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Atmospheric deposition of reactive nitrogen (Nr) has enriched oligotrophic lakes with nitrogen (N) in many regions of the
world and elicited dramatic changes in diatom community structure. The lakewater concentrations of nitrate that cause these
community changes remain unclear, raising interest in the development of diatom-based transfer functions to infer nitrate.
We developed a diatom calibration set using surface sediment samples from 46 high-elevation lakes across the Rocky Mountains
of the western US, a region spanning an N deposition gradient from very low to moderate levels (<1 to 3.2 kg Nr ha−1 year−1 in wet deposition). Out of the fourteen measured environmental variables for these 46 lakes, ordination analysis identified
that nitrate, specific conductance, total phosphorus, and hypolimnetic water temperature were related to diatom distributions.
A transfer function was developed for nitrate and applied to a sedimentary diatom profile from Heart Lake in the central Rockies.
The model coefficient of determination (bootstrapping validation) of 0.61 suggested potential for diatom-inferred reconstructions
of lakewater nitrate concentrations over time, but a comparison of observed versus diatom-inferred nitrate values revealed
the poor performance of this model at low nitrate concentrations. Resource physiology experiments revealed that nitrogen requirements
of two key taxa were opposite to nitrate optima defined in the transfer function. Our data set reveals two underlying ecological
constraints that impede the development of nitrate transfer functions in oligotrophic lakes: (1) even in lakes with nitrate
concentrations below quantification (<1 μg L−1), diatom assemblages were already dominated by species indicative of moderate N enrichment; (2) N-limited oligotrophic lakes
switch to P limitation after receiving only modest inputs of reactive N, shifting the controls on diatom species changes along
the length of the nitrate gradient. These constraints suggest that quantitative inferences of nitrate from diatom assemblages
will likely require experimental approaches. 相似文献
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David D. Bosch Alisa W. Coffin Joseph Sheridan Oliva Pisani Dinku M. Endale Tim C. Strickland 《水文研究》2021,35(8):e14334
The US Department of Agriculture-Agricultural Research Service Southeast Watershed Research Laboratory (SEWRL) initiated a hydrologic research program on the Little River Experimental Watershed (LREW) in 1967. Long-term (52 years) streamflow data are available for nine sites, including rainfall-runoff relationships and hydrograph characteristics regularly used in research on interactive effects of climate, vegetation, soils, and land-use in low-gradient streams of the US EPA Level III Southeastern Plains ecoregion. A summary of prior research on the LREW illustrates the impact of the watershed on building a regional understanding of hydrology and water quality. Climatic and streamflow data were used to make comparisons of scale across the nine nested LREW watersheds (LRB, LRF, LRI, LRJ, LRK, LRO, LRN, LRM, and LRO3) and two regional watersheds (Alapaha and Little River at Adel). Annual rainfall for the largest LREW, LRB, was 1200 mm while average annual streamflow was 320 mm. Annual rainfall, streamflow, and the ratio between annual streamflow and rainfall (Sratio) were similar (α = 0.05) across LREWs LRB, LRF, LRI, LRJ, LRK, and LRO. While annual rainfall within the 275 ha LRO3 was found to be similar to LRO and LRM (α = 0.05), annual streamflow and Sratio were significantly different (α = 0.05). Comparisons of annual rainfall, streamflow, and Sratio between LRB and the regional watersheds indicated no differences (α = 0.05). Based upon this analysis, most regional watersheds shared similar hydrologic characteristics. LRO3 was an exception, where increases in row crops and decreases in forest coverage resulted in increased streamflow. LREW data have been instrumental in building considerable scientific understanding of flow and transport processes for these stream systems. Continued operation of the LREW hydrologic network will support hydrologic research as well as environmental quality and riparian research programs that address emerging and high priority natural resource and environmental issues. 相似文献
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Concentrations of weathering products in streams often show relatively little variation compared to changes in discharge, both at event and annual scales. In this study, several hypothesized mechanisms for this “chemostatic behavior” were evaluated, and the potential for those mechanisms to influence relations between climate, weathering fluxes, and CO2 consumption via mineral weathering was assessed. Data from Loch Vale, an alpine catchment in the Colorado Rocky Mountains, indicates that cation exchange and seasonal precipitation and dissolution of amorphous or poorly crystalline aluminosilicates are important processes that help regulate solute concentrations in the stream; however, those processes have no direct effect on CO2 consumption in catchments. Hydrograph separation analyses indicate that old water stored in the subsurface over the winter accounts for about one-quarter of annual streamflow, and almost one-half of annual fluxes of Na and SiO2 in the stream; thus, flushing of old water by new water (snowmelt) is an important component of chemostatic behavior. Hydrologic flushing of subsurface materials further induces chemostatic behavior by reducing mineral saturation indices and increasing reactive mineral surface area, which stimulate mineral weathering rates. CO2 consumption by carbonic acid mediated mineral weathering was quantified using mass-balance calculations; results indicated that silicate mineral weathering was responsible for approximately two-thirds of annual CO2 consumption, and carbonate weathering was responsible for the remaining one-third. CO2 consumption was strongly dependent on annual precipitation and temperature; these relations were captured in a simple statistical model that accounted for 71% of the annual variation in CO2 consumption via mineral weathering in Loch Vale. 相似文献
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Aaron Barkatt Mervet S Boulos Alisa Barkatt William Sousanpour Morad A Boroomand Pedro B Macedo John A O&#x;Keefe 《Geochimica et cosmochimica acta》1984,48(2):361-371
Leach tests carried out on tektite specimens (indochinites and australites) under high-dilution conditions show a common behavior characterized by low leach rates (1.8 × 10?5g · m?2 · d?1or 7.2 × 10?12m · d?1 at 23°C) and an activation energy of (79.6 ± 0.7) × 103J · mol?1. The extent of selective leaching is very small, of the order of 10?8 m. Extrapolation of test results over the lifetime of the tektites gives an excellent agreement with field observations on the extent of corrosion, and this is an important step in establishing the validity of laboratory tests as a basis for the development of models and predictions concerning long-term durabilities at least in the limiting case of high dilution or rapid flow. The results are also shown to be in agreement with various previous observations on the corrosion resistance of tektites. The chemical durability of tektites is observed to be consistent with their composition, highlighting requirements for high corrosion resistance in glasses; these requirements include a silica content in excess of 67 mol%, an extremely low water content and an alkali content which is low both absolutely and relative to the di- and poly-valent metal oxide levels. It is shown that artificial glasses which fulfil these criteria are no less corrosion-resistant than the corresponding natural glasses. These conclusions have bearing on the development as well as on the evaluation of glasses intended for very long service, such as radioactive waste vitrification media. 相似文献
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Timothy P. Wangler Alisa Stratulat Philippa Duffus Jean H. Prévost George W. Scherer 《Environmental Earth Sciences》2011,63(7-8):1565-1572
Many historically and culturally significant buildings have sandstones that contain swelling clay inclusions in the binding phase. Differential strains that evolve during wetting and drying cycles can generate stresses that are on the order of the strength of the stone, leading to degradation. Most damage observed in the field is surface delamination and buckling of the stone over a flaw, indicating that the damage is occurring during wetting. Classical buckling theory predicts buckling to occur at a particular aspect ratio, or flaw size. The results of this study confirm buckling theory experimentally. Through finite-element simulation and experiment, the study then explores a potential flaw propagation mechanism whereby nonuniform wetting patterns generate stress intensities capable of flaw propagation. As a result, small natural flaws can grow to the critical size necessary for buckling. 相似文献
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Linking transit times to catchment sensitivity to atmospheric deposition of acidity and nitrogen in mountains of the western United States 
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下载免费PDF全文 Transit times are hypothesized to influence catchment sensitivity to atmospheric deposition of acidity and nitrogen (N) because they help determine the amount of time available for infiltrating precipitation to interact with catchment soil and biota. Transit time metrics, including fraction of young water (Fyw) and mean transit time (MTT), were calculated for 11 headwater catchments in mountains of the western United States based on differences in the amplitude of the seasonal signal of δ18O in streamflow and precipitation. Results were statistically compared with catchment characteristics to elucidate controlling mechanisms. Transit times also were compared with stream solute concentrations to test the hypothesis that transit times are a primary influence on weathering rates and biological assimilation of atmospherically deposited N. Results indicate that transit times in the study catchments are strongly related to soil, vegetation, and topographic characteristics, with barren terrain (bare rock and talus) and steep slopes linked to high Fyw and short MTT, whereas forest soil (hydrogroup B) was linked to low Fyw and greater MTT. Concentrations of silicate weathering products (Na+ and Si) were negatively related to Fyw and barren terrain, and positively related to MTT and forest soil, supporting the concept that weathering fluxes and buffering capacity tend to be low in alpine areas due to short transit times. Nitrate concentrations were positively related to N deposition, catchment slope, and barren terrain, and negatively related to forest, indicating that hydrologic and/or biogeochemical processes associated with steep slopes limit uptake of atmospherically deposited N by biota. Interannual and seasonal variability in transit times and source water contributions in the study catchments was substantial, reflecting the influence of strong temporal variations in snowmelt inputs in high‐elevation catchments of the western United States. Results from this study confirm that short transit times in these areas are a key reason they are highly sensitive to atmospheric pollution and climate change. 相似文献
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M. Alisa Mast Sheila F. Murphy David W. Clow Colin A. Penn Graham A. Sexstone 《水文研究》2016,30(12):1811-1823
Water quality of the Big Thompson River in the Front Range of Colorado was studied for 2 years following a high‐elevation wildfire that started in October 2012 and burned 15% of the watershed. A combination of fixed‐interval sampling and continuous water‐quality monitors was used to examine the timing and magnitude of water‐quality changes caused by the wildfire. Prefire water quality was well characterized because the site has been monitored at least monthly since the early 2000s. Major ions and nitrate showed the largest changes in concentrations; major ion increases were greatest in the first postfire snowmelt period, but nitrate increases were greatest in the second snowmelt period. The delay in nitrate release until the second snowmelt season likely reflected a combination of factors including fire timing, hydrologic regime, and rates of nitrogen transformations. Despite the small size of the fire, annual yields of dissolved constituents from the watershed increased 20–52% in the first 2 years following the fire. Turbidity data from the continuous sensor indicated high‐intensity summer rain storms had a much greater effect on sediment transport compared to snowmelt. High‐frequency sensor data also revealed that weekly sampling missed the concentration peak during snowmelt and short‐duration spikes during rain events, underscoring the challenge of characterizing postfire water‐quality response with fixed‐interval sampling. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Francis P Miknis Daniel A Netzel J.Ward Smith M.Alisa Mast Gary E Maciel 《Geochimica et cosmochimica acta》1982,46(6):977-984
Cross polarization, magic-angle spinning 13C NMR measurements have been made on raw oil shales that represent a variety of geologic ages, origins, depositional environments and source locations. A high degree of correlation was established between the fraction of aliphatic carbon measured by 13C NMR, and the genetic potential, calculated from Fischer assay data. The correlation is independent of the type of kerogen in the raw shale, and its degree of evolution. A short discussion on the validity of various correlations between physical/chemical properties of oil shales and Fischer assay oil yields is given. 相似文献