns identified here into predictive models may increase predictive accuracy for dynamic system response to climate change.

The oceans play a pivotal role in mitigating climate change by sequestering approximately 25% of annually emitted carbon dioxide (CO₂). High-latitude oceans, especially the Arctic continental shelves, emerge as crucial CO₂ sinks due to their cold, low saline, and highly productive ecosystems. However, these heterogeneous regions remain inadequately understood, hindering accurate assessments of their carbon dynamics. This study investigates variation in pCO₂ levels during peak ice sheet melt, in the Greenland coastal ocean and estimates rates of air-sea exchange across 6° of latitude. The East and West coast of Greenland displayed distinct regions with unique controlling factors. Though, both coasts represent CO₂ sinks in summer. Geographical variation in pCO₂ and air-sea exchange was linked intricately to freshwater export from the Greenland ice sheet and levels of primary production in these ecosystems. Air-sea exchange of CO₂ ranged from 0.23 to −64 mmol m⁻² day⁻¹. However, we found that flux estimation faces substantial uncertainties (up to 672%) due to wind product averaging and gas exchange formula selection. Upscaling only heightens this uncertainty leading to wide ranging estimates of Greenland coastal CO₂ uptake between −16 and −26 Tg C year⁻¹ (This study, Dai et al., 2022, https://doi.org/10.1146/annurev-earth-032320-090746; Laruelle et al., 2014, https://doi.org/10.1002/2014gb004832). Obtaining a reliable assessment of air-sea CO₂ exchange necessitates data collection across seasons, and, even more so, refinement of the gas transfer velocity estimations in the Arctic coastal zone.

Inland aquatic systems, such as reservoirs, contribute substantially to global methane (CH₄) emissions; yet they are among the most uncertain contributors to the total global carbon budget. Reservoirs generate significant amounts of CH₄ within their bottom sediment, where the gas is stored and can easily escape via ebullition. Due to the large spatial and temporal variability associated with ebullition, CH₄ fluxes from these aquatic systems are challenging to quantify. To address these uncertainties, six different water storage reservoirs, with average flux rates ranging between 20 and 678 mg CH₄ m⁻² d⁻¹, were hydro-acoustically surveyed using a previously established technique to investigate the spatial variability of free gas stored at the sediment surface that could be released as bubbles. Sediment samples and vertical profiles of temperature and dissolved oxygen were also collected to understand their respective influences on sediment gas formation. We found that the established relation used to determine sediment gas storage via the sonar technique, which relied solely on acoustic backscatter (Sv_max), tended to underestimate gas storage in shallower, siltier sediment zones and overestimate gas storage in coarser (>2 mm) sediment zones. In response, we introduce an improved model, incorporating gas and sediment type as correction factors for gas attenuation effects on Sv_max values. The extended model is able to elucidate patterns within the gas volume data, revealing clearer trends across different sediment types. This research provides valuable data and methodological insights that can enhance the accuracy of greenhouse gas modeling and budget assessments for reservoirs.

In-stream biogeochemical processing, typically associated with base flow conditions, has recently been assessed at higher discharges, aided by high frequency monitoring. However, the potential for nutrient and carbon processing is still largely unknown in streams impacted by agriculture, representing major pathways for eutrophication and diffuse pollution. In this study, we measured solute concentrations and gross primary productivity (GPP) and ecosystem respiration (ER) to infer nitrate (NO₃⁻) and dissolved organic carbon (DOC) supply and demand across contrasting hydrological conditions. As expected, solute supply greatly surpassed in-stream biological demand for both NO₃⁻ and DOC for intermediate to large discharges. However, during four consecutive weeks in summer, lowered NO₃⁻ supply and high metabolic activity led to a 60% and 31% reduction in stream NO₃⁻ and DOC export. We also compared metabolism-discharge versus solute concentration-discharge patterns during storm events to better understand biogeochemical responses to high flows. Metabolic rates showed a contrasting response to storm events: ER increased while GPP decreased following declines in NO₃⁻ concentrations. The positive correlation between GPP and NO₃⁻ concentrations suggests that GPP suppression can be partially attributed to decreased NO₃⁻ availability during storm events. This study supports the idea that agricultural streams have a limited capacity to biologically process DOC and NO₃⁻. However, it also emphasizes that the balance between supply and demand can vary from severe saturation to limitation, depending on seasonal fluctuations in discharge and metabolic activity, highlighting the crucial role of mitigating pollution at its source during hydrologically active periods to improve water quality.

Temperate lakes worldwide are losing ice cover but the implications for under-ice thermal dynamics are poorly constrained. Using a 92-year record of ice phenology from a temperate and historically dimictic lake, we examined trends, variability, and drivers of ice phenology and under-ice temperatures. The onset of ice formation decreased by 23 days century⁻¹, which can be largely attributed to warming air temperatures. Ice-off date has become substantially more variable with spring air temperatures and cumulative February through April snowfall explaining over 80% of the variation in timing. As a result of changing ice phenology, total ice duration contracted by a month and more than doubled in interannual variability. Using weekly under-ice temperature profiles for the most recent 36 years, we found that shorter ice duration decreased winter inverse stratification and was associated with an extended spring mixing period. We illustrate the limitations of relying on discrete ice clearance dates in our assumptions around under-ice thermal dynamics by presenting high-frequency under-ice observations in two recent winters: one with intermittent ice cover and a year with slow spring ice clearance.

We evaluate the use of clumped isotopes of methane (CH₄) to fingerprint local atmospheric sources of methane. We focus on a regenerative stormwater conveyance (RSC) stream wetland site running through the University of Maryland campus, which emits methane due to its engineering. Air samples in the RSC were collected at different heights above the surface and at different times of the day including both early in the morning, after methane accumulated below the nocturnal boundary layer, and late in the afternoon when convection mixed air to the cloud layer. Measured Δ¹²CH₂D₂ values of air samples record mixing between locally produced methane with low D/H and ambient air. The Δ¹²CH₂D₂ of the near surface air collected at the RSC during the early morning ranges from ∼+23‰ to ∼+35‰ which is lower than the ∼+50‰ values of tropospheric air. Mixing between background air (with Δ¹²CH₂D₂ ∼+50‰) and methane captured from chamber and bubble samples, as well as produced in incubation (all with negative Δ¹²CH₂D₂), explains the observed values of Δ¹²CH₂D₂ and Δ¹³CH₃D of near surface RSC air samples. The effect of mixing with biogenic sources on Δ¹³CH₃D is much smaller. The findings demonstrate how methane isotopologues can be used as a tool not only to fingerprint local contributions to these greenhouse gas emissions but also to identify sources of near-surface methane hot spots.