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"Robison, Andrew"
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Superlinear scaling of riverine biogeochemical function with watershed size
River networks regulate carbon and nutrient exchange between continents, atmosphere, and oceans. However, contributions of riverine processing are poorly constrained at continental scales. Scaling relationships of cumulative biogeochemical function with watershed size (allometric scaling) provide an approach for quantifying the contributions of fluvial networks in the Earth system. Here we show that allometric scaling of cumulative riverine function with watershed area ranges from linear to superlinear, with scaling exponents constrained by network shape, hydrological conditions, and biogeochemical process rates. Allometric scaling is superlinear for processes that are largely independent of substrate concentration (e.g., gross primary production) due to superlinear scaling of river network surface area with watershed area. Allometric scaling for typically substrate-limited processes (e.g., denitrification) is linear in river networks with high biogeochemical activity or low river discharge but becomes increasingly superlinear under lower biogeochemical activity or high discharge, conditions that are widely prevalent in river networks. The frequent occurrence of superlinear scaling indicates that biogeochemical activity in large rivers contributes disproportionately to the function of river networks in the Earth system.
River networks play an important role in biogeochemical processes of the earth system. Here the authors show that cumulative river network function increases faster than watershed size for many biogeochemical processes, particularly at higher river flow, indicating large rivers contribute disproportionately to network function in the Earth System.
Journal Article
Glacier loss and vegetation expansion alter organic and inorganic carbon dynamics in high-mountain streams
High-mountain ecosystems are experiencing the acute effects of climate change, most visibly through glacier recession and the greening of the terrestrial environment. The streams draining these landscapes are affected by these shifts, integrating hydrologic, geologic, and biological signals across the catchment. We examined the organic and inorganic carbon dynamics of streams in four Alpine catchments in Switzerland to assess how glacier loss and vegetation expansion are affecting the carbon cycle of these high-mountain ecosystems. We find that the organic carbon concentration and fluorescence properties associated with humic-like compounds increase with vegetation cover within a catchment, demonstrating the increasing importance of allochthonous dissolved organic carbon sources following glacier retreat. Meanwhile, streams transitioned from carbon dioxide sinks to sources with decreasing glacier coverage and increased vegetation coverage, with chemical weathering and soil respiration likely determining the balance. Periods of sink behavior were also observed in non-glaciated streams, possibly indicating that the chemical consumption of carbon dioxide could be more common in high-mountain, minimally vegetated catchments than previously known. Together, these results demonstrate the dramatic shifts in carbon dynamics of high-mountain streams following glacier recession, with significant changes to both the organic and inorganic carbon cycles. The clear link between the terrestrial and aquatic zones further emphasizes the coupled dynamics with which all hydrologic and biogeochemical changes in these ecosystems should be considered, including the carbon sink or source potential of montane ecosystems.
Journal Article
Predicting climate-change impacts on the global glacier-fed stream microbiome
The shrinkage of glaciers and the vanishing of glacier-fed streams (GFSs) are emblematic of climate change. However, forecasts of how GFS microbiome structure and function will change under projected climate change scenarios are lacking. Combining 2,333 prokaryotic metagenome-assembled genomes with climatic, glaciological, and environmental data collected by the
Vanishing Glaciers
project from 164 GFSs draining Earth’s major mountain ranges, we here predict the future of the GFS microbiome until the end of the century under various climate change scenarios. Our model framework is rooted in a space-for-time substitution design and leverages statistical learning approaches. We predict that declining environmental selection promotes primary production in GFSs, stimulating both bacterial biomass and biodiversity. Concomitantly, predictions suggest that the phylogenetic structure of the GFS microbiome will change and entire bacterial clades are at risk. Furthermore, genomic projections reveal that microbiome functions will shift, with intensified solar energy acquisition pathways, heterotrophy and algal-bacterial interactions. Altogether, we project a ‘greener’ future of the world’s GFSs accompanied by a loss of clades that have adapted to environmental harshness, with consequences for ecosystem functioning.
Little is known about how climate change impacts glacier-fed streams (GFSs) microbiomes. Here, using a modelling framework based on global GFS metagenomic, climatic and environmental data the authors predict future increases in GFS bacterial biomass and diversity, but potential loss of clades adapted to extreme conditions.
Journal Article
Spatial and temporal heterogeneity of methane ebullition in lowland headwater streams and the impact on sampling design
Headwater streams are known sources of methane (CH₄) to the atmosphere, but their contribution to global scale budgets remains poorly constrained. While efforts have been made to better understand diffusive fluxes of CH₄ in streams, much less attention has been paid to ebullitive fluxes. We examine the temporal and spatial heterogeneity of CH₄ ebullition from four lowland headwater streams in the temperate northeastern United States over a 2-yr period. Ebullition was observed in all monitored streams with an overall mean rate of 1.00 ± 0.23 mmol CH₄ m−2 d−1, ranging from 0.01 to 1.79 to mmol CH₄ m−2 d−1 across streams. At biweekly timescales, rates of ebullition tended to increase with temperature. We observed a high degree of spatial heterogeneity in CH₄ ebullition within and across streams. Yet, catchment land use was not a simple predictor of this heterogeneity, and instead patches scale variability weakly explained by water depth and sediment organic matter content and quality. Overall, our results support the prevalence of CH₄ ebullition from streams and high levels of variability characteristic of this process. Our findings also highlight the need for robust temporal and spatial sampling of ebullition in lotic ecosystems to account for this high level of heterogeneity, where multiple sampling locations and times are necessary to accurately represent the mean rate of flux in a stream. The heterogeneity observed likely indicates a complex set of drivers affect CH₄ ebullition from streams which must be considered when upscaling site measurements to larger spatial scales.
Journal Article
Roles of sulfate adsorption and base cation supply in controlling the chemical response of streams of western Virginia to reduced acid deposition
Decreased acid deposition over recent decades has led to reductions in streamwater acidity on a widespread basis throughout the U.S. and Europe. A notable exception has been the southern Appalachian Mountains of the southeastern U.S., where declines in acid deposition have not translated into similar trends in stream chemistry in these watersheds with highly-weathered soils. To better characterize this observed behavior, streamwater samples collected at 64 sites in western Virginia on a quarterly basis from 1987 to 2011 were analyzed for chemical properties. Individual watershed response was strongly influenced by the dominant underlying bedrock, which affected sulfate $\\left({\\mathrm{S}\\mathrm{O}}_{4}^{2-}\\right)$ adsorption and base cation supply. Overall, pH increased at a majority of sites across all bedrock types. However, acid neutralizing capacity (ANC) decreased at most sites underlain by base-poor bedrock, suggesting the susceptibility to episodic acidification remains a serious threat to these streams. The declines in ANC were more closely related the depletion of base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) rather than increased ${\\mathrm{S}\\mathrm{O}}_{4}^{2-}$ concentration. Sites with higher relative ${\\mathrm{S}\\mathrm{O}}_{4}^{2-}$ adsorption exhibited little change in ANC. A mass balance analysis of sulfur at a base-poor watershed revealed that exports have recently surpassed inputs for the first time within the several-decade period of record. This pattern appears likely to continue, and if sustained, the depletion of the stored pool of sulfur signifies an important precursor for further improvements in streamwater acidity in the region.
Journal Article
Dominance of Diffusive Methane Emissions From Lowland Headwater Streams Promotes Oxidation and Isotopic Enrichment
Inland waters are the largest natural source of methane (CH 4 ) to the atmosphere, yet the contribution from small streams to this flux is not clearly defined. To fully understand CH 4 emissions from streams and rivers, we must consider the relative importance of CH 4 emission pathways, the prominence of microbially-mediated production and oxidation of CH 4 , and the isotopic signature of emitted CH 4 . Here, we construct a complete CH 4 emission budgets for four lowland headwater streams by quantifying diffusive CH 4 emissions and comparing them to previously published rates of ebullitive emissions. We also examine the isotopic composition of CH 4 along with the sediment microbial community to investigate production and oxidation across the streams. We find that all four streams are supersaturated with respect to CH 4 with diffusive emissions accounting for approximately 78–100% of total CH 4 emissions. Isotopic and microbial data suggest CH 4 oxidation is prevalent across the streams, depleting approximately half of the dissolved CH 4 pool before emission. We propose a conceptual model of CH 4 production, oxidation, and emission from small streams, where the dominance of diffusive emissions is greater compared to other aquatic ecosystems, and the impact of CH 4 oxidation is observable in the emitted isotopic values. As a result, we suggest the CH 4 emitted from small streams is isotopically heavy compared to lentic ecosystems. Our results further demonstrate streams are important components of the global CH 4 cycle yet may be characterized by a unique pattern of cycling and emission that differentiate them from other aquatic ecosystems.
Journal Article