Search for: All records

Abstract Headwater wetlands are important sites for carbon storage and emissions. While local- and landscape-scale factors are known to influence wetland carbon biogeochemistry, the spatial and temporal heterogeneity of these factors limits our predictive understanding of wetland carbon dynamics. To address this issue, we examined relationships between carbon dioxide (CO₂) and methane (CH₄) concentrations with wetland hydrogeomorphology, water level, and biogeochemical conditions. We sampled water chemistry and dissolved gases (CO₂and CH₄) and monitored continuous water level at 20 wetlands and co-located upland wells in the Delmarva Peninsula, Maryland, every 1–3 months for 2 years. We also obtained wetland hydrogeomorphologic metrics at maximum inundation (area, perimeter, and volume). Wetlands in our study were supersaturated with CO₂(mean = 315 μM) and CH₄(mean = 15 μM), highlighting their potential role as carbon sources to the atmosphere. Spatial and temporal variability in CO₂and CH₄concentrations was high, particularly for CH₄, and both gases were more spatially variable than temporally. We found that groundwater is a potential source of CO₂in wetlands and CO₂decreases with increased water level. In contrast, CH₄concentrations appear to be related to substrate and nutrient availability and to drying patterns over a longer temporal scale. At the landscape scale, wetlands with higher perimeter:area ratios and wetlands with higher height above the nearest drainage had higher CO₂and CH₄concentrations. Understanding the variability of CO₂and CH₄in wetlands, and how these might change with changing environmental conditions and across different wetland types, is critical to understanding the current and future role of wetlands in the global carbon cycle. 

Abstract Wetland soils are a key global sink for organic carbon (C) and a focal point for C management and accounting efforts. The ongoing push for wetland restoration presents an opportunity for climate mitigation, but C storage expectations are poorly defined due to a lack of reference information and an incomplete understanding of what drives natural variability among wetlands. We sought to address these shortcomings by (1) quantifying the range of variability in wetland soil organic C (SOC) stocks on a depressional landscape (Delmarva Peninsula, USA) and (2) investigating the role of hydrology and relative topography in explaining variability among wetlands. We found a high degree of variability within individual wetlands and among wetlands with similar vegetation and hydrogeomorphic characteristics. This suggests that uncertainty should be presented explicitly when inferring ecosystem processes from wetland types or land cover classes. Differences in hydrologic regimes, particularly the rate of water level recession, explained some of the variability among wetlands, but relationships between SOC stocks and some hydrologic metrics were eclipsed by factors associated with separate study sites. Relative topography accounted for a similar portion of SOC stock variability as hydrology, indicating that it could be an effective substitute in large-scale analyses. As wetlands worldwide are restored and focus increases on quantifying C benefits, the importance of appropriately defining and assessing reference systems is paramount. Our results highlight the current uncertainty in this process, but suggest that incorporating landscape heterogeneity and drivers of natural variability into reference information may improve how wetland restoration is implemented and evaluated. 

Abstract Methane (CH₄) dynamics in wetlands are spatially variable and difficult to estimate at ecosystem scales. Patches with different plant functional types (PFT) represent discrete units within wetlands that may help characterize patterns in CH₄variability. We investigate dissolved porewater CH₄concentrations, a representation of net CH₄production and potential source of atmospheric flux, in five wetland patches characterized by a dominant PFT or lack of plants. Using soil, porewater, and plant variables we hypothesized to influence CH₄, we used three modeling approaches—Classification and regression tree, AIC model selection, and Structural Equation Modeling—to identify direct and indirect influences on porewater CH₄dynamics. Across all three models, dissolved porewater CO₂concentration was the dominant driver of CH₄concentrations, partly through the influence of PFT patches. Plants in each patch type likely had variable influence on CH₄via root exudates (a substrate for methanogens), capacity to transport gas (both O₂from and CH₄to the atmosphere), and plant litter quality which impacted soil respiration and production of CO₂in the porewater. We attribute the importance of CO₂to the dominant methanogenic pathway we identified, which uses CO₂as a terminal electron acceptor. We propose a mechanistic relationship between PFT patches and porewater CH₄dynamics which, when combined with sources of CH₄loss including methanotrophy, oxidation, or plant‐mediated transport, can provide patch‐scale estimates of CH₄flux. Combining these estimates with the distribution of PFTs can improve ecosystem CH₄flux estimates in heterogenous wetlands and improve global CH₄budgets. 

River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.